Comparative study of simple and modified rice husk biochar for cadmium removal: Adsorption performance and possible mechanisms

The anthropogenic and developmental activities are responsible for the degradation of water qualities that lead to the accumulation of several toxic contaminants including heavy metals in the water bodies. Along with that, the effluents and wastewater released from the industries and urban cities are used for irrigating the agricultural fields which lead to the accumulation of heavy metals in the soil and food crops (Singh et al. 2010). The presence of toxic metal ions in water and other systems is responsible for several health issues (Wang et al. 2012). This problem could be rectified by using some techniques like membrane filtration, chemical precipitation, and oxidation/reduction (Ali 2010). However, because of low removal efficiency and high cost, these techniques are not preferred at a large scale. Over these techniques, recently, the application of biochar has attracted more attention (Li et al. 2023). The biochar is a lucrative adsorbent produced by using agricultural residues through pyrolysis at moderate temperatures (300–700 °C). Among different agricultural residues, rice husk is one of the major residues that can be used for biochar production. It will solve the problem of managing the waste generated during the rice milling process as well as it could be acting as a good adsorbing agent for metal adsorption (Hoslett et al. 2019; Quyen et al. 2021).

The biochar obtained from rice husk has several functional groups and is highly porous in nature that could be able to adsorb organic and inorganic compounds including heavy metals from water and other systems (Obemah & Baowei 2014; Li et al. 2017a, 2017b; Wang et al. 2017). In earlier studies, it has been reported that by using biochar, the heavy metal ions can be reduced from contaminated water with the help of different processes like the exchange of ions, binding with functional groups, physical sorption, and surface precipitation (Chen et al. 2011; Inyang et al. 2012). Furthermore, with chemical and physical modifications, the adsorption rate of biochar could be increased by changing the surface properties (He et al. 2019).

There are different ways of modifying, like treating with acids or bases and saturating with minerals (Rajapaksha et al. 2016; Sarkar et al. 2019). The acid treatment removes the contaminants from the surface of biochar and introduces more functional groups (Xu et al. 2020). The basic treatments also affect the surface properties of biochar to enhance the precipitation of metals (Inyang 2013). The chemical modification may lead to the addition of a functional group and also affect the adsorption properties of biochar (Zhang et al. 2017; An et al. 2019).

With the above context, the current study will try to find out the comparative action of simple and chemically modified biochar obtained from rice husk in the adsorption of Cd²⁺. Here, only Cd²⁺ is considered because its concentration is above the safe limit in water, soil, and food crops at wastewater-irrigated areas of Varanasi city, UP, India (Singh et al. 2010; Verma et al. 2015). Among different modifying agents, the chemicals which have oxygen-containing functional groups have been reported to be effective in increasing sorption capacity (Kasera et al. 2022). Furthermore, the modifying agents (KMnO₄ and HNO₃) have been selected based on their different properties to show variation in the adsorption behavior such as KMnO₄ which has a strong oxidizing potential of +1.5 V and HNO₃ has a less strong oxidizing potential of +0.17 V. KMnO₄ acts not only as a potent oxidizing agent but also as the precursor of the MnO_x group and provides the more active sites (Fu et al. 2022). With the variation in the chemical nature of modifying agents, the adsorption nature of biochar can be varied. Compared with the previous study, the current study discusses in detail the pattern of the adsorption behavior and mechanistic action of simple and modified biochar to find out which action is more responsible for the adsorption behavior of biochar. Compared with other studies, here the experiment is repeated to check its efficiency for practical application in natural systems by using the chemically modified form biochar to adsorb toxic metals in wastewater collected from different sites (Banaras Locomotive Works (BLW), Bhagwanpur, Lohta) of Varanasi, UP, India. The study will promote the current strategy to be applied at a large scale to reduce metal contamination in the water, and further, in the soil, and in the food chain.

Rice husk biochar (RHB) obtained with 500–600 °C pyrolytic temperature was collected from the paper and pulp industry at Ramnagar, Varanasi, Uttar Pradesh, India. After collecting the RHB, it was washed by using distilled water (DW) and dried at 80 °C. The dried RHB (2.0 g) was treated with KMnO₄ and HNO₃ solutions (100 mL; 0.01 M), to modify the biochar. After incubation for 8 h at 30 °C, the modified forms of biochar by using KMnO₄ and HNO₃ solutions (0.01 M) were filtered and washed by using DW, and finally dried at 80 °C for 24 h. The simple and modified biochar were examined by proximate and ultimate analyses (Table 1).

Thermogravimetric analysis (TGA) was conducted by Model-GPC 5140 Perkin Elmer (Singapore). The temperature ranged from 25 to 1,000 °C with 10 °C min⁻¹ of heating rate and with a steady flow rate of N₂ gas.

Spectra of the biochar for structural changes were obtained by total reflectance-Fourier transform infrared (ATR-FTIR) Thermo Fisher (Nicolet, iS5, USA) FTIR spectrometer. The groups present at the surface were identified between the range of 4,000 and 400 cm⁻¹.

A field emission scanning electron microscopy (FE-SEM, QUANTA FEG 200, FEI, USA) was used for analyzing the morphological characteristics of the biochar surface.

To get the idea about specific surface area, N₂ adsorption–desorption tests were performed depending upon Brunauer–Emmett–Teller (BET) and pore volume using model-Autosorb (IQ2) (Quantachrome, USA).

The elemental composition was analyzed in simple and modified biochar with the help of the CHNS analyzer (Euro EA, Elemental Analyzer, Spain). The elemental composition and volatile matter, fixed carbon, are given in Table 1.

To find out the elemental composition and their binding state, a quantitative technique was used like X-ray photoelectron spectroscopy (XPS) by using K-alpha (Thermo Fisher Scientific, USA).

Wastewater samples (five replicates) were collected from various wastewater-irrigated sites at Varanasi, such as Lohta (untreated), BLW (12 MLD), and Bhagwanpur (9.8 MLD), Uttar Pradesh, India. The simple and modified biochar (KMnO₄ and HNO₃) were mixed in the 50 mL wastewater sample and agitated at 150 rpm for 300 min (5 h). Afterward, the Cd²⁺ concentrations were estimated in the filtered sample of simple and modified biochar.

Cd²⁺ was estimated in the simple and modified biochar through the digestion experiments, which were performed by the mixture of triacid with a combination of HNO₃, H₂SO₄, and HClO₄ in 5:1:1 ratio (Allen et al. 1986). The Cd²⁺ concentration was determined by AAS (Perkin Elmer, Aanalyst 800, USA).

To establish the relationship between Cd²⁺ solution and biochar, absorption isotherms were analyzed. The analysis was done by using the level of Cd²⁺ solution, and the data were analyzed with respect to the Langmuir and Freundlich adsorption isotherms.

The data of adsorption kinetics were used to find out the best model that could describe the adsorption process, whether it is a pseudo-first- or pseudo-second-order kinetic model

With modification by HNO₃ and KMnO₄, there was an increase in the sharpness and shifting of each functional group (Figure 3(c)–3(e)). Furthermore, with the adsorption of Cd²⁺ in HNO₃-modified biochar, –OH group shifted to 3,450 cm⁻¹, the alkoxy (–C–O) group shifted to 1,067 cm⁻¹, the Si–O–Si group shifted to 801 cm⁻¹, and O–Si–O group shifted to 457 cm⁻¹ (Figure 3(d)). After Cd²⁺ adsorption in KMnO₄-modified biochar, there was further shifting of the functional groups like –OH group shifted to 3,437 cm⁻¹, alkoxy (–C–O) group shifted to 1,058 cm⁻¹, Si–O–Si group shifted to 805 cm⁻¹, and O–Si–O group shifted to 460 cm⁻¹ (Figure 3(f)). One more functional group was observed with KMnO₄ modification, at 1,619 cm⁻¹ and it was probably because of expansion of lignin-containing groups (C = C) and after Cd²⁺ adsorption, this group shifted to 1,609 cm⁻¹ (Li et al. 2017a, 2017b). Metal adsorption was facilitated by the redox-based active binding groups at the surface of the biochar, which also aids in electron transfer (Zhang et al. 2015). With modification, there were some changes in the adsorption band of the functional groups as discussed above. This shifting assigned the vibration-specific functional group in the modified biochar compared to the simple biochar, which are basically responsible for more Cd²⁺ adsorption. Song et al. (2014) have also reported increased O–H absorption band sharpness at 3,428 and 1,100 after KMnO₄ modification. With modification, the surface Si–O–Si group is deprotonated and shifted to 794 cm⁻¹ in HNO₃-modified biochar and 792 cm⁻¹ in KMnO₄-modified biochar. These shiftings provided surface complexation, constituting a specific Cd²⁺ adsorption site in the modified biochar over simple biochar (Wang et al. 2017; Xiang et al. 2018). Overall, the modification with the HNO₃ and KMnO₄ resulted in more oxygen groups at the biochar surface that helped in the binding of Cd²⁺ ions (Figure 3(d)–3(f)).

BET analysis provides information about surface area and other properties of simple and modified biochar (Table 2). The surface areas of simple biochar were 23.218 m²/g, whereas KMnO₄- and HNO₃-modified biochar have surface areas of 53.07 and 30.705 m²/g, respectively. The pore volumes were estimated as 0.082 cc/g for KMnO₄-modified biochar, 0.108 cc/g for HNO₃-modified biochar, and 0.093 cc/g for simple biochar (Table 2). Modification of biochar affected the porosity and surface area (Cui et al. 2016a, 2016b). This increase was the maximum for KMnO₄-modified biochar compared to the simple and HNO₃-modified biochar (Table 2). The modifying agents (KMnO₄ and HNO₃) removed the impurities and added more heterogeneous pores at the surface of the biochar which ultimately increased the surface area (Li et al. 2023). Compared to HNO₃, KMnO₄ has a higher oxidizing nature, which may cause the conversion of nanopores into mesopores and macropores that resulted in more surface area and pore volume (Chen et al. 2019; Xu et al. 2020).

