Alkaline leachate, dust generation, and foul smell during the stacking process of natural rubber biosludge (NRBS) can pollute surrounding water, soil, and air. In this study, natural rubber chemically activated carbon (NRCAC) has been synthesized for the first time from NRBS by pyrolysis using ZnCl2 at 700 °C for adsorptive removal of Cr(VI) and methylene blue (MB) from aqueous solutions. Both NRBS and NRCAC were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Brunauer–Emmett–Teller (BET), and thermogravimetric analyzer (TGA). FTIR and SEM-EDS suggested significant functional and morphological transformations in NRCAC. Experimental investigations of different process parameters, such as pH, concentration, contact time, salt concentration, etc., were conducted to study their influences on adsorption. Adsorption and desorption kinetics followed a pseudo-second-order model, while adsorption equilibrium followed Liu isotherm. Maximum uptake calculated from the Liu model was 81.28 and 211.90 mg/g for Cr(VI) and MB, respectively. Thermodynamic analysis established spontaneous and endothermic adsorption. Up to five adsorption/desorption cycles were conducted using eluents such as 1 M NaOH and water for Cr(VI) and MB, respectively. Electrostatic attraction and ion-exchange favored Cr(VI)/MB adsorption, while hydrogen bonding and π–π stacking were significant in MB uptake. Overall findings suggest that NRBS (a renewable agro-industrial, abundant, and freely available) could be employed to synthesize biochar for adsorptive removal of wastewater containing Cr(VI)/MB.

  • Natural rubber chemically activated carbon (NRCAC) was prepared for first time from natural rubber biosludge (NRBS).

  • BET surface area of 4.62 and 414.16 m2/g was reported for NRBS and NRCAC, respectively.

  • Maximum experimental uptake of Cr(VI) and MB by NRCAC was 78 and 203 mg/g, respectively.

  • NaCl and CaCl2 showed a negative effect on Cr(VI) removal but a positive effect on MB removal.

  • Five regeneration cycles of NRCAC using 1 M NaOH for Cr(VI) and water for MB.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Due to persistent nature of the pollutants, discharge of heavy metals and dye ions into aquatic environment has drawn worldwide attention (Ding et al. 2021). Hexavalent chromium (Cr(VI)) is commonly spilled into water streams by wastewater from tanneries, textile printing, metallurgy, batteries, electroplating, and other industries (Ding et al. 2021). It can enter cells via isostructural anion (such as phosphate) transfer channels, and Cr(VI) metabolism intermediates can produce severe DNA adducts and cross-links, leading to irreversible impairment in animals, plants, and humans (Guo et al. 2021). The United States Environmental Protection Agency has set a maximum limit of 0.05 mg/L for Cr(VI) concentration in drinking water (USEPA 2016). Thus, it is critical to investigate feasible methods for removing Cr(VI) from wastewater to safeguard human health and the aquatic ecosystem.

Similarly, effluent-containing dyes, generated mainly by textiles, paper, food, cosmetics, medicines, and leather industries, are a significant source of water pollution as they are recalcitrant to conventional treatment methods (Tang et al. 2021). This is because dyestuffs have a complicated chemical structure and are resistant to light, heat, and oxidizing agents. Over 10,000 dyes were claimed to be commercially available, with an annual output of over 7 × 105 tons (Tang et al. 2021). In addition, the literature has stated that up to 20% of colors used in industry are lost in industrial effluents (Tang et al. 2021). Dye and dyeing effluents have a high chromaticity and chemical oxygen demand (COD) content. Even a tiny amount of these dyes discharged into water resources can impact aquatic life and the food web. For example, ingestion of methylene blue (MB), a cationic dye, may lead to inflammation of leptomeninges, neuronal apoptosis, increased heart rate, nausea, and vomiting (Hosseini et al. 2022). As a result, high-performance approaches for removing Cr(VI) and MB are urgently needed to protect water quality and human health.

Conventional techniques, such as chemical coagulation, chemical precipitation, ion exchange, adsorption, and membrane filtration, have been applied to remove various pollutants (Xu et al. 2020). Most of these treatment methods become too costly to meet local regulations for high-volume dilute discharge (Hosseini et al. 2022). Adsorption, on the other hand, is considered the most effective technique for removal of heavy metals and dye because of its high efficiency, ease of use, and cost-effective nature (Xu et al. 2020).

The ‘activation’ of raw carbon material refers to its transformation into a porous material with a high adsorptive surface area. This activated carbon (AC) can be obtained from two key routes: physical and chemical activations. The former involves carbonizing the substance in an inert environment before being activated at high temperatures with steam or CO2 as an activating reagent. In chemical activation, before co-carbonization, the base material is impregnated with appropriate chemicals such as salts (e.g., ZnCl2, FeCl3, and MgCl2) or acids (e.g., H2SO4, HNO3, and H3PO4) (Ramirez-Gutierrez et al. 2020). Zinc chloride (ZnCl2) is one of the most promising modifiers due to its low melting point. The melting point (290 °C) of ZnCl2 is much less than the activation temperature (700 °C) for the preparation of natural rubber chemically activated carbon (NRCAC) and its boiling point (732 °C) is less than 700 °C. Therefore, ZnCl2 remains in liquid condition at the time of the pyrolysis process, and liquid ZnCl2 is intercalated into the carbon matrix of NRBS, resulting in pore development. The reaction between the carbon atoms and dehydrating agent is promoted in the extended interlayers of carbon (Hu et al. 2017).

A number of recent articles focus on preparation of chemically AC from various waste materials such as activated sludge (van Veenhuyzen et al. 2021), rice and coffee husk (Paredes-Laverde et al. 2021), walnut shells (Roquia et al. 2021), pea peels (Sahlabji et al. 2022), silver berry seeds (Benmahdi et al. 2021), waste paper (Tang et al. 2021), vinegar residue (Ding et al. 2021), sea buckthorn stones (Guo et al. 2021), and aerobic granular sludge (Yan et al. 2020). Therefore, one of the sources of raw materials utilized by researchers is waste sludge for adsorbent preparation. Nowadays, sludge waste management is a big problem concerning its disposal. The methods for dumping lead groundwater contamination and, thereby, other socio-economic impacts (Suksiripattanapong et al. 2015; Rigby et al. 2016). Many studies (Suksiripattanapong et al. 2015; Rigby et al. 2016; Rahman et al. 2017; Heniegal et al. 2020) have investigated this area and found that contamination of pollutants is high in groundwater and a nuisance due to dumping ETP area premises. There is a growing need to find alternative solutions for sludge management. One of the alternative solutions is its utilization as raw materials for adsorbent preparation. The present study and authors’ previous study (Mahapatra et al. 2021, 2022) have also attempted to use effluent treatment plant (ETP) sludge collected from nearby natural rubber processing industries in Tripura, the second largest natural rubber (Hevea brasiliensis) producing state in India. Environmental awareness was not adequate at the time of planning and establishing these industries. ETP sludge waste is generated in ETPs of these small and medium-scale manufacturing units during the production of Indian standard natural rubber (ISNR), rubber latex thread, gloves, etc., using latex scarp rubber and rubber latex. These industries are also not in a position to treat and dispose of the ETP sludge wastes (named natural rubber biosludge (NRBS)) they generate through proper utilization. Our research group attempted for the first time to prepare AC from NRBS to remove Cr(VI) and MB (Mahapatra et al. 2021). As per our literature review, the work presented in this manuscript is the first initiation of the utilization of NRBS for preparation of porous activated carbon (NRCAC) with a high specific surface area via a ZnCl2-activated pyrolysis process. The other objectives of this study were to characterize the chemical properties and surface morphology of NRBS and NRCAC; to evaluate the efficiency of removal of Cr(VI) and MB in a batch adsorption by investigating effect of variation of different process parameters, kinetics, isotherms, and thermodynamic properties; and to investigate the desorption and reusability of NRCAC using several solvents.

Source of raw materials and reagents

The wet solid waste was collected from Brite Rubber Processors Private Limited (N 23° 53.3′ E 91° 21.9′) at Bodhjungnagar, West Tripura, India. The collected solid waste was dried at 70 °C for 2 h in a tray dryer, followed by further drying at 105 °C in an air dryer. The resulting dried mass was named natural rubber industry biosludge (NRBS). NRBS was stored as a fine size passing through a 100 BSS mesh. This procedure was published elsewhere (Mahapatra et al. 2021). All used analytical grade chemicals were purchased from MERCK except MB (Sigma Aldrich).

Synthesis of NRCAC

Adsorption experiments were conducted to compare the adsorption capacities of eight carbonized materials prepared under different conditions (Table 1) separately in a four-legged vertical reactor (FLVR, Mecco Engineering Co., India) with 100 mL/min N2 purge or in a muffle furnace. Briefly, NRBS was ground and passed through a 100 BSS mesh. Then ground NRBS was impregnated with ZnCl2 in ratios (NRBS:ZnCl2) of 1:0, 1:1, and 1:2.5. Each impregnated mass was mixed with deionized water (DW) in the ratio of 3:2 and stirred for 24 h at 40 °C. Then, each suspension was oven-dried at 105 °C. The resulting solid mass was heated separately in FLVR or muffle furnace. The temperature was raised to 550 or 700 °C at a ramping rate of 10 °C/min, and then the temperature remained constant for 2 h, subsequently cooling to an ambient temperature. The product was mixed with 3 M HCl and stirred for 4 h, followed by continuous DW washing and filtering using a Buchner funnel under vacuum until a constant filtrate pH was achieved. The wet mass was dried, ground, passed through a 100 BSS mesh, and stored for selection of ideal conditions for AC preparation through adsorption experiments.

