Synthesis and characterization of amine functionalized cellulose-silica composites for heavy metal adsorption from contaminated water

Abstract

The pressing challenge of heavy metal pollution in water sources demands innovative and sustainable solutions. This project explored recent advancements in heavy metal remediation techniques, focusing on the utilization of cellulose–silica composites and tailored surface modification techniques. The synthesis strategies and properties of cellulose–silica adsorbents highlight their enhanced adsorption capacities and structural robustness for removing heavy metal pollutants from aqueous environments. The study investigated various surface modification approaches, including thiol functionalization, amino acid grafting, and silane coupling agents, for optimizing the surface chemistry and morphology of cellulose–silica composites. Mechanistic insights into the adsorption processes and kinetics of modified adsorbents were studied, along with considerations for optimizing adsorption performance under different environmental conditions. The adsorption method for hexavalent chromium (Cr (VI) removal from domestic and industrial wastewater is widely desirable due to public health concerns about the heavy metal. The study aimed to investigate the adsorption of Cr (VI) using a novel adsorbent: an amine-functionalized cellulose-silica composite derived from banana pseudo-stem. The in-situ sol-gel method was used to create cellulose-silica silane functionalized composites and analyzed them through different characterization techniques such as attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), Scanning Electron Microscopy (SEM), and transmission electron microscopy (TEM) techniques. ATR-FTIR depicted key organic constituents in raw banana pseudo-stem fibers (BF) and the formation of Si–O bonds in Bleached Cellulose-Silica (BC–SiO2) composite and further enhanced by the grafting of N-[3- (trimethoxysilyl)propyl]ethylenediamine (DAPTMS) onto the BC-SiO2 surface in BC-SiO2- DAPTMS. Functionalization with varying DAPTMS concentrations (2, 4, and 10%) was employed to enhance the composites' adsorption capacity, binding affinity, and thermal stability. Comprehensive characterization using ATR-FTIR, XRD, TGA, BET, SEM, and TEM revealed structural and thermal modifications, with higher DAPTMS concentrations improving adsorption performance. The modifications of BC with SiO2 followed by DAPTMS result in the BC-SiO2-DAPTMS composite, which has reduced crystallinity as shown by XRD and enhanced thermal stability as demonstrated by TGA, while BET analysis showed altered surface area and pore characteristics in BC-SiO2-DAPTMS (2%). The SEM and TEM imaging provided visual evidence of structural modifications and improved dispersion in BC-SiO2-DAPTMS composites. The effects of initial Cr (VI) concentration, adsorbent weight dosage, contact time, and pH on the removal efficiency of Cr (VI) using amine-functionalized cellulose–silica composites were also investigated. The results highlighted significant differences in adsorption performance based on the composite formulation and operating conditions. The initial Cr (VI) concentration effect revealