Chemically modified cellulose nanomaterial for remediation of nickel and lead from secondary runoff industrial wastewater

Water is paramount for the sustainable development of all healthy life throughout the world (Berger et al. 2017; Brandes et al. 2019; Maharana et al. 2020; Shahnaz et al. 2020; Tsade et al. 2020; Tshikovhi et al. 2020). However, water pollution with toxic metals is recently considered the greatest stimulating issue all over the world (Anush et al. 2019). Nowadays, the presence of various contaminants including toxic inorganic nutrients, different organic species and fuel-related products have been reported throughout the world in aquatic systems (Briffa et al. 2020; Sharma et al. 2021; Tsade et al. 2021). As a result of their toxicity and bioaccumulation potential, the above-stated pollutants pose serious hazards for human beings and other environments (He et al. 2017; Amanda et al. 2020; Ibrahim et al. 2020). Especially, water contamination arising from toxic inorganic metals results in serious adverse impacts on the environment and human health (Bhanjana et al. 2017; Shahnaz et al. 2020; Tshikovhi et al. 2020). As a result of this, unlike organic contaminants, toxic inorganic nutrients are not biodegradable and can be accumulated for a long time within the tissues of living organisms, producing many illnesses and syndromes, such as cancer (Atkovska et al. 2018; Nguyen et al. 2019). Inorganic nutrients including, nickel, lead, arsenic, mercury and cadmium, to name a few, may be considered the most toxic of heavy metals in marine bodies. Increased amounts of these metals discharged into marine systems cause severe well-being and ecological risks and possibly will lead to a rise in water purification prices. For example, the toxicity of lead can affect the reproductive systems, intelligence index, and kidney and liver illnesses (Gaurav et al. 2017; Xu et al. 2017). Moreover, regarding the studies conducted in toxicological on nephrotoxicity, the introduction of lead into the human body can be related to renal tubular necrosis, and this introduction is directly related to the failure of glomeruli (Rathnasekara et al. 2021). Exposure to nickel, on the other hand, can harm the lungs, stomach, kidneys, and cause cancer (Ibrahim et al. 2020). Because of this, eliminating heavy metals, in general, and lead and nickel, in particular, from wastewater before discharging it into the environment, is of the highest interest. Traditional methods for the elimination of these types of toxic inorganic nutrients include adsorption, coagulation, precipitation, filtration, solvent extraction, reverse osmosis, and ion exchange (Matsis and Grigoropoulou 2008; Anjum et al. 2019; Agnes et al. 2020; Mohib et al. 2020). Nevertheless, some of these cause high operational costs, with relatively low eradication capacities and more often treated waters need an extra polishing step or steps to achieve desired removals, thus making it even more expensive (Mohd Nor et al. 2021). Adsorption, however, has demonstrated lower running costs, and large removal capacities and has presented itself as a plausible alternative. Even further, the use of adsorbent materials is widespread including activated carbon, carbon nanotubes (CNTs), and nanocomposites (Alipour et al. 2020), making it the technology of choice (Hokkanen et al. 2016).

But, from these polymeric materials, cellulose has more merits such as easy to obtain, not toxic, as well as more available hydroxyl functional groups taht are accessible for different chemical amendment reactions (Jianhua et al. 2020; Yeit et al. 2020; Kara et al. 2021a). In spite of its pronounced merits, the demerits of cellulose in wastewater cleaning are its delayed hydrophilicity, less physical and chemical stability, and limited pollutant uptake capability (Anirudhan & Deepa 2015). Due to this reason, nowadays, the synthesis of nanomaterials for adsorbing adsorbate is growing (Ahankari et al. 2020). However, nanomaterials, including titania-doped nanotube, carbon-based nanotube, and zerovalent iron nanomaterials are toxic and not suitable for pollutant elimination (Ahankari et al. 2020). These drawbacks were easily resolved by synthesizing adsorbent materials by disintegrating cellulose materials into cellulose nanomaterials (CNMs) using different preparation methods (Kim et al. 2015; Kara et al. 2021b). Regarding its synthesis methods and origin materials, nanocellulose could be categorized into cellulose nano-fibers (CNFs), cellulose nano-crystals (CNCs), and bacterial-cellulose (BC) (Hokkanen et al. 2016). In this study, CNC was prepared by using the sulfuric acid hydrolysis method.