Table 1

(A) Activated carbon preparation followed by adsorption experiments (m/V= 1 g/L, Co = 100 mg/L, Sags = 150 rpm, t = 24 h, T = 30 °C, and V = 0.1 L). (B) Uptake capacities of NRBS, NRAC, and NRCAC (m/VNRBS/NRAC (Cr(VI)/MB) = 10 g/L, m/VNRCAC,Cr(VI) = 3 g/L, m/VNRCAC,MB = 1 g/L, Sags = 150 rpm, t = 24 h, T = 30 °C, and V = 0.1 L)

(A)
Activated carbon preparationFinal heating temperature (°C)550
700
 Impregnation ratio (NRBS:ZnCl21:0 1:1 1:1 1:2.5  1:0 1:1 1:1 1:2.5 
 Apparatus for conversion FLVR Muffle furnace FLVR FLVR  FLVR Muffle furnace FLVR FLVR 
 N2 flow rate (mL/min) 100 100 100  100 100 100 
 Yield after final heatinga (%) (Equation (3)) 52.14 ± 0.43 60.04 ± 0.94 68.25 ± 0.30 75.62 ± 0.32  24.23 ± 0.42 41.55 ± 0.40 59.78 ± 0.27 66.92 ± 0.43 
Adsorption experiments qe,exp,Cr(VI)b (mg/g) 1.45 ± 0.04 16.90 ± 0.28 26.97 ± 0.43 25.46 ± 0.44  3.98 ± 0.06 26.87 ± 0.16 35.83 ± 0.33 35.79 ± 0.28 
qe,exp,MBb (mg/g) 8.31 ± 0.10 47.83 ± 0.19 43.45 ± 0.31 40.61 ± 0.62  26.67 ± 0.34 47.81 ± 0.14 49.89 ± 0.41 49.42 ± 0.37 
(B)
qeb (mg/g)
100 mg/L
500 mg/L
900 mg/L
Adsorbent typeSBET (m2/g)Cr(VI)MBCr(VI)MBCr(VI)MB
NRBS 4.62  2.39 ± 0.02 5.80 ± 0.07 9.94 ± 0.14 12.46 ± 0.19 15.83 ± 0.18 17.80 ± 0.25  
NRAC 20.66  4.48 ± 0.08 8.26 ± 0.06 21.15 ± 0.38 19.52 ± 0.34 34.11 ± 0.45 23.53 ± 0.43  
NRCAC 414.16  28.64 ± 0.30 89.95 ± 0.43 69.10 ± 0.35 186.42 ± 0.27 72.59 ± 0.78 188.38 ± 0.76  
(A)
Activated carbon preparationFinal heating temperature (°C)550
700
 Impregnation ratio (NRBS:ZnCl21:0 1:1 1:1 1:2.5  1:0 1:1 1:1 1:2.5 
 Apparatus for conversion FLVR Muffle furnace FLVR FLVR  FLVR Muffle furnace FLVR FLVR 
 N2 flow rate (mL/min) 100 100 100  100 100 100 
 Yield after final heatinga (%) (Equation (3)) 52.14 ± 0.43 60.04 ± 0.94 68.25 ± 0.30 75.62 ± 0.32  24.23 ± 0.42 41.55 ± 0.40 59.78 ± 0.27 66.92 ± 0.43 
Adsorption experiments qe,exp,Cr(VI)b (mg/g) 1.45 ± 0.04 16.90 ± 0.28 26.97 ± 0.43 25.46 ± 0.44  3.98 ± 0.06 26.87 ± 0.16 35.83 ± 0.33 35.79 ± 0.28 
qe,exp,MBb (mg/g) 8.31 ± 0.10 47.83 ± 0.19 43.45 ± 0.31 40.61 ± 0.62  26.67 ± 0.34 47.81 ± 0.14 49.89 ± 0.41 49.42 ± 0.37 
(B)
qeb (mg/g)
100 mg/L
500 mg/L
900 mg/L
Adsorbent typeSBET (m2/g)Cr(VI)MBCr(VI)MBCr(VI)MB
NRBS 4.62  2.39 ± 0.02 5.80 ± 0.07 9.94 ± 0.14 12.46 ± 0.19 15.83 ± 0.18 17.80 ± 0.25  
NRAC 20.66  4.48 ± 0.08 8.26 ± 0.06 21.15 ± 0.38 19.52 ± 0.34 34.11 ± 0.45 23.53 ± 0.43  
NRCAC 414.16  28.64 ± 0.30 89.95 ± 0.43 69.10 ± 0.35 186.42 ± 0.27 72.59 ± 0.78 188.38 ± 0.76  

aIn Equation (3), WAC is the weight of activated carbon after final heating.

bAdsorption capacity of Cr(VI)/MB. (NRAC – natural rubber activated carbon, which is named as AC in Mahapatra et al. 2021).

Characterization

The dried samples (NRBS, NRCAC, NRCAC-Cr(VI) (NRCAC adsorbed with Cr(VI)), and NRCAC-MB (NRCAC adsorbed with MB)) were characterized by using different instrumental analyses as described in the following. The X-ray diffraction (XRD) analysis was obtained exploiting a diffractometer (Smartlab, Rigaku Technologies, Japan) with Cu-Kα radiation (λ = 1.54 Å) in the 2θ range of 10°–80°. Functional groups were detected using Fourier transform infrared spectroscopy (FTIR) (IRAffinity-1, Shimadzu, Japan) within wavenumber 4,000–400 cm−1. SEM images were taken using a field emission scanning electron microscope (FESEM) connected with a EDS (energy-dispersive X-ray spectroscopy) (Sigma 300, Zeiss, Germany). Nitrogen adsorption–desorption experiments were studied at –196.15 °C in a surface area analyzer (TriStar II, Micromeritics, USA) to know the textural properties. A thermogravimetric analyzer (TGA; TG 209 F1 Libra, Netzsch, Germany) was used to report the thermal behavior under 40 mL/min nitrogen flow rate, 10 °C/min ramping rate, and 21–1,000 °C temperature range.

Proximate analysis was estimated following standard ASTM-D1762 analysis methods (ASTM-D1762-84 2011) (Table 2). A CHNS elemental analyzer (Flash 2000, Thermo Scientific, UK) was employed for ultimate analysis (Table 2). The chemical composition of oxides was analyzed with an X-ray fluorescence spectrometer (DY 2942 Zetium, Malvern Panalytical, UK), and the major components (SiO2, Al2O3, CaO, Fe2O3) are detected as shown in Table 2.

Table 2

Properties of NRBS and NRCAC

pHaTKN (%)TGA weight lossb (%)
NRBS  7.32 ± 0.04 3.57 ± 0.02  60.35 
NRCAC  4.03 ± 0.03 2.16 ± 0.03  19.37 
     Textural properties 
SBET (m2/g)t-plot external surface area (m2/g)Total area (DFT surface energy) (m2/g)Cumulative pore volume (BJH adsorption)c (cm3/g)Cumulative pore volume (BJH desorption)c (cm3/g)Average pore diameter (BJH adsorption) (4 V/A) (nm)Average pore diameter (BJH desorption) (4 V/A) (nm)
NRBS  4.62 6.72 17.84 0.014 0.009 14.21 7.23 
NRCAC  414.16 223.59 671.64 0.164 0.153 10.29 3.81 
Proximate analysis (wt.%)
MoisturedAsheVolatile mattereFixed carbone,f
NRBS  5.46 ± 0.04  51.74 ± 0.09  43.95 ± 0.82   4.31 ± 0.05 
NRCAC  10.62 ± 0.15  92.15 ± 0.95  5.62 ± 0.10  2.23 ± 0.04 
Ultimate analysis (wt.%)
Elemental composition (wt.%)
Atomic ratio
CHNSOgO/CH/C(O + N)/C
NRBS  35.58 5.83 3.59 BDLh 55.0  1.55 0.1639 1.64 
NRCAC  49.47 1.83 2.61 BDLh 53.91  1.09 0.0370  1.14 
Chemical composition of oxides (wt.%)
SiO2Al2O3Fe2O3 (T)MnOMgOCaONa2OK2OTiO2P2O5
NRBS  30.13 10.59 7.00 0.18 5.71 24.19 BDLh 3.20 0.63 18.38 
NRCAC  79.78 10.85 0.74 BDLh 3.68 0.31 BDLh 1.94 2.36 0.35 
Solubility testSoluble metallic and non-metallic contents (mg/kg)
Solid-LiquidSolid:Liquid (w:w)Solubility (mg/L)Soluble (%)NaKCaMgZnCuFeMnCrSi
NRBS-DW 1:15  12.53 ± 0.13 18.79 ± 0.07 40.96 ± 0.18 94.76 ± 0.28 632.43 ± 0.14 210.73 ± 0.28 1.22 ± 0.03 10.09 ± 0.15 1.51 ± 0.01 BDLh 1.32 ± 0.03 19.32 ± 0.13 
NRCAC-DW 1:15  10.26 ± 0.63 15.39 ± 0.07 37.53 ± 0.26 77.14 ± 0.21 46.38 ± 0.14 150.45 ± 0.23 1.54 ± 0.02 BDLh 2.17 ± 0.04 1.54 ± 0.01 0.02 ± 0.00 68.45 ± 0.23 
NRBS-Aqua regia 1:10  70.3 ± 0.26 70.3 ± 0.18 75.22 ± 0.33 120.34 ± 0.18 978.45 ± 0.25 456.20 ± 0.25 65.65 ± 0.08 61.45 ± 0.25 567.89 ± 0.05 67.31 ± 0.18 7.76 ± 0.04 134.51 ± 0.32 
NRCAC-Aqua regia 1:10  23.28 ± 0.02 23.28 ± 0.02 50.66 ± 0.37 105.56 ± 0.18 90.89 ± 0.18 30.21 ± 0.13 843.78 ± 0.24 52.12 ± 0.19 87.21 ± 0.13 BDLh 5.01 ± 0.05 396.45 ± 0.18 
NRBS-Cr(VI) soln. 1:10  17.75 ± 0.25 17.75 ± 0.09 45.53 ± 0.18 96.21 ± 0.09 616.56 ± 0.18 197.65 ± 0.28 1.03 ± 0.01 8.77 ± 0.04 1.34 ± 0.03 BDLh 74.97 ± 0.20 15.46 ± 0.10 
NRCAC-Cr(VI) soln. 1:10  19.02 ± 1.13 19.02 ± 0.06 20.20 ± 0.18 74.8 ± 0.33 43.21 ± 0.14 148.43 ± 0.07 1.23 ± 0.02 BDLh 1.78 ± 0.03 1.64 ± 0.01 41.50 ± 0.12 62.31 ± 0.03 
NRBS-MB soln. 1:10  18.66 ± 0.08 18.66 ± 0.10 50.56 ± 0.16 99.34 ± 0.34 645.23 ± 0.09 194.32 ± 0.11 1.15 ± 0.02 6.52 ± 0.10 1.45 ± 0.02 BDLh BDLh 13.78 ± 0.03 
NRCAC-MB soln. 1:10  19.65 ± 0.62 19.65 ± 0.13 48.08 ± 0.19 79.87 ± 0.32 42.91 ± 0.19 145.21 ± 0.16 1.08 ± 0.03 BDLh 1.73 ± 0.03 1.59 ± 0.01 0.03 ± 0.00 65.12 ± 0.27 
pHaTKN (%)TGA weight lossb (%)
NRBS  7.32 ± 0.04 3.57 ± 0.02  60.35 
NRCAC  4.03 ± 0.03 2.16 ± 0.03  19.37 
     Textural properties 
SBET (m2/g)t-plot external surface area (m2/g)Total area (DFT surface energy) (m2/g)Cumulative pore volume (BJH adsorption)c (cm3/g)Cumulative pore volume (BJH desorption)c (cm3/g)Average pore diameter (BJH adsorption) (4 V/A) (nm)Average pore diameter (BJH desorption) (4 V/A) (nm)
NRBS  4.62 6.72 17.84 0.014 0.009 14.21 7.23 
NRCAC  414.16 223.59 671.64 0.164 0.153 10.29 3.81 
Proximate analysis (wt.%)
MoisturedAsheVolatile mattereFixed carbone,f
NRBS  5.46 ± 0.04  51.74 ± 0.09  43.95 ± 0.82   4.31 ± 0.05 
NRCAC  10.62 ± 0.15  92.15 ± 0.95  5.62 ± 0.10  2.23 ± 0.04 
Ultimate analysis (wt.%)
Elemental composition (wt.%)
Atomic ratio
CHNSOgO/CH/C(O + N)/C
NRBS  35.58 5.83 3.59 BDLh 55.0  1.55 0.1639 1.64 
NRCAC  49.47 1.83 2.61 BDLh 53.91  1.09 0.0370  1.14 
Chemical composition of oxides (wt.%)
SiO2Al2O3Fe2O3 (T)MnOMgOCaONa2OK2OTiO2P2O5
NRBS  30.13 10.59 7.00 0.18 5.71 24.19 BDLh 3.20 0.63 18.38 
NRCAC  79.78 10.85 0.74 BDLh 3.68 0.31 BDLh 1.94 2.36 0.35 
Solubility testSoluble metallic and non-metallic contents (mg/kg)
Solid-LiquidSolid:Liquid (w:w)Solubility (mg/L)Soluble (%)NaKCaMgZnCuFeMnCrSi
NRBS-DW 1:15  12.53 ± 0.13 18.79 ± 0.07 40.96 ± 0.18 94.76 ± 0.28 632.43 ± 0.14 210.73 ± 0.28 1.22 ± 0.03 10.09 ± 0.15 1.51 ± 0.01 BDLh 1.32 ± 0.03 19.32 ± 0.13 
NRCAC-DW 1:15  10.26 ± 0.63 15.39 ± 0.07 37.53 ± 0.26 77.14 ± 0.21 46.38 ± 0.14 150.45 ± 0.23 1.54 ± 0.02 BDLh 2.17 ± 0.04 1.54 ± 0.01 0.02 ± 0.00 68.45 ± 0.23 
NRBS-Aqua regia 1:10  70.3 ± 0.26 70.3 ± 0.18 75.22 ± 0.33 120.34 ± 0.18 978.45 ± 0.25 456.20 ± 0.25 65.65 ± 0.08 61.45 ± 0.25 567.89 ± 0.05 67.31 ± 0.18 7.76 ± 0.04 134.51 ± 0.32 
NRCAC-Aqua regia 1:10  23.28 ± 0.02 23.28 ± 0.02 50.66 ± 0.37 105.56 ± 0.18 90.89 ± 0.18 30.21 ± 0.13 843.78 ± 0.24 52.12 ± 0.19 87.21 ± 0.13 BDLh 5.01 ± 0.05 396.45 ± 0.18 
NRBS-Cr(VI) soln. 1:10  17.75 ± 0.25 17.75 ± 0.09 45.53 ± 0.18 96.21 ± 0.09 616.56 ± 0.18 197.65 ± 0.28 1.03 ± 0.01 8.77 ± 0.04 1.34 ± 0.03 BDLh 74.97 ± 0.20 15.46 ± 0.10 
NRCAC-Cr(VI) soln. 1:10  19.02 ± 1.13 19.02 ± 0.06 20.20 ± 0.18 74.8 ± 0.33 43.21 ± 0.14 148.43 ± 0.07 1.23 ± 0.02 BDLh 1.78 ± 0.03 1.64 ± 0.01 41.50 ± 0.12 62.31 ± 0.03 
NRBS-MB soln. 1:10  18.66 ± 0.08 18.66 ± 0.10 50.56 ± 0.16 99.34 ± 0.34 645.23 ± 0.09 194.32 ± 0.11 1.15 ± 0.02 6.52 ± 0.10 1.45 ± 0.02 BDLh BDLh 13.78 ± 0.03 
NRCAC-MB soln. 1:10  19.65 ± 0.62 19.65 ± 0.13 48.08 ± 0.19 79.87 ± 0.32 42.91 ± 0.19 145.21 ± 0.16 1.08 ± 0.03 BDLh 1.73 ± 0.03 1.59 ± 0.01 0.03 ± 0.00 65.12 ± 0.27 

