(690h) Adsorption of Hexavalent Chromium from Water Utilizing Amine-Grafted Silica Nanoparticles

The study focuses on addressing hexavalent chromium contamination in water through the utilization of a novel amine (NH₂) functionalized silica-based adsorbent, designated as PEI-Si. Comprehensive characterization including FTIR, FESEM, EDS, TGA, BET, and zeta potential analyses was conducted on the material. PEI-Si exhibited a notable adsorption capacity (q_L) of 133.3 mg/g at 25ËšC and pH 6. The impact of factors such as dose, pH, and contact time was investigated, highlighting the significant role of amine groups in the modified adsorbent structure in enhancing Cr (VI) uptake. Overall, PEI-Si demonstrates considerable promise as an effective adsorbent for the treatment of chromium-contaminated water.

Introduction

The escalation of anthropogenic activities worldwide has led to an alarming increase in the presence of heavy metal contamination in water bodies. Chromium (Cr), in particular, stands out as a potent carcinogen and ranks among the top twenty hazardous substances due to its extreme toxicity. Its most stable oxidation states, +3 and +6, particularly the latter, pose significant health risks, causing severe damage to vital organs such as kidneys, liver, and respiratory systems, thus compromising the overall immune system [1]. Regulatory bodies have set stringent limits for Cr concentrations, allowing a maximum of 2 mg/L in industrial effluents and 0.05 mg/L in drinking water [1,2]. Among the various methods available for heavy metal removal from water, adsorption proves to be highly effective. Silica-based adsorbents, readily accessible and abundant, have gained significant attention. Particularly, silica nanoparticles modified through grafting are emerging as a leading choice globally due to their unique structural properties [3]. These nanoparticles feature surfaces adorned with silanol groups, strategically utilized for surface modification, enhancing their dispersibility in solvents and polymeric matrices based on the chosen modifier. This study focuses on addressing aqueous Cr (VI) contamination remediation using a novel amine (NH₂) functionalized grafted silica adsorbent, denoted as PEI-Si. The material was synthesized employing a "grafting to" technique and subjected to thorough characterization [4]. The adsorption efficacy of the material was evaluated through batch adsorption studies, demonstrating its potential for chromium removal from aqueous solutions.

Method

Initially, a solution of polyethyleneimine (PEI) was prepared in ethanol. Subsequently, a modified liquid polymer was synthesized by combining this PEI solution with 3-glycidoxypropyltrimethoxysilane (GLYMO) under continuous stirring at room temperature for 12 hours. Following this, the PEI-GLYMO polymer was added dropwise to a homogeneous dispersion of oven-dried bare silica nanoparticles in ethanol, with the aid of sonication, and left to react for 24 hours. Finally, the resulting material was separated via centrifugation and then dried at 80ËšC overnight, yielding a powder that could be stored. Fig. 1(a,b) depicts a schematic representation of the synthesis process.

Results and Discussion

The infrared (IR) analysis, as depicted in Fig. 1(c-d), confirms the successful grafting process, evident from the presence of characteristic epoxy (1270 cm^-1) and amine (1585 cm^-1) peaks in the PEI-Si spectra [5]. Additionally, the Si-O-Si stretching peak at 1126 cm^-1 is prominent. BET analysis (Fig. 1e) indicates a reduction in specific surface area (SBET) in PEI-Si (SBET: 112 m²/g) compared to bare, unmodified silica nanoparticles (SBET: 311 m²/g). The amine modification in PEI-Si is further evidenced by a significant change in zeta potential, with the point of zero charge (pHZPC) shifting from 3.2 in bare silica to 9.6 in PEI-Si, attributed to the presence of NH₃+ in the solution (refer Fig. 1f) [6]. Thermogravimetric analysis (TGA), shown in Fig. 1g, was utilized to calculate the grafting density (0.77 chains/nm²) using a previously reported method [7]. The chemical composition of PEI-Si, both before and after adsorption, is analyzed using energy-dispersive X-ray spectroscopy (EDS) (Fig. 1(h-i)). The presence of Cr in the post-adsorption EDS (Fig. 1i) profile confirms Cr uptake on the adsorbent.