A number of investigations have been carried out previously on the nanocellulose-based adsorbent materials prepared from different natural resources such as date palm (Phoenix dactylifera L.) (Adel et al. 2018), residue from cotton (Do Nascimento et al. 2015), banana linters (Harini et al. 2018), husk obtained from corn (Yang et al. 2017), and others. As per the knowledge of the researcher, few works were reported using CNMs prepared from the dried stem of Eichhornia crassipes weed collected from Lake Abaya. Also, for the improvement of the CNM surface area, it was modified using succinic anhydride chemical in the carboxyl group. In addition to this, to the greatest of the researcher's knowledge, most of the research carried out in the reported references were using aqueous solution merely, but there is a research gap on real wastewater treatment. Therefore, this study aims at looking at the CNM prepared from dried stem of E. crassipes weed resources for the elimination of nickel and lead found in SERWW.

The analytical grade chemicals were purchased and used for all the experiments. They were namely, NaClO₂ (80%, Shanghai ZZ New Material Tech. Co., Ltd, China), CH₃CH₂OH (97%, Tradewell International Pvt. Ltd, India), toluene (99%, Loba Chemie Pvt. Ltd, India), NaOH (99%, Shraddha Associates (GUJ) Pvt. Ltd, India), conc. HCl (35%, Loba Chemie Pvt. Ltd, India), conc. H₂SO₄ (69%, Loba Chemie Pvt. Ltd, India), conc. HNO₃ (69%, Loba Chemie Pvt. Ltd, India), Ni(NO₃)₂ (99%, Shraddha Associates (GUJ) Pvt. Ltd, India), Pb(NO₃)₂ (99%, Shraddha Associates (GUJ) Pvt. Ltd, India), and NaHCO₃ (99%, Shraddha Associates (GUJ) Pvt. Ltd, India).

The stem of the weed found in the aqueous system, namely E. crassipes for CNM fabrication was collected randomly from Lake Abaya ‘Arba Minch’, Gamo Zone, Ethiopia. Next, the secondary runoff industrial wastewater (SERWW) was acquired from the Modjo area, Oromia Region, Ethiopia. After this, the randomly collected and dried stem of the E. crassipes weed sample was washed with distilled water many times and dried with air at room temperature and grinded with a clean mill. Then, cellulose was fabricated by taking 10 g of carefully dried stem of E. crassipes plus a mixture of 1:1.5 of toluene to ethanol solvent for about 48 h at 50 °C followed by cleaning with hot water and dried out in oven for 10 h at 50 °C. Then, the fabricated and dried-out fibers were adjusted into short fibers of roughly 5 mm in length. Following, the adjusted cellulose fibers were cured with 90 mL of 2.25 M NaOH solution at 60 °C for 2.5 h for the removal of lignin and hemicelluloses found in E. crassipes biomass. This solution was cleaned carefully with deionized water repetitively till a solution with pH of 7 was obtained. Again, the obtained solution with pH of 7 was clarified, centrifuged and dried out in the oven at 60 °C for 12 h. At this stage, the solution reached a pulp form through a careful grinding procedure and the bleaching procedure was performed with a 3.5:1.5 mixture of sodium chlorite (NaClO₂) and glacial acetic acid for 5 h at 70 °C through mechanical stirring. Then, the obtained residue solution was constantly centrifuged, clarified and dried out to get the suspension of cellulose. The suspension of cellulose was cleaned from any impurities such as lignin, hemicellulose, fluid by careful treatment with 90 mL of 2.25 M NaOH solution. In order to obtain cellulose-free from any non-cellulose components, this procedure was reiterated by half of the initial amount of bleaching reagent. Following this, the solution was hydrolyzed using 90 mL of 6.5 M H₂SO₄ for 3 h to carefully break down the cell wall into smaller fibers and to isolate the fibrils from each other. At this stage, the white suspension of CNM was centrifuged at 12,000 rpm to separate the CNM and homogenized in a homogenizer at about 12,000 rpm for 3 h. Finally, the fabricated CNM was stored in an appropriate place and analyzed for characterization of different required techniques. These experimental steps were taken by Kara et al. (2021a, 2021b) through slight modification.