aParameters resulted during extraction from NRBS and NRCAC by DW at 30 °C.

bLoss% from 21 to 700 °C as per TGA.

cPore diameter: 2–300 nm.

dOn as-received basis.

eDry basis after heating for 24 h at 105 °C.

fDetermined by difference.

gCalculated by mass balance (BET – Brunauer, Emmett, and Teller; BJH – Barrett–Joyner–Halenda).

hBelow detection limit.

For metallic and non-metallic analysis, dried 5 g of grounded solid material (NRBS or NRCAC) was digested with 50 mL of aqua regia (HCl:HNO3 as 3:1 v/v) for 24 h. Then 400 mL of DW was mixed and boiled for 2.5 h to obtain a solution of 500 mL, followed by filtration with Whatman GF/C (Juel et al. 2017). In addition, NRBS and NRCAC were also mixed with liquid solvents, namely DW, 100 mg/L of Cr(VI), and MB solutions separately and then incubated at 30 °C for 24 h, followed by filtration after centrifugation. All the above-collected filtrates were analyzed by atomic absorption spectroscopy (AAS) (iCE 3000 Series, Thermo Fischer Scientific, India). The KjelTRON (Tulin Equipments, India), a nitrogen/protein analyzer, was exploited to measure total Kjeldahl nitrogen (TKN) (APHA/AWWA/WEF 2017).

The pH drift method was used to determine point of zero charge (pHPZC), which refers to the pH at which the net charge on NRCAC surface is equal to zero (Mahapatra et al. 2021). In Erlenmeyer flasks, 50 mL of 0.01 M NaCl solutions with pH 2–12 were prepared. In each flask, 0.15 g of NRCAC sample was added. Following this, all flasks were shaken at 30 °C, 150 rpm for 48 h. A pHfinal value was recorded for each solution.

Adsorption experiments

2.828 g of K2Cr2O7 and 1.0 g of MB were dissolved separately with DW in a 1,000 mL volumetric flask and shaken well to obtain 1,000 mg/L of Cr(VI) and MB standard solutions. The desired solutions of various concentrations were prepared by dilution for performing experiments. Investigation of adsorption efficiency of NRCAC was carried out using a thermostatically regulated oscillating shaker, taking 100 mL of Cr(VI)/MB solution by varying process parameters: particle size (dp,avg = 0.04–1.27 mm), dosage (m/V = 0.5–10 g/L), agitation speed (Sags = 0–250 rpm), pH (1–12), initial concentration (Co = 50–900 mg/L), contact time (t = up to 24 h), temperature (T = 20–50 °C), and salt (NaCl, CaCl2, and (NH4)2Fe(SO4)2·6H2O) concentration (0.025–0.30 mol/L). A portion of each adsorbate solution was filtered using Whatman GF/C fitted syringe filter (inner diameter: 25 mm). The filtrate absorbance was measured by exploiting a UV-Vis spectrophotometer (GENESYS 10S, Thermo Scientific, USA) at λCr(VI) = 540 nm (using the diphenyl carbazide method) and λMB = 665 nm (APHA/AWWA/WEF 2017). All the adsorption experiments (except contact time and pHPZC) were performed for 24 h, taking original Cr(VI) solutions (pH = 3.94–4.61) and MB solutions (pH = 5.43–5.75). 0.1 M HCl/NaOH was used when pH change was required. The average results obtained from triplicate experiments were reported as experimental data (error bars displayed in the graphs). The following two equations were applied to calculate adsorption capacity, qt (mg/g) (Equation (1)) and removal rate, R (%) (Equation (2)):
formula
(1)
i.e., where
formula
(2)
where qt (mg/g) and Ct (mg/L) are the uptake capacity and concentration of adsorbate at any given time t; qe (mg/g) is the equilibrium uptake; m (g) is the amount of adsorbent; V (L) is the volume of solution; Co and Ce (mg/L) are the initial and equilibrium concentrations of adsorbate, respectively.

Desorption and reusability

Experimental batch study on desorption was performed to investigate the reusability of NRCAC. Initially, saturated NRCAC was prepared using 100 mg/L Cr(VI)/MB following batch adsorption conditions: dp,avg of –100 BSS, m/VCr(VI) of 3.0 g/L, m/VMB of 1.0 g/L, Sags at 150 rpm, t for upto 24 h, T at 30 °C, and V of 0.1 L. The suspension was centrifuged for 10 min at 10,000 rpm to recover Cr(VI)/MB-loaded NRCAC followed by drying at 105 °C. This dried solid was thoroughly mixed with eluting solvents (DW, and 0.01–1 M of HCl/NaOH) to investigate the best solvent for the desorption study. The entire adsorption–desorption study was accomplished in five cycles using the best solvent.

Error functions

Error functions were applied to confirm the best fitting of kinetic and isotherm models. For further details, see Supplementary material (Lima et al. 2015).

Preparation of NRCAC

From Table 1(A), the adsorption capacity (qe) of prepared ACs in FLVR using impregnated ratios of 1:1 and 1:2.5 at 700 °C was more than that of the other conditions. But qe of 1:1 ratio is more or less similar to that of 1:2.5. Therefore, 1:1 impregnated ratio was chosen for the preparation of NRCAC. An average NRCAC yield of 29.21% under these conditions was calculated from Equation (3).
formula
(3)
where WNRBS (g) is the weight of dried NRBS; WAC (g) is the weight of dried NRCAC after washing and filtering.

Both NRAC and NRCAC were prepared from same raw material NRBS. Brunauer–Emmett–Teller (BET) surface area (SBET) was increased by 22 times during the conversion of NRBS to NRAC in muffle furnace at 550 °C (Mahapatra et al. 2021), whereas 90 folds increased SBET during the conversion of NRBS to NRCAC by the method discussed in Section 2.2. Therefore, qe accordingly increased by the order qe,NRCAC>qe,NRAC>qe,NRBS. for a particular initial adsorbate concentration. For a specific adsorbent (NRBS/NRAC/NRCAC), qe gradually increased with an increase in initial concentration, which can be explained in Section 3.3.5.