The optimization of adsorbent dose was initially conducted in the range of 0.1 g/L to 0.6 g/L (Fig. 1j). Cr (VI) rejection increases steeply from 22.3% to 44.7% as the dose increases from 0.1 g/L to 0.3 g/L. At higher doses (0.4-0.6 g/L), rejection increases marginally (7%). Hence, the optimum dose is selected as 0.3 g/L. The effect of solution pH on adsorption is elucidated over a wide pH range of 2 to 9 (Fig. 1k). Cr (VI) rejection decreases gradually with increasing pH (from 2 to 7) and then falls steeply above pH 7. This is attributed to the high positive zeta potential of PEI-Si, which gradually reduces with pH (in the range of 2 to 7) as observed in Fig 1f. Protonation of NH₂ groups on the adsorbent surface to NH₃⁺ form in highly acidic solutions facilitates strong electrostatic interactions with Cr existing primarily as HCrO₄ˉ and Cr₂O₇^2- in the solution between pH 2 and 6, while CrO₄^2- is predominant above pH 7 [6]. Thus, as the pH is further increased, the extent of protonated amine species reduces and the adsorbent approaches pH_ZPC with mild positive charge, which is not capable of binding the Cr moieties. As a result, Cr removal falls sharply in the pH range of 7 to 9. The effect of feed Cr (VI) concentration on uptake is shown in Fig. 1l. The equilibrium capacity (q_e) increases gradually with feed concentration and eventually approaches a steady value indicating material saturation. The corresponding maximum Langmuir capacities (q_L) for Si and PEI-Si are 17.3 mg/g and 133.3 mg/g at 25ËšC, which corroborates the enhanced capacity (by almost 8 times) of amine-modified PEI-Si. Furthermore, it is also noted that the Langmuir model provides a better fit (higher R²) compared to the Freundlich model, indicating monolayer adsorption [8]. The effect of contact time on Cr uptake, as shown in Fig. 1(m-n), reveals that the adsorption follows pseudo-second-order kinetics (R²: 0.999) compared to the pseudo-first-order model (R²: 0.875). This suggests that the principal mechanism of adsorption is chemisorption [6].

Conclusion

A novel synthesis approach successfully demonstrated the grafting of amine groups onto bare silica nanoparticles. This modification visibly impacted the material's porosity and specific surface area. The synthesized amine-functionalized PEI-Si exhibited remarkable adsorption of aqueous hexavalent chromium (with a q_L value of 133.3 mg/g), facilitated by strong chemical interactions between the NH₂ groups and Cr(VI). This interaction was bolstered by strong electrostatic attraction.

References

[1] A.U. Rajapaksha, R. Selvasembian, A. Ashiq, V. Gunarathne, A. Ekanayake, V.O. Perera, H. Wijesekera, S. Mia, M. Ahmad, M. Vithanage, Y.S. Ok, A systematic review on adsorptive removal of hexavalent chromium from aqueous solutions: Recent advances, Sci. Total Environ. 809 (2022) 152055.

[2] C.P.C.B. Govt. of India, The Environment (Protection) Rules, 1986.

[3] M. Tizzotti, A. Charlot, E. Fleury, M. Stenzel, J. Bernard, Modification of polysaccharides through controlled/living radical polymerization grafting-towards the generation of high performance hybrids, Macromol. Rapid Commun. 31 (2010) 1751–1772.

[4] M. Sokolowski, C. Bartsch, V.J. Spiering, S. Prévost, M.S. Appavou, R. Schweins, M. Gradzielski, Preparation of Polymer Brush Grafted Anionic or Cationic Silica Nanoparticles: Systematic Variation of the Polymer Shell, Macromolecules. 51 (2018) 6936–6948.

[5] W. Choi, K. Min, C. Kim, Y.S. Ko, J.W. Jeon, H. Seo, Y.K. Park, M. Choi, Epoxide-functionalization of polyethyleneimine for synthesis of stable carbon dioxide adsorbent in temperature swing adsorption, Nat. Commun. 7 (2016) 1–8.

[6] E.H. Jang, S.P. Pack, I. Kim, S. Chung, A systematic study of hexavalent chromium adsorption and removal from aqueous environments using chemically functionalized amorphous and mesoporous silica nanoparticles, Sci. Rep. 10 (2020) 1–20.

[7] T. Mondal, R. Ashkar, P. Butler, A.K. Bhowmick, R. Krishnamoorti, Graphene Nanocomposites with High Molecular Weight Poly(ε-caprolactone) Grafts: Controlled Synthesis and Accelerated Crystallization, ACS Macro Lett. 5 (2016) 278–282. https://doi.org/10.1021/acsmacrolett.5b00930.

[8] H. Wang, S. Wang, S. Wang, L. Fu, L. Zhang, Efficient metal-organic framework adsorbents for removal of harmful heavy metal Pb(II) from solution: Activation energy and interaction mechanism, J. Environ. Chem. Eng. 11 (2023) 109335.