The crystallite size of the S-CNM adsorbent was analyzed with the help of an XRD instrument (Cu-Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA under a 2θ diffraction angle from 0° to 40° at a scan rate of 2°/min). The functional group found on the fabricated S-CNM adsorbent was recognized through the help of FT-IR (Perkin-Elmer, RXI model). The morphology characteristic of the fabricated S-CNM adsorbent surface was observed using a Hitachi S-4100 scanning electron microscope (SEM). The specific surface area (SSA) determination was performed by Brunauer–Emmett–Teller (BET) analysis. The particle size of the S-CNM adsorbents was determined using a transmission electron microscope (TEM).

First, the 1,000-ppm stock solution of nickel and lead ions was prepared from their soluble salts and the stock solution with the necessary amount of the synthesized and characterized adsorbent (S-CNM) dose was mixed in a 100-mL flask by diluting to the required adsorbate concentrations and stirred with an orbital for 150 min. The solution pH of nickel and lead was justified with the help of dilute HCl and NaOH solutions through pH meter. The use of one separation technique, namely filtration separates the solid/liquid phases of the solution. After this, the absorbance measurement of nickel and lead ions was performed with the help of a flame atomic absorption spectrophotometer (FAAS, Shimadzu, USA). The above all experiments were performed by measuring different constituents that affect the removal capability of nickel and lead ions, including, contact time ranging from 20 to 180 min, solution pH ranged from 2 to 10, adsorbent dose ranged from 0.02 to 3.0 g, and initial adsorbate concentration ranged from 5 to 40 mg/L. The influences of every component on the removal capability of nickel and lead ions were performed by putting the rest components at a constant system.

The rate of adsorption mostly depends on the contact time and is termed the most significant parameter providing valuable data for the velocity of sorption. It also evaluates the Ni(II) and Pb(II) ions uptake capacity of S-CNM adsorbent from SERWW. Thus, the adsorption kinetics mechanism of Ni(II) and Pb(II) ions from SERWW was studied at a contact time of 20–180 min and maintaining constituents (pH of the solution, S-CNM dose, and Pb(II) and Ni(II) ions starting levels) at optimum.

The reusability study of the previously used S-CNM adsorbent in the adsorption of nickel and lead ions in the SERWW was done by duplicating the experimental procedures through the use of similar S-CNM adsorbent for four consecutive circles. Desorption of nickel and lead ions was performed through the addition of the optimum S-CNM value (0.3 g) previously used for eradicating nickel and lead ions from SERWW in a conical flask with 10.5 mL of 0.1 M HCl solution for 19.5 min and sonication for 6 min was performed. Afterwards, the material (S-CNM) was easily alienated from the mixture by centrifugation. The alienated material (S-CNM) was washed away with the help of distilled water six times, dried out and reapplied for four consecutive cycles.