Characterization of adsorbents

X-ray diffraction

In Figure 1(a), NRBS showed three high-intensity sharp peaks between 20° and 30°, i.e., SiO2 (21.01° and 26.76°) and CaMgCO3 (29.52°) (Mahapatra et al. 2021). However, two high-intensity peaks for NRCAC were detected in the same region for only SiO2 (21.02° and 26.80°), without any XRD peak for CaMgCO3, which was also proved by X-ray fluorescence (XRF) analysis (Table 2). XRF analysis also showed a remarkable increase of SiO2 wt.% after conversion of NRBS to NRCAC, which was reflected in the XRD analysis through the number of peaks. As per TGA (Figure 3(c)), %mass loss was recorded to be 61.16% when temperature increased to 700 °C (pyrolysis temperature of NRCAC preparation from NRBS). This loss is mainly due to the volatile matter, moisture, etc., but a loss of SiO2 as volatile matter is impossible; thus, SiO2 remains to ash. The existence of peaks of Fe2O3, CaMg(CO3)2, CaCO3, MgO, CaO, K2CO3, and NaCl was not detected in the diffractogram of NRCAC (Figure 1(a)) because most of these compounds were removed due to high-temperature thermal conversion followed by leaching out with 3 M HCl during preparation of NRCAC from NRBS. Due to the above reasons, metal (Ca, Fe, K, Na, Mg) concentrations in NRCAC analyzed by the AAS method were found to be very low compared to NRBS and XRF analysis of NRCAC indicated low wt.% of metal oxides (Table 2). The appearance of ZnCl2 peak was due to its utilization during NRCAC preparation (Wu & Zhang 2012).
Figure 1

(a) X-ray diffraction patterns of NRBS and NRCAC; (b) FTIR spectroscopy of NRBS, NRCAC, NRCAC-Cr(VI), and NRCAC-MB.

Figure 1

(a) X-ray diffraction patterns of NRBS and NRCAC; (b) FTIR spectroscopy of NRBS, NRCAC, NRCAC-Cr(VI), and NRCAC-MB.

Close modal

FTIR spectroscopy

Figure 1(b) compares FTIR spectrums of samples, namely NRBS, NRCAC, NRCAC-Cr(VI), and NRCAC-MB. Supplementary material, Table S1 shows the wavenumbers of different peaks and their corresponding assignments. Some changes in the frequencies of peaks ranging from 4,000 to 400 cm−1 were found before and after adsorption. In addition, a few peaks disappeared, while some peaks appeared. The spectra confirmed the presence of –OH (Zhao et al. 2020; Mahapatra et al. 2022), C = O, and C = C (Hu et al. 2021), Si–O–Si, and C–O in all four samples (Silva et al. 2016).

The two peaks in the NRBS spectrum at 2,924 and 2,853 cm−1 corresponded to C–H asymmetrical and symmetrical stretching vibrations of methylene groups, respectively, revealing the presence of aliphatic structures in the NRBS. But these peaks disappeared in NRCAC because ZnCl2 activation at high temperature (500 °C) leads to breakdown of cellulose, hemicellulose, lignin, etc., into aromatic carbon-rich structures in NRCAC, which was correlated with the decrease of H content in ultimate analysis (Table 2) (Paredes-Laverde et al. 2021). In addition, –OH stretching peak was much broader after conversion. Another two peaks assigned for Si–O–Si at 532 and 464 cm−1 (Fan et al. 2017) were merged during pyrolysis of NRBS to NRCAC. The broadened and deep peaks at 459 cm−1 was assigned to angular bending of Si–Mg–O (Silva et al. 2016). On the other hand, one peak in NRCAC at 770 cm−1 corresponded to C–H aromatic plane out of plane vibration (Guo et al. 2018).

After Cr(VI) adsorption onto NRCAC, nominal shifting from 3,358 to 3,347 cm−1 might be due to complexation between –OH and Cr(VI) (Nigam et al. 2019). After adsorption of MB through π–π stacking interaction between aromatic backbone of MB and NRCAC with other mechanisms such as hydrogen bonding, electrostatic attractions, etc., significant new peaks appeared at 1,386, 1,318, and 1,221 cm−1 for C–N bond in MB heterocycle, Ar–N deformation vibration, and C = S respectively, which were present in MB (Gong et al. 2015). Nitrogen and sulfur atoms in MB were responsible for intermolecular hydrogen bonding (Fan et al. 2017).

FESEM and EDS spectra

The surface morphologies of NRBS, NRCAC, NRCAC-Cr(VI), and NRCAC-MB are shown in Figure 2(a)–2(d). After activation of NRBS with ZnCl2, SEM images of NRCAC exhibited highly developed pores than pristine NRBS, which were also confirmed by BET surface area (SBET) (Table 2). The conversion of NRBS to NRCAC was carried out in N2, resulting in NRCAC containing higher C and Si and lower O and N, which were reflected in elemental analysis, including the color mappings of NRBS and NRCAC (Figure 2(e) and 2(f), Supplementary material, Fig. S1). These results were also supported in ultimate analysis (for C, O, and N), TKN analysis (for N), and XRF and AAS analysis (for Si) (Table 2). EDS analysis also indicated that Zn and Cl were insignificantly present in NRBS compared to NRCAC. As a result, 100% ZnCl2 could not be removed from pyrolyzed material during the acid-leaching process. The existence of Zn in NRCAC was also proved by AAS analysis (Table 2) (Paredes-Laverde et al. 2021). On the other hand, phosphorous (P) disappeared during the carbonization process due to its absence in NRCAC (Figure 2(f)), which was also reflected in XRF analysis (Table 2).
Figure 2

FESEM images (5,000 × , 15.0 kV): (a) NRBS; (b) NRCAC; (c) NRCAC-Cr(VI); and (d) NRCAC-MB. FESEM images with elemental EDS composition in inset: (e) NRBS and (f) NRCAC.

Figure 2

FESEM images (5,000 × , 15.0 kV): (a) NRBS; (b) NRCAC; (c) NRCAC-Cr(VI); and (d) NRCAC-MB. FESEM images with elemental EDS composition in inset: (e) NRBS and (f) NRCAC.

Close modal
Figure 3

Nitrogen adsorption–desorption isotherm plot with inset showing pore size distribution (BJH model) for (a) NRBS and (b) NRCAC. TG analysis in the presence of nitrogen with temperature for (c) NRBS and (d) NRCAC showing variation in mass and derivative thermogravimetry.

Figure 3

Nitrogen adsorption–desorption isotherm plot with inset showing pore size distribution (BJH model) for (a) NRBS and (b) NRCAC. TG analysis in the presence of nitrogen with temperature for (c) NRBS and (d) NRCAC showing variation in mass and derivative thermogravimetry.

Close modal

N2 adsorption–desorption isotherm

The textural properties of NRBS and NRCAC were evaluated through N2 adsorption–desorption isotherm study (Figure 3(a) and 3(b), Table 2). As per IUPAC classification, the isotherm shape of NRBS, as represented in Figure 3(a), followed hysteresis loop H3 with Type IV, whereas NRCAC showed H4, Type IV isotherm (Figure 3(b)), indicating the capillary condensation with monolayer–multilayer adsorption in both cases that is the characteristics of mesoporous structures (Silva et al. 2016). N2 adsorption is increased with an increase in P/P0 value. The initial portion of type IV (up to P/P0 0.69 for NRBS and 0.48 for NRCAC) is attributed to monolayer adsorption, whereas the second portion represented the multilayer N2 adsorption (Guo et al. 2021). The inset plot within Figure 3(a) and 3(b) shows the pore size distributions of NRBS and NRCAC following BJH (Barrett–Joyner–Halenda) theory. NRBS had pore diameters ranging from 2 to 50 nm where the pores were distributed with not much more variation (2–50 nm). In NRCAC, major distribution of mesopores is within 3–5 nm, a moderate distribution between 5 and 12 nm, and a much lower distribution in the range of 12–50 nm as well. Table 2 shows the values of SBET and total mesopore volume increased almost 90 and 12 times, respectively, after NRBS was converted to NRCAC. The impregnating agent (ZnCl2) and higher pyrolysis temperature (700 °C) had a significant role in this tremendous increase in pore volume and surface area. ZnCl2 acts as a pore fabricating agent through mechanisms as follows: swelling nature of ZnCl2 by impregnation of NRBS (raw material); the abundant pores are developed during pyrolysis due to release of volatile matters, which is accelerated by dehydrating effect of ZnCl2; then these pores are the passage for ZnCl2 for creation of mesopores (Xu et al. 2017; Paredes-Laverde et al. 2021), which is reflected in the BET analysis and resulted in higher BET surface area; the ZnCl2 occupied in the pores was removed by HCl leaching (Xu et al. 2017).

Thermogravimetric analysis

TGA determines the thermal stability of any given sample in a specific atmosphere. Weight loss and derivative thermogravimetry (DTG) of NRBS and NRCAC are shown in Figure 3(c) and 3(d), where the plotted data were evaluated from the results of TGA experiments.

The TGA plots (Figure 3(c)) of NRBS can be divided into four sections. In the first section (25–200 °C), a significant mass loss of 11.18% was observed due to loss of interstitial water (25–200 °C) and moisture (up to 105 °C), which corresponded to the first valley of the DTG profile (Tian et al. 2019). Temperatures ranging from 200 to 500 °C covered the second section, where a weight loss of 45.24% was observed due to carbonaceous matrix breakdown (hemicelluloses, cellulose, lignin, etc.). Within the same temperature range, a high intense and broad valley of DTG profile showed two minimum peaks (at 320 and 365 °C), representing a maximum mass loss rate at 320 and 365 °C. On the other hand, 4.74% weight loss in the third section (500–720 °C) was ascribed to carbon dioxide release due to thermal dissociation of MgCO3 (minor) and CaCO3 (major) (Qi et al. 2010). DTG profile having a valley of low intense and narrow shape (peak at 700 °C) was represented in the same section. In the last section of 700–1,000 °C, a low and fixed value of DTG within this temperature range signified that a rate of mass loss reached a low constant value, which was reflected in the weight loss of only 4.11%. This low loss indicated that all carbon had been removed, and only inorganic ashes remained (Lima et al. 2019a).

The TG analysis of NRCAC in N2 could be classified into three sections. The plot (Figure 3(d)) showed a significant mass loss of 11.97%, with one deep DTG peak (70 °C) in the first region (22–105 °C). A mass loss of 7.39% can be seen in the second section (105–700 °C) due to the decomposition of remaining cellulose and hemicellulose. A loss of 5.11% can be seen in the third stage (700–1,000 °C), where the remaining MgCO3 (major) and CaCO3 (minor) might be decomposed. However, the DTG profile (105–1,000 °C) remained parallel to the temperature axis, which explained the low %loss (12.50%) in NRCAC as compared to that of 59.91% in NRBS for the same temperature range (Tian et al. 2019). Volatile matter produced during thermal pyrolysis of NRBS during preparation of NRCAC at 700 °C resulting this low %loss in NRCAC.