The analysis of the O1s spectra of simple biochar (Figure 5(ii-D)) showed peaks at 532.64/532.2/532.8 eV that correspond to silicate (⁠⁠) and carbonate (⁠⁠) groups (Zhang et al. 2020). Similar groups were observed in the modified biochar by HNO₃ (Figure 5(ii-E)). In the KMnO₄-modified biochar (Figure 5(ii-F)), extra peaks were observed at 529.78 eV that represented quinines carbonyl (C = O) and MnO₂ group, whereas the peaks at 531.09 eV represented –OH, COOR (ester anhydride), and the peaks at 533.17 and 535.22 eV showed ester anhydride (C = O) and carboxyl groups (COOR), respectively (Qian & Chen 2014; Fan et al. 2018a, 2018b). In Figure 5(iii-G), Mn2p spectra confirmed the successfully loaded MnO₂ on the KMnO₄-modified biochar. It was represented by the peaks at 642.2 and 653.94 eV (Figure 5(iii-G)). The separation of 11.4 eV between these two peaks indicated the manganese that exhibited either the oxidation state of Mn³⁺ or Mn⁴⁺. In the present study, Mn⁴⁺ was predominant as shown by a strong peak at 642.2 eV in Figure 5(iii-G) (Han et al. 2006). The XPS analysis verified the existence of the various groups, which were specifically accountable for the more Cd²⁺ ion's adsorption.

Based on the supporting data of adsorption kinetics, it has been observed that the maximum adsorption of the Cd²⁺ occurs during the initial phase, i.e., up to 120 min due to more availability of vacant sites, and later on, the adsorption rate was finally reached at the equilibrium stage up to 5 h, and thereafter there was no adsorption. The adsorption process is relatively slow with the passage of time, as Cd²⁺ may occupy all the vacant sites at the surface and interact with the internal active sites of carbon in the biochar (Li et al. 2017a, 2017b; Tan et al. 2018).

Earlier, several studies were done to show the role of modified forms of biochar in metal adsorption but all are in the synthetic solutions under laboratory conditions (Liang et al. 2017; Liu et al. 2020). Therefore, to check the practicality of such a strategy, adsorption rate of the modified forms of biochar has been evaluated in the wastewater sample (five replicates) collected from various wastewater-irrigated sites at Varanasi, such as Lohta (untreated), BLW (12 MLD), and Bhagwanpur (9.8 MLD), Uttar Pradesh, India. At all the sites, the KMnO₄-modified biochar showed a maximum reduction of Cd²⁺ metal ranging from 92 to 95%, the HNO₃-modified biochar at reduced Cd²⁺ metal from 83 to 85%, and the least reduction of Cd²⁺ metal was obtained through the simple biochar, i.e., from 68 to 80% (Table 5). With repetition of this experiment in naturally collected wastewater sample, it has been confirmed that the KMnO₄-modified form of biochar can be promoted for wider application because of its more adsorption capacity, i.e., 92.95%.

This current study focused on the Cd²⁺ ions adsorption with the application of simple and modified biochar (KMnO₄ and HNO₃). The KMnO₄- and HNO₃-modified biochar showed maximum adsorption of Cd²⁺ by 15.3 and 13.79 mg/g, respectively, calculated via the Langmuir model. The behavior of Cd²⁺ adsorption on the adsorbent was best described by the pseudo-second-order kinetics model based on the R² value. The SEM and BET analyses confirmed that the KMnO₄-modified biochar improved the pore size and pore volume significantly compared to HNO₃ and simple biochar. The FTIR characterization indicated that the functional groups were responsible for increasing the rate of adsorption in the modified biochar. The repetition of this experiment in naturally collected wastewater from different sites of Varanasi, UP, India, also confirmed that the KMnO₄-modified biochar can be an efficient adsorbent to increase the removal of Cd²⁺ from the contaminated system. It can be utilized as a simple, efficient, and environmentally beneficial management technique to reduce metal load. With further studies, the feasibility of its long-term application can be estimated in wastewater-irrigated soil to check the availability of metals in the food chain and consequently reduce the risk to human health.

The authors are thankful to the Head, CAS, Botany, Institute of Science, B.H.U., Varanasi, for providing all needed support.

K.G. performed the experiments and wrote the manuscript. A.S. analyzed the data and finalized the content. N.G. and D.K.P. helped to explain characterization data. R.P.S. helped to finalize the manuscript.

A.S. is financially supported by the IoE seed grant scheme (Dev. IoE Scheme No. 6031). The funding has been provided to K.G. by the UGC Non-Net Fellowship scheme (R/Dev./IX-Sch./21-22/33615).

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.