Figure 2(b) displayed the X-ray diffractometer spectra of the synthesized CNP to govern the crystallinity size. As shown, the spectral main peaks at 15.99°, 22.22° and 34.97° indicate the respective cellulose-I structure and correspond to the crystals of the representative plane description of 110, 200, and 004, respectively (Anirudhan & Deepa 2015; Kara et al. 2020). The S-CNP adsorbent crystallite size was calculated and the value was 2.45 nm. Likewise, its width was 3.65 nm. Similar to this, Anirudhan & Deepa (2015) reported similar results that entitled on correlating nanocrystalline cellulose particle size and its surface area. Generally as seen from the peaks, a semi-crystalline with unstructured broadened hemp and crystalline peaks was observed. This is owing to the circumstance that the S-CNP was fabricated from natural-based materials, which could not show a 100% crystallinity environment. Additionally, the esterification process of the cellulose was achieved with the reaction of succinic anhydride to form an esterified cellulose nanoparticle (S-CNP). The semi-crystallinity structure of CNM was confirmed by XRD analysis (Kara et al. 2020).

Figure 2(c) and 2(d) depicts the morphology of S-CNP adsorbent. As shown in the figure, a rod-shaped spectral peak, which exhibited an organized and importantly penetrable cooperated assembly with indelicately even pore sizes with augmented definite surface areas, was detected. This consequently resulted due to the extreme homogenization of the adsorbent constituents throughout fabrication development and this meaningfully increased the surface area to volume ratio for S-CNP adsorbent. Additionally, the fibers of S-CNP are particularly disconnected from each other. The succinic anhydride modification process showed the actual rough surface S-CNP adsorbent detected by SEM micrographs. As a result, the surface area of S-CNP is augmented through the S-CNP adsorbent roughness of the surface and reducing the particle size (Zhou et al. 2012). Therefore, excess vacant sites exist for nickel and lead metal ions adsorption. The morphological feature of S-CNM was measured by TEM interpretations (Figure 2(d)). The image shows the nanosized fibril with a wide heterogeneous distribution both in width and in length.

In the above equation, P_o and P represent saturated and partial vapor pressure of N₂ gas at equilibrium in pa, respectively; n represents the N₂ gas volume adsorbed at STP by using mL as a unit; X_m represents BET monolayer capability; C represents a dimensionless constant associated with the enthalpy of N₂ gas adsorption on the adsorbent.

In the above equation, L represents Avogadro constant (6.022 × 10²³ mol⁻¹), a_s represents the S-CNM adsorbent SSA of the mass m in grams for BET analysis, σ_m represents N₂ gas molecular adsorptive cross-sectional area in the complete monolayer (equal to 0.162 nm² for N₂ gas) analysis, the 22,400 represents the amount of occupied volume in mL by 1 mole of N₂ gas at STP. The cylindrical shape of the S-CNP adsorbent was observed from the BET plot given in Figure 3(b). Findings showed that the synthesized S-CNP adsorbent has a higher SSA of 124.82 m²g⁻¹. Therefore, S-CNP adsorbent has shown higher metal adsorption competence owing to the decline and increase of S-CNP adsorbent particle size and SSA, respectively, observed from BET data analysis.

The collected SERWW physicochemical analysis values are presented in Table 1. As seen in Table 1, the pH average value was 5.7 ± 0.04. From the pH result, nearly acidic SERWW was collected from the Modjo River. Subsequently, the SERWW average values for COD and BOD were 70.9 ± 0.03 and 65.04 ± 0.02 mg/L, respectively. Following this, the SERWW average values for Fe²⁺, TDS, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, and Cu²⁺ were 0.59 ± 0.05, 40.4 ± 0.02, 39.3 ± 0.02, 90.5 ± 0.05, 89.6 ± 0.2 and 0.50 ± 0.02 mg/L, respectively. As seen from the result, cations and anions are present in the collected SERWW and reacts with the Ni(II) and Pb(II) ions in the adsorption mechanisms.