Effect of adsorption process parameters

Effect of particle size

Figure 4(a) is drawn from the experimental data obtained from studying the effect of particle size (dp,avg) on adsorption. It was observed in Figure 4(a) that the R% was increased (RCr(VI)% = 70.82–85.84 and RMB% = 83.24– 90.07) with a decrease in dp,avg due to grinding of large particles into smaller ones resulting in additional surface area (Al-Zboon & Al-Harahsheh 2019). Figure 4(a) also showed that R% reached a maximum at a dp,avg of 0.13 mm (corresponding to particles passing through 100 BSS and retaining on 150 BSS, i.e., 0.15–0.10 mm). After that, R% attained a negligible increase with a decrease in dp,avg. Therefore, all remaining adsorption experiments were carried out taking particles passing through a 100 BSS mesh. A similar trend was shown by Li et al. (2011).
Figure 4

Experimental investigation of (a) particle size; (b) adsorbent dosage; (c) agitation speed; (d) pHPZC (conditions for pHPZC: dp,avg= –100 BSS, m/V = 3.0 g/L, Sags = 150 rpm, t = 48 h, T = 30 °C, V = 0.05 L); (e) Cr(VI) speciation diagram in aqueous solution as a function of pH simulated by Visual MINTEQ software; (f) pH; (g) initial concentration; (h) contact time; (i) temperature; and (j) salt concentration. (dp,avg = −100 BSS [except (a), (e)], m/VCr(VI) [except (b), (e)] = 3.0 g/L and m/VMB [except (b), (e)] = 1.0 g/L, Sags [except (c), (e)] = 150 rpm, Co [except (d), (e), (g), (h), (i), (j)] = 100 mg/L, t = up to 24 h [except (d), (e)], T [except (e), (g), (i)] = 30 °C, and V = 0.1 L [except (d), (e)]

Figure 4

Experimental investigation of (a) particle size; (b) adsorbent dosage; (c) agitation speed; (d) pHPZC (conditions for pHPZC: dp,avg= –100 BSS, m/V = 3.0 g/L, Sags = 150 rpm, t = 48 h, T = 30 °C, V = 0.05 L); (e) Cr(VI) speciation diagram in aqueous solution as a function of pH simulated by Visual MINTEQ software; (f) pH; (g) initial concentration; (h) contact time; (i) temperature; and (j) salt concentration. (dp,avg = −100 BSS [except (a), (e)], m/VCr(VI) [except (b), (e)] = 3.0 g/L and m/VMB [except (b), (e)] = 1.0 g/L, Sags [except (c), (e)] = 150 rpm, Co [except (d), (e), (g), (h), (i), (j)] = 100 mg/L, t = up to 24 h [except (d), (e)], T [except (e), (g), (i)] = 30 °C, and V = 0.1 L [except (d), (e)]

Close modal

Effect of adsorbent dosage

The effect of NRCAC dosage (m/V) varied from 0.5 to 10.0 g/L on adsorption efficiency, as depicted in Figure 4(b). The achieved R% was recorded to be 96.82% (Cr(VI)) and 99.15% (MB), which corresponded to 5.0 and 2.0 g/L dose, respectively, and then almost attained constant with further increase in dose till 10.0 g/L in both adsorbates. The availability of more pores and surface area with higher functional groups and active sites could explain better removal with an increased dose (Fan et al. 2017). In contrast to R%, adsorption capacity (mg/g) was reduced from 46.20 to 9.93 for Cr(VI) and 103.84–9.98 for MB as the dose was increased from 0.5 to 10.0 g/L. This is due to, according to Equation (1), qe is related inversely to m and proportional to C (C = CoCe). If m increases, Ce decreases, i.e., C increases. But C does not increase in the same proportion as m, and it is observed that C/m value gradually decreases with an increase of m. Since R% was close to 96–99% at 5.0 g/L dose for Cr(VI) and 2.0 g/L dose for MB, percent removal is hard to change with the variation of different process parameter values because R% has already reached almost 100% if experiments are conducted at the above doses. Therefore, adsorption experiments in the present study were performed considering an optimum dose of 3.0 g/L (RCr(VI)% = 85.79) and 1.0 g/L (RMB% = 89.62).

Effect of agitation speed

Figure 4(c) shows the effect of agitation speed (Sags) on the adsorption efficiency of Cr(VI)/MB onto NRCAC. The R% of Cr(VI) (12.96–84.21%) and MB (16.22–89.72%) varied when Sags value increased from 0 to 150 rpm, and then R% almost remained constant with further increased Sags value up to 250 rpm. A similar trend was observed by Cherdchoo et al. (2019). This is due to a reduction in thickness of boundary layer around particles with a consequent decrease in mass transfer resistance (Guechi & Hamdaoui 2016). Therefore, the optimum agitation speed was recommended at 150 rpm and used in all other experiments.

pHPZC and effect of solution pH

The point of zero charge (pHPZC) of NRCAC was 3.83 (Figure 4(d)). Below pHPZC, surface charge will be positive, whereas surface will be a negative charge above the pHPZC. It is found that pH affects adsorbate speciation and surface properties of adsorbent, which can influence their interaction. Islam et al. (2017) displayed a similar pattern graph with a pHPZC value close to our result.

The percentage of various Cr(VI) species (H2CrO4, HCrO4, Cr2O72−, and CrO42−) in its solution only depends on the pH value. When pH < pHPZC (3.83), the adsorbent surface became a positive charge, and surface positive charge gradually increased with a decrease in pH value. As a result, the electrostatic attraction forces (between negative Cr(VI) species and positively charged NRCAC) gradually increased to a maximum value (i.e., RCr(VI)% = 95.49 at pH 2.0) with a decrease in pH from pHPZC to 2.0 due to negative species HCrO4 and Cr2O72− at that pH range (3.83–2.0). When further pH decreased from 2.0 to 1.0, the negative species HCrO4was converted to uncharged H2CrO4 (HCrO4 + H+⇌ H2CrO4), resulting decrease in electrostatic attraction forces that enhanced the reduction of R% (92.00% at pH 1.0).

When pH > pHPZC (3.83), the NRCAC surface became negatively charged, and the gradual deposition of negative charge on adsorbent surface continued with a change of pH from 3.83 to 12.0. In this pH range (3.83–12.0), the negative species HCrO4 and Cr2O72− are converted to CrO42− (HCrO4⇌ CrO42− + H+, Cr2O72− + H2O ⇌ 2CrO42− + 2H+) from pH 5.0, and their fraction in the solution becomes zero at pH 7.0 for HCrO4 and 9.0 for Cr2O72−. Therefore, R% (85.38% at pH 4.0 and 46.62% at pH 12.0) gradually decreased due to an increase in electrostatic repulsion between negatively charged adsorbent surface and negative species (pH 3.8–7.0: HCrO4, Cr2O72−, and CrO42−; pH 7.0 to 9.0: HCrO4 and CrO42−; pH 9.0 to 12.0: CrO42−) as displayed in Figure 4(e). Interestingly, R% (81.72% at pH 5.0–59.72% at pH 7.0) decreased remarkably for a small change of pH (5.0–7.0) due to conversion of unit negative charged species (HCrO4) to negative divalent ions (CrO42−) resulting in much rise of electrostatic repulsive forces. As pHfinal of the solution suddenly jumped from 6.96 to 11.96, corresponding to pH 11.0–12.0 (as shown in Figure 4(f)), the negative charge deposition on NRCAC occurred so quickly that enhancement of electrostatic repulsive forces with only negative species (CrO42−) present in the solution resulting in a decrease of R% (55.35% at pH 11.0 and 46.62% at pH 12.0). Several authors have observed similar trends in the literature (Nigam et al. 2019; Shakya et al. 2019).

Similarly, variation of R% and pHfinal with pHinitial is plotted in Figure 4(f) for MB. When pH < 3.83 (pHPZC), the NRCAC surface became protonated, and H+ concentration on the NRCAC surface gradually increased with a decrease in pH (3.83–1.0). At the same time, MB was strongly protonated (Figure 5) in solution that easily replaced the adsorbed H+ ion on NRCAC surface by the ion-exchange process (Mahapatra et al. 2021). As a result, R% (90.92–97.36%) enhanced with decreased pH (3.83–1.0). On the other hand, when pH > pHPZC (3.83), NRCAC surface deposited negative charge and MB+ was continuously deprotonated with an increase in pH value. At the same time, with varying pH (3.83–10.0), the values of R% and pHfinal changed negligibly from 90.92–90.07% and 3.92–5.17, respectively. As the pH of the MB solution was adjusted from 4.0 to 10.0, the pHfinal was only moderately increased from 3.92 to 5.17. This implied a limited deposition of negative charge on NRCAC surface along with negligible deprotonation of MB+. Thus, the electrostatic attraction between negatively charged NRCAC and MB+ remained almost constant, reflecting the negligible change in R%. In contrast, a remarkable change in pHfinal (5.17–10.89) value was observed with a slight change in pH (10.0–12.0), leading to a substantial negative charge deposition on the NRCAC surface. Therefore, a significant increase in the electrostatic attraction between the negatively charged NRCAC surface and MB+ led to a remarkable enhancement in R% (90.07 at pH 10.0–99.95 at pH 12.0) (He et al. 2018).

Effect of initial concentration

The effect of initial adsorbate concentration (Co = 50–900 mg/L) on R% was studied at different temperatures for both adsorbates. The R% decreased for Cr(VI) and MB with an increasing Co as demonstrated in Figure 4(g). The active sites responsible for adsorption remain constant, but the number of adsorbate molecules rises with an increase in the value of Co, which reduces the R% according to Equation (2) (Tang et al. 2021).

Effect of contact time

The adsorption of Cr(VI)/MB on NRCAC was investigated with a variation of time at different Co and the findings are depicted in Figure 4(h). With an increase in contact time, the adsorption capacities of both Cr(VI) and MB were enhanced until equilibrium was attained. In the first stage, rapid adsorption was observed within 60 min, achieving uptake > 80% of its saturation value. In contrast, the second stage was a slow adsorption process and took another 60 min to reach above 90% of equilibrium uptake value. Both Cr(VI) and MB curves exhibited a flat plateau in the third stage, indicating that adsorption capacity did not change significantly after 120 min. The reason for various rates of adsorption at different stages: the rapid mass transfer of adsorbate molecules to the film of boundary layer occurs in the first stage, then they tend to slowly diffuse in the second stage from the boundary layer onto the adsorbent surface with higher mass transfer resistance, and in the third stage diffused molecules enter into the porous structures for which molecule movement is hindered (Chen et al. 2021).

Effect of temperature

The plot of R% versus temperature (20–50 °C) at various initial adsorbate concentrations (Co: 100, 200, 300, 500, and 900 mg/L) is shown in Figure 4(i). In the case of each Co, R% increased with temperature because (i) the mechanism of adsorption was endothermic (Section 3.6), implying higher driving force at higher temperature and (ii) reduction of adsorbate transport resistance due to easier diffusion at an increased solution temperature (Fan et al. 2017).

Effect of salt concentration

The effect of salt on adsorption using 200 mg/L solution of Cr(VI)/MB was studied with varying salt concentrations (0.025–0.30 mol/L). For Cr(VI) removal, an interesting observation from Figure 4(j) was that NaCl and CaCl2 showed a negative effect, but Mohr's salt ((NH4)2Fe(SO4)2·6H2O) exhibited a positive effect. Equilibrium pH for NaCl and CaCl2 was recorded as between 5.01 and 5.29, indicating that negative chromium species (HCrO4, Cr2O72−, and CrO42−) were present in the solution and anionic species (Cl) came from the salt at same time. As a result, competition of both Cl and chromium species occurred during adsorption on NRCAC, resulting in gradual reduction of R% (74.49–48.69% for NaCl and 74.49–46.94% for CaCl2) with an increase in salt concentration (Norouzi et al. 2018). The influence of CaCl2 on Cr(VI) adsorption had a more negative effect than NaCl because Cl ion concentration was increased twice in CaCl2 when compared with NaCl. On the contrary, with an increasing Mohr's salt concentration (0.025–0.30 mol/L) as shown in Figure 4(j), R% was increased (74.49–96.76%), and acidic equilibrium pH was observed 2.32–2.48. The positive effect of Mohr's salt may be due to conversion of Cr(VI) to Cr(III) in presence of oxidizing agent Fe2+ in an acidic solution (6Fe2+ + Cr2O72− + 14H+ = 6Fe3+ + 2Cr3+ + 7H2O) and finally, chromium removed as precipitate of FeCr2O4 (Fe2+ + 2Cr3+ + 8OH = FeCr2O4+ 4H2O) (Yang et al. 2019). Thus, R% in presence of Mohr salt refers total removal percent of Cr(VI) by adsorption and precipitation process.

But on the other hand, positive influence in MB adsorption with a change in salt (NaCl or CaCl2) concentration may be explained as follows: (i) both salts enhanced the solution ionic strength with varying salt concentrations from 0.025 to 0.30 mol/L that increased R% (73.05–95.03% for NaCl and 73.05–98.91% for CaCl2 (Figure 4(j)) following the principle of electrostatic interaction (Afroze et al. 2016); (ii) another possibility may be due to MB dimerization or MB aggregation induced by the presence of salt ions due to ‘salting out effect’ (Kuo et al. 2008). Weak physical interactions such as Van der Waals forces, ion-dipole, and dipole–dipole are responsible for dimerization (Kuo et al. 2008). In both cases, the boosting of R% was higher for CaCl2 than NaCl due to higher ionic strength.

Adsorption kinetics

Contact time experiments for Cr(VI)/MB adsorption on NRCAC were performed to study adsorption kinetics. Four models were fitted to the time-dependent adsorption capacity data using the following equations: pseudo-first-order (PFO) (Equation (4)) (Langergren & Svenska 1898), pseudo-second-order (PSO) (Equation (5)) (Ho & McKay 1999), Avrami-fractional order (AV) (Equation (6)) (Avrami 1939), and intra-particle diffusion (IPD) (Equation (7)) (Weber & Morris 1963).
formula
(4)
formula
(5)
formula
(6)
formula
(7)
where kPFO, kPSO, kAV, and kIPD are the PFO, PSO, AV, and IPD rate constants, respectively; nAV is the fractional adsorption order; CIPD is the intercept.
Figure 5

Schematic diagram of MB speciation (Mahapatra et al. 2021).

Figure 5

Schematic diagram of MB speciation (Mahapatra et al. 2021).

Close modal
The contact time experimental data were used to fit four kinetic models (PFO, PSO, AV, and IPD), which were represented through graphs (Figure 6(a)–6(d)). The obtained kinetic parameters and error values are tabulated in Table 3. A model with a lower RMSE and SSE and higher was more suitable to fit the kinetic data. Comparing the error values of three models (PFO, PSO, and AV), the PSO model was the best model for describing the kinetic study.
Table 3

Fitting parameters of kinetic models for adsorption of Cr(VI)/MB onto NRCAC

Kinetic modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
100a200a300a500a900a100a200a300a500a900a
 qe,exp (mg/g)  28.46 48.83 62.88 68.90 72.59  89.83 146.34 173.78 187.12 188.38 
Pseudo-first-order qe (mg/g)  27.60 47.48 60.89 66.32 68.74  87.79 143.60 168.50 181.70 181.90 
kPFO× 102 (/min)  11.45 6.07 5.30 5.15 4.37  10.72 5.59 5.52 4.80 6.06 
  0.9749 0.9866 0.9853 0.9734 0.9826  0.9785 0.9924 0.9915 0.9911 0.9780 
RMSE (mg/g)  1.26 1.76 2.42 3.54 3.09  3.73 4.08 5.13 5.81 8.69 
SSE (mg2/g2 19 37 70 151 114  167 200 315 405 906 
Pseudo-second-order qe (mg/g)  28.60 49.64 63.90 69.69 72.59  90.90 150.30 176.90 191.30 190.70 
kPSO× 104 (g/mg/min)  0.72 0.19 0.12 0.11 8.56  0.21 5.64 4.61 3.62 4.79 
  0.9997 0.9974 0.9982 0.9980 0.9992  0.9993 0.9949 0.9940 0.9968 0.9987 
RMSE (mg/g)  0.13 0.78 0.85 0.97 0.66  0.66 3.34 4.32 3.49 2.12 
SSE (mg2/g2 0.2 11  133 224 146 54 
Avrami-fractional order qe (mg/g)  28.21 39.75 49.93 68.23 70.22  79.39 118.60 138.70 183.90 152.20 
kAV (/min)  0.12 0.76 0.88 0.04 0.04  2.41 1.51 1.69 0.04 0.91 
nAV (-)  0.57 2.50 2.72 0.61 0.69  3.23 1.85 2.28 0.78 3.57 
  0.9978 0.3954 0.3446 0.9979 0.9960  0.6405 0.3580 0.3406 0.9963 0.3890 
RMSE (mg/g)  0.37 11.82 16.19 1.00 1.48  15.25 37.57 45.26 3.71 45.75 
SSE (mg2/g2 1,537 2,882 11 24  2,559 15,500 22,500 152 23,000 
Intra-particle diffusion model
First Stage
Second Stage
Third Stage
AdsorbateCo (mg/L)KIPD1 (mg/g /min/2)CIPD1 (mg/g)(R1)2kIPD2 (mg/g /min/2)CIPD2 (mg/g)(R2)2kIPD3 (mg/g /min/2) × 102CIPD3 (mg/g)(R3)2
Cr(VI) 100 2.88 9.76 0.9627  0.24 24.16 0.8775  0.7 28.21 0.9932  
200 4.95 7.87 0.9267  0.36 41.25 0.9082  1.49 48.27 0.9987  
300 6.46 7.85 0.9361  0.49 51.68 0.9010  1.13 62.35 0.9207  
500 6.36 10.53 0.9447  0.80 52.26 0.8822  3.14 67.67 0.9465  
900 7.30 4.72 0.9567  1.22 49.45 0.9287  21.23 64.73 0.9865  
MB 100 10.39 24.37 0.9547  1.03 74.89 0.8151  1.70 89.20 0.9567  
200 16.04 16.02 0.9395  1.23 123.63 0.8903  4.33 144.68 0.9405  
300 19.23 15.87 0.9076  1.21 145.34 0.9124  6.93 171.14 1.0000  
500 14.12 36.66 0.8612  1.09 162.33 0.9329  11.31 182.71 0.9418  
900 17.65 34.79 0.8811  2.04 146.91 0.8843  3.77 186.91 0.9418  
Kinetic modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
100a200a300a500a900a100a200a300a500a900a
 qe,exp (mg/g)  28.46 48.83 62.88 68.90 72.59  89.83 146.34 173.78 187.12 188.38 
Pseudo-first-order qe (mg/g)  27.60 47.48 60.89 66.32 68.74  87.79 143.60 168.50 181.70 181.90 
kPFO× 102 (/min)  11.45 6.07 5.30 5.15 4.37  10.72 5.59 5.52 4.80 6.06 
  0.9749 0.9866 0.9853 0.9734 0.9826  0.9785 0.9924 0.9915 0.9911 0.9780 
RMSE (mg/g)  1.26 1.76 2.42 3.54 3.09  3.73 4.08 5.13 5.81 8.69 
SSE (mg2/g2 19 37 70 151 114  167 200 315 405 906 
Pseudo-second-order qe (mg/g)  28.60 49.64 63.90 69.69 72.59  90.90 150.30 176.90 191.30 190.70 
kPSO× 104 (g/mg/min)  0.72 0.19 0.12 0.11 8.56  0.21 5.64 4.61 3.62 4.79 
  0.9997 0.9974 0.9982 0.9980 0.9992  0.9993 0.9949 0.9940 0.9968 0.9987 
RMSE (mg/g)  0.13 0.78 0.85 0.97 0.66  0.66 3.34 4.32 3.49 2.12 
SSE (mg2/g2 0.2 11  133 224 146 54 
Avrami-fractional order qe (mg/g)  28.21 39.75 49.93 68.23 70.22  79.39 118.60 138.70 183.90 152.20 
kAV (/min)  0.12 0.76 0.88 0.04 0.04  2.41 1.51 1.69 0.04 0.91 
nAV (-)  0.57 2.50 2.72 0.61 0.69  3.23 1.85 2.28 0.78 3.57 
  0.9978 0.3954 0.3446 0.9979 0.9960  0.6405 0.3580 0.3406 0.9963 0.3890 
RMSE (mg/g)  0.37 11.82 16.19 1.00 1.48  15.25 37.57 45.26 3.71 45.75 
SSE (mg2/g2 1,537 2,882 11 24  2,559 15,500 22,500 152 23,000 
Intra-particle diffusion model
First Stage
Second Stage
Third Stage
AdsorbateCo (mg/L)KIPD1 (mg/g /min/2)CIPD1 (mg/g)(R1)2kIPD2 (mg/g /min/2)CIPD2 (mg/g)(R2)2kIPD3 (mg/g /min/2) × 102CIPD3 (mg/g)(R3)2
Cr(VI) 100 2.88 9.76 0.9627  0.24 24.16 0.8775  0.7 28.21 0.9932  
200 4.95 7.87 0.9267  0.36 41.25 0.9082  1.49 48.27 0.9987  
300 6.46 7.85 0.9361  0.49 51.68 0.9010  1.13 62.35 0.9207  
500 6.36 10.53 0.9447  0.80 52.26 0.8822  3.14 67.67 0.9465  
900 7.30 4.72 0.9567  1.22 49.45 0.9287  21.23 64.73 0.9865  
MB 100 10.39 24.37 0.9547  1.03 74.89 0.8151  1.70 89.20 0.9567  
200 16.04 16.02 0.9395  1.23 123.63 0.8903  4.33 144.68 0.9405  
300 19.23 15.87 0.9076  1.21 145.34 0.9124  6.93 171.14 1.0000  
500 14.12 36.66 0.8612  1.09 162.33 0.9329  11.31 182.71 0.9418  
900 17.65 34.79 0.8811  2.04 146.91 0.8843  3.77 186.91 0.9418  

aInitial concentration of Cr(VI) and MB (mg/L).