As seen in Figure 4(a), the effects of S-CNP dosage on the (%R) of nickel and lead metal ions from the SERWW showed that 82.6 and 93.5% of nickel and lead metal ions, respectively, can be eliminated from SERWW through the use of S-CNP adsorbent with an optimum adsorbent dosage of 0.5 g at room temperature. The relatively greater %R for lead ions was observed through the use of S-CNP adsorbent may be because of the solubility nature of the metals. The more soluble the adsorbate the less adsorbed onto the adsorbent. In conclusion, it can be seen that for nanosorbent the %R of both metal ions increases through the increase of the S-CNP adsorbent dose up to the optimum values. After the optimum S-CNP adsorbent, the adsorption competence declined. This is predicted as a result of the fact that at increased concentration levels of nanosorbent (S-CNP), large number of vacant spaces are available for the metal ions. However, at augmented concentration levels, there is no additional augmentation in the adsorption mechanism because of the amount of nickel and lead ions bonded to the S-CNP adsorbent and the number of movable ions in the SERWW turns into constant even with extra accumulation of the S-CNP adsorbent dose (Tana et al. 2018; Igberase et al. 2019).

Effects of initial concentration (C_i) of nickel and lead metal ions on the adsorption mechanism by S-CNP adsorbent is represented in Figure 5(b). At the starting point, the adsorption reaction goes as fast as and in a very short period of time it attains the equilibrium value of 30 mgL⁻¹. The quicker metal uptake processes at the starting point were a result of the large unoccupied spaces of the S-CNP adsorbent surface. At this stage, the S-CNP adsorbent clearly displayed higher uptake competence of nickel and lead metal ions. This happens as a result of the comparatively augmented SSA of S-CNP adsorbent which was observed by the reaction of CNP with carboxyl and ester-yielding chemicals (Gopal Reddi et al. 2017).

The regression investigational data found through the use of PFO kinetics model was invalid for the adsorption process due to the investigational q_e number being much dissimilar than the measured q_e value for S-CNP adsorbent. The findings specified that the rate of adsorption mechanism did not fit PFO kinetics. Thus, Figures 8(a) and 8(b) show a PSO kinetics model that fits the study data well for nickel and lead metal ions adsorption on S-CNP sorbent. In line with this, similar research was reported by Tsade et al. (2021) on the adsorption of lead metal ions from wastewater through the use of CNMs. The kinetic data were found in Figures 8(a) and 8(b) and given in Table 3. The kinetics study provides a higher regression value (R² = 0.985 and 0.991, for PSO) and below satisfactory correlation coefficient value (R² = 0.764 and 0.972, for PFO) in S-CNP adsorbent for the removal of nickel and lead metal ions, correspondingly.

Contamination by nickel and lead metal ions in an aqueous system is great owing to an enormous quantity of crude discharge of industrial wastewater from different industrial sites. As a result, the health of aqueous habitats is affected time by time. Thus, this study makes a solution to remove these contaminants before discharging them into an aqueous system. Therefore, the synthesized S-CNP adsorbent makes use of low-cost, renewable, and high uptake capability for the adsorption of nickel and lead metal ions from SERWW. The results showed that the removal capability of S-CNP toward lead metal ions in comparison with nickel metal ions was surprisingly increased due to the solubility nature of lead metal ions. From the adsorption mechanism, the lead metal ion adsorption fits the Langmuir isotherm and the nickel metal ion adsorption fits the Freundlich isotherm model. Furthermore, both metal ion adsorption fits by the PSO kinetics model. Finally, from the regeneration study, it is possible to conclude that S-CNP is an efficient adsorbent for the removal of heavy metals. Thus, from the adsorption mechanism point of view, reader can have a clue about the S-CNP adsorbent adsorption process, its characteristics, and its efficiency. Generally, as seen from the results, the synthesized S-CNP adsorbent was very efficient in removing heavy metal ions, in general, and nickel and lead metal ions, in particular.

The researcher of this study thanks Adama Science and Technology University for providing the funds and accommodation to perform this research. This research has been sponsored by the Project (ASTU, SoANS/JV-259298/2019), Research and Technology Transfer Office, Adama Science and Technology University.

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

The authors declare there is no conflict.