Figure 6

Fitting of pseudo-first-order, pseudo-second-order, and Avrami-fractional order kinetic models for (a) Cr(VI) and (b) MB onto NRCAC; intra-particle diffusion for (c) Cr(VI) and (d) MB onto NRCAC; fitting of adsorption isotherm models for Cr(VI) and MB onto NRCAC at (e) 20 °C, (f) 30 °C, (g) 40 °C, and (h) 50 °C; (i) Van't Hoff plot for Cr(VI) and MB. (dp,avg= –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, Co [except (a), (b), (c), (d), (i)] = 100 mg/L t = up to 24 h, T [except (e), (g), (h), (i)] = 30 °C, V = 0.1 L).

Figure 6

Fitting of pseudo-first-order, pseudo-second-order, and Avrami-fractional order kinetic models for (a) Cr(VI) and (b) MB onto NRCAC; intra-particle diffusion for (c) Cr(VI) and (d) MB onto NRCAC; fitting of adsorption isotherm models for Cr(VI) and MB onto NRCAC at (e) 20 °C, (f) 30 °C, (g) 40 °C, and (h) 50 °C; (i) Van't Hoff plot for Cr(VI) and MB. (dp,avg= –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, Co [except (a), (b), (c), (d), (i)] = 100 mg/L t = up to 24 h, T [except (e), (g), (h), (i)] = 30 °C, V = 0.1 L).

Close modal

The transport mechanism of adsorbate is best described by three stages (film diffusion, IPD, and intrinsic adsorption) which are elaborately described elsewhere (Hosseini et al. 2022). These stages were represented by tri-linear lines drawn based on IPD model for each adsorbate (Cr(VI)/MB) concentration, i.e., first, second, and third lines depicted film diffusion, IPD, and intrinsic adsorption, respectively. For a particular concentration, the slope, i.e., rate constant (kIPD) followed the order: kIPD1 (first stage) > kIPD2 (second stage) > kIPD3 (third stage) signifying the decreasing adsorption rate, whereas the intercept (CIPD) values showed reverse order, i.e., CIPD1 (first stage) < CIPD2 (second stage) < CIPD3 (third stage). All the CIPD values were positive (Figure 6(c)–6(d), Table 3), i.e., no line was passing through the origin. This indicated adsorption of Cr(VI)/MB on NRCAC followed the IPD, but it was not the sole rate control step (Chen et al. 2021).

Adsorption isotherms

Isotherm studies for Cr(VI)/MB onto NRCAC were conducted at 20, 30, 40, and 50 °C. Three recognized isotherm models, namely the Langmuir model (Equation (8)) (Langmuir 1916), the Freundlich model (Equation (9)) (Freundlich 1906), and the Liu model (Equation (10)) (Liu et al. 2003) were applied to analyze the adsorption equilibrium data quantitatively.
formula
(8)
formula
(9)
formula
(10)
where qmax represents the maximum quantity of Cr(VI)/MB uptake onto NRCAC; KL (Langmuir constant) relates to the energy constant; KF (Freundlich constant) is coefficient associated with adsorption capacity; nF is the coefficient related to the intensity of adsorption; Kg is the Liu equilibrium constant; nL is the exponent of the Liu equation.

Isotherm experiments were investigated to know the best fitting isotherm adsorption model in the present study, with the findings displayed in Figure 6(e)–6(h). The obtained model parameters with error values are reported in Table 4. The Liu isotherm model fitted better with the experimental data, considering the lowest values of RMSE and SSE and highest value. However, in the Langmuir model, RMSE and SSE values were also low, and value was very close to one as well as qe,exp was close to qmax of Langmuir isotherm. Thus, this model could also be applied to the present investigation.

Table 4

The non-linear fitting parameters of isotherm models and the thermodynamic analysis for adsorption of Cr(VI)/MB onto NRCAC.

Isotherm modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
20 °C30 °C40 °C50 °C20 °C30 °C40 °C50 °C
  qe,exp (mg/g)  69.68 72.59 75.02 78.43  185.11 188.38 198.18 202.85 
Langmuir  qmax (mg/g)  72.47 74.70 76.93 81.96  188.20 189.70 198.30 202.30 
 KL (L/mg)  0.0350 0.0426 0.0546 0.0647  0.0548 0.0817 0.1144 0.1728 
   0.9949 0.9951 0.9979 0.9969  0.9873 0.9843 0.9865 0.9867 
 RMSE (mg/g)  1.69 1.73 1.17 1.55  6.98 8.05 7.90 8.12 
 SSE (mg2/g2 26 27 12 21  438 583 561 593 
Freundlich  KF (L(1/nF)/(g mg(1/nF-1)))  17.47 19.54 22.28 25.41  59.07 66.99 78.00 87.97 
 nF (-)  4.40 4.61 4.92 5.18  5.30 5.74 6.31 6.94 
   0.9289 0.9262 0.9116 0.8826  0.9308 0.9308 0.9171 0.9078 
 RMSE (mg/g)  6.28 6.70 7.67 9.53  16.30 16.87 19.56 21.37 
 SSE (mg2/g2 355 404 530 817  2,392 2,560 3,443 4,109 
Liu  qmax (mg/g)  75.19 77.60 78.37 81.28  199.90 205.10 209.70 211.90 
 Kg (L/mg)  0.03 0.04 0.05 0.07  0.05 0.07 0.11 0.17 
 nL(–)  0.87 0.86 0.92 1.05  0.77 0.71 0.73 0.74 
   0.9962 0.9969 0.9985 0.9968  0.9929 0.9960 0.9954 0.9952 
 RMSE (mg/g)  1.45 1.36 1.01 1.58  5.24 4.07 4.63 4.90 
 SSE (mg2/g2 17 15 20  220 133 172 192 
Thermodynamic parameters
Adsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
Temperature (K)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol)
293  7.44 –18.13 18.57 125.05 –18.08  9.70 –23.63 31.67 188.29 –23.53 
303  7.63 –19.24   –19.33  10.03 –25.28   –25.41 
313  7.92 –20.62   –20.59  10.45 –27.21   –27.29 
323  8.13 –21.85   –21.84  10.90 –29.28   –29.18 
Isotherm modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
20 °C30 °C40 °C50 °C20 °C30 °C40 °C50 °C
  qe,exp (mg/g)  69.68 72.59 75.02 78.43  185.11 188.38 198.18 202.85 
Langmuir  qmax (mg/g)  72.47 74.70 76.93 81.96  188.20 189.70 198.30 202.30 
 KL (L/mg)  0.0350 0.0426 0.0546 0.0647  0.0548 0.0817 0.1144 0.1728 
   0.9949 0.9951 0.9979 0.9969  0.9873 0.9843 0.9865 0.9867 
 RMSE (mg/g)  1.69 1.73 1.17 1.55  6.98 8.05 7.90 8.12 
 SSE (mg2/g2 26 27 12 21  438 583 561 593 
Freundlich  KF (L(1/nF)/(g mg(1/nF-1)))  17.47 19.54 22.28 25.41  59.07 66.99 78.00 87.97 
 nF (-)  4.40 4.61 4.92 5.18  5.30 5.74 6.31 6.94 
   0.9289 0.9262 0.9116 0.8826  0.9308 0.9308 0.9171 0.9078 
 RMSE (mg/g)  6.28 6.70 7.67 9.53  16.30 16.87 19.56 21.37 
 SSE (mg2/g2 355 404 530 817  2,392 2,560 3,443 4,109 
Liu  qmax (mg/g)  75.19 77.60 78.37 81.28  199.90 205.10 209.70 211.90 
 Kg (L/mg)  0.03 0.04 0.05 0.07  0.05 0.07 0.11 0.17 
 nL(–)  0.87 0.86 0.92 1.05  0.77 0.71 0.73 0.74 
   0.9962 0.9969 0.9985 0.9968  0.9929 0.9960 0.9954 0.9952 
 RMSE (mg/g)  1.45 1.36 1.01 1.58  5.24 4.07 4.63 4.90 
 SSE (mg2/g2 17 15 20  220 133 172 192 
Thermodynamic parameters
Adsorbent–adsorbate
NRCAC-Cr(VI)
NRCAC-MB
Temperature (K)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol/K)ΔG° (kJ/mol)
293  7.44 –18.13 18.57 125.05 –18.08  9.70 –23.63 31.67 188.29 –23.53 
303  7.63 –19.24   –19.33  10.03 –25.28   –25.41 
313  7.92 –20.62   –20.59  10.45 –27.21   –27.29 
323  8.13 –21.85   –21.84  10.90 –29.28   –29.18 

Moreover, maximum uptakes of NRCAC for Cr(VI) and MB were found to be 78.43 and 202.85 mg/g, respectively, which were much more than those of adsorbents prepared by chemical activation using ZnCl2 in the other studies (Table 5).

Table 5

Recent literature review of various ZnCl2-based ACs

Raw materialsAdsorption capacity (mg/g)
pHReferences
Cr(VI)MB
Sea buckthorn stones 19.29 – 5.0 Guo et al. (2021)  
Fox nutshell 43.45 – 2.0 Kumar & Jena (2017)  
Sludge 67.70 – 2.0 Nguyen et al. (2018)  
Teakwood sawdust 72.46 – 2.0 Ramirez et al. (2020)  
Natural rubber sludge 78.43 – Original solution pH Present study 
Sesame seed shells – 103.00 Original solution pH Sharif et al. (2018)  
Silver berry seeds – 120.48 Original solution pH Benmahdi et al. (2021)  
Activated sludge – 144.80 Original solution pH Ding et al. (2019)  
Tea waste – 147.06 Original solution pH Tuli et al. (2020)  
Natural rubber sludge – 202.85 Original solution pH Present study 
Raw materialsAdsorption capacity (mg/g)
pHReferences
Cr(VI)MB
Sea buckthorn stones 19.29 – 5.0 Guo et al. (2021)  
Fox nutshell 43.45 – 2.0 Kumar & Jena (2017)  
Sludge 67.70 – 2.0 Nguyen et al. (2018)  
Teakwood sawdust 72.46 – 2.0 Ramirez et al. (2020)  
Natural rubber sludge 78.43 – Original solution pH Present study 
Sesame seed shells – 103.00 Original solution pH Sharif et al. (2018)  
Silver berry seeds – 120.48 Original solution pH Benmahdi et al. (2021)  
Activated sludge – 144.80 Original solution pH Ding et al. (2019)  
Tea waste – 147.06 Original solution pH Tuli et al. (2020)  
Natural rubber sludge – 202.85 Original solution pH Present study 

Adsorption thermodynamics

The Liu isotherm constant (Kg) (Table 4) was used to calculate the thermodynamic equilibrium constant applying Equation (11) because this isotherm model was found to be the best fit for the experimental equilibrium data at various temperatures (20–50 °C) for NRCAC (Lima et al. 2019b).
formula
(11)
where Madsorbate (g/mol) is the atomic mass of Cr(VI)/MB; [Adsorbate]° (1 mol L−1) is the standard concentration of adsorbate; γ is the coefficient of activity. To calculate ΔG° (standard Gibbs energy change), Equations (12) and (13) are used:
formula
(12)
formula
(13)
where T (K) is the temperature; R (8.314 J/K mol) is the universal gas constant. The combination of Equations (12) and (13) has resulted in Equation (14):
formula
(14)

A linear plot of ln versus 1/T from Equation (14) is drawn in Figure 6(i) to calculate changes in standard enthalpy (ΔH°) and standard entropy (ΔS°) from slope and intercept (tabulated in Table 4). Positive ΔH° and negative ΔG° values indicated endothermic and spontaneous adsorption of Cr(VI)/MB onto NRCAC, while positive ΔS° values suggested the increased randomness at NRCAC interface during adsorption (Lima et al. 2019b).

Desorption and reusability

The desorbed uptake, qt,d (mg/g) (Equation (15)) and desorption efficiency, Dt(%) (Equation (16)) (Daneshvar et al. 2017) for each cycle were evaluated.
formula
(15)
formula
(16)
where Ct,d (mg/L) is Cr(VI)/MB concentration in the desorbing solution at time t; Vd (L) is the desorbing solution volume; md (g) is the mass of NRCAC saturated with Cr(VI)/MB; De (%) is the desorption efficiency at equilibrium. The kinetic model equations (Equations (4)–(6) were also used for desorption kinetic study (using the best-selected solvents) for the first cycle by replacing qt, qe, kPFO, kPSO, kAV, and nAV with qt,d, qe,d, kPFO,d, kPSO,d, kAV,d, and nAV,d, respectively.
The reusability of NRCAC for Cr(VI) and MB adsorption was examined. The results are demonstrated in Figure 7. It was found that 1 M NaOH and DW were the best eluents with the highest desorption efficiency, De(Cr(VI)): 80.98% and De(MB): 85.79%, respectively (Figure 7(a)).
Figure 7

(a) Desorption efficiency of various eluents; (b) cycles of adsorption–desorption of Cr(VI) and MB; (c) contact time for desorption (first cycle); (d) kinetic model fitting for Cr(VI) and MB onto NRCAC (first cycle desorption). (Adsorption conditions: dp,avg = –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, Co = 100 mg/L, t = 24 h (except (c), (d)), T = 30 °C, V = 0.1 L; desorption conditions: eluentCr(VI) –1 M NaOH and EluentMB – DW (except (a)), dp,avg = –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, t = 24 h (except (c), (d)), T = 30 °C, V = 0.1 L).

Figure 7

(a) Desorption efficiency of various eluents; (b) cycles of adsorption–desorption of Cr(VI) and MB; (c) contact time for desorption (first cycle); (d) kinetic model fitting for Cr(VI) and MB onto NRCAC (first cycle desorption). (Adsorption conditions: dp,avg = –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, Co = 100 mg/L, t = 24 h (except (c), (d)), T = 30 °C, V = 0.1 L; desorption conditions: eluentCr(VI) –1 M NaOH and EluentMB – DW (except (a)), dp,avg = –100 BSS, m/VCr(VI) = 3.0 g/L, m/VMB = 1.0 g/L, Sags = 150 rpm, t = 24 h (except (c), (d)), T = 30 °C, V = 0.1 L).

Close modal

Adsorption efficiencies (R%) and (De%) calculated from Equations (2) and (16 were gradually reduced from first cycle (RCr(VI)%: 86.33 and RMB%: 89.55; De(Cr(VI))%: 82.29, and De(MB)%: 85.86) to last cycle (RCr(VI)%: 25.19 and RMB%: 39.34; De(Cr(VI))%: 14.58, and De(MB)%: 23.31) as shown in Figure 7(b). Pore plugging and complex formation are the major reasons for this reduction (Qu et al. 2021).

Figure 7(c) on the first cycle showed De(Cr(VI)) of 80.31% and De(MB) of 84.54% reached maximum desorption within 4 h. The study of three kinetic models (PFO, PSO, and AV) also in the first cycle was depicted in Figure 7(d), and obtained kinetic parameters are summed up in Table 6. The PSO was the best-approximated desorption kinetic model for Cr(VI) and MB (Daneshvar et al. 2017). Based on the obtained model PSO, the desorption process was found to be the rate-limiting step (Njikam & Schiewer 2012).

Table 6

Fitting parameters of kinetic models for first desorption cycle of Cr(VI)/MB from NRCAC

Kinetic modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)NRCAC-MB
 qe,d,exp (mg/g) 23.71 76.71 
Pseudo-first-order qe,d (mg/g) 22.93 74.08 
kPFO,d× 102 (/min) 3.46 8.09 
 0.9882 0.9515 
RMSE (mg/g) 0.90 5.11 
SSE (mg2/g2261 
Pseudo-second-order qe,d (mg/g) 24.75 77.61 
kPSO,d× 103 (g/mg/min) 1.86 1.74 
 0.9959 0.9888 
RMSE (mg/g) 0.53 2.46 
SSE (mg2/g22.82 60.36 
Avrami-fractional order qe,d (mg/g) 16.54 63.21 
kAV,d (/min) 1.51 2.59 
nAV,d (–) 5.54 3.39 
 0.1806 0.5356 
RMSE (mg/g) 7.53 15.79 
SSE (mg2/g2510 2,245 
Kinetic modelsParametersAdsorbent–adsorbate
NRCAC-Cr(VI)NRCAC-MB
 qe,d,exp (mg/g) 23.71 76.71 
Pseudo-first-order qe,d (mg/g) 22.93 74.08 
kPFO,d× 102 (/min) 3.46 8.09 
 0.9882 0.9515 
RMSE (mg/g) 0.90 5.11 
SSE (mg2/g2261 
Pseudo-second-order qe,d (mg/g) 24.75 77.61 
kPSO,d× 103 (g/mg/min) 1.86 1.74 
 0.9959 0.9888 
RMSE (mg/g) 0.53 2.46 
SSE (mg2/g22.82 60.36 
Avrami-fractional order qe,d (mg/g) 16.54 63.21 
kAV,d (/min) 1.51 2.59 
nAV,d (–) 5.54 3.39 
 0.1806 0.5356 
RMSE (mg/g) 7.53 15.79 
SSE (mg2/g2510 2,245 

The regeneration cost calculation shows that approximately US$ 67.02 is required to purchase NAOH to desorb 1 kg of Cr(VI)-loaded NRCAC in the first cycle. On the other hand, the cost involved is negligible for other eluting agent (water), i.e., for MB-loaded NRCAC.

Fate of the loaded NRCAC

Dumping Cr(VI)/MB-loaded NRCAC in the open environment is challenging due to the possibility of Cr(VI)/MB leaching. This exhausted NRCAC was incinerated at 700 °C resulting in the decomposition of MB along with the residual ash. 10 g of this ash was mixed with 50 mL of DW, maintaining the solid–liquid ratio 1:5, and stirred for 24 h followed by filtering (Nag et al. 2017). The analysis of the resulting filtrate did not trace any Cr(VI). Therefore, ash could be used in brick preparation, road construction, or landfilling purposes. As a result, incineration of Cr(VI)/MB-loaded NRCAC will reduce secondary pollution and be a step forward to a greener environment.

An extensive literature search revealed that NRBS has never been explored for the synthesis of chemically activated carbon (NRCAC). Accordingly, waste NRBS has been utilized for the first time in preparation of NRCAC for the adsorptive removal of hazardous pollutants such as heavy metals (Cr(VI)) and dyes (MB). NRCAC was prepared from NRBS by ZnCl2-activation that led to a substantial increase in adsorption capacity, i.e., six-fold for Cr(VI) and nine-fold for MB because of a considerable enhancement in BET surface area (4.62 m2 g−1 for NRBS and 414.16 m2 g−1 for NRCAC), i.e., almost 90-fold increase. The N2 adsorption–desorption isotherm study showed mesoporous pore distribution in NRCAC. FTIR and scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS) suggested significant functional and morphological transformations in NRCAC. NRCAC showed 95.49% (pH = 2.0) of Cr(VI) removal, while MB removal was 97.36% (pH = 1.0) and 99.95% (pH = 12.0). Mass transfer during adsorption was explained through the IPD model. Unlike conventional procedure, the thermodynamic equilibrium constant was derived by considering the Liu isotherm at four different temperatures. The desorption/reusability study of consecutive five cycles indicated that sludge-derived NRCAC had the potential to be a reusable adsorbent for Cr(VI) and MB removal. Thus, the present study helps to address environmental problems as an efficient and eco-friendly NRCAC has a high capacity to remove Cr(VI) and MB from wastewater and dilute discharge of pollutants can be concentrated through adsorption/desorption cycles with consequent regeneration of adsorbent.

The authors would like to thank IIT Guwahati for providing characterization facility of XRD, FTIR, SEM-EDS, BET, and TGA analyzer. Authors acknowledge Guwahati Biotech Park for CHNS analysis and Guwahati University for XRF analysis. Authors are also thankful to NIT Agartala for AAS analysis. Authors also greatly thank Brite Rubber Processors Private Ltd, Agartala, in India for providing raw sludge. Authors gratefully acknowledge the Department of Biotechnology (DBT), Govt. of India, for the financial support (BT/COE/34/SP28408/2018).

This is to certify that the authors are not affiliated with or involved with any organisation or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this paper.

U.M. conceptualized the study, did data curation, formal analysis, investigated the study, performed methodology, collected resources, did software analysis, validated the study, visualized the study, wrote the original draft, wrote the review and edited the article. A.C. conceptualized the study, did data curation, collected resources, supervised, validated the study, wrote the review and edited the article. C.D. conceptualized the study, acquired funds, did data curation, formal analysis, wrote the review and edited the article. A.K.M. conceptualized the study, acquired funds, supervised the study, wrote the review and edited the article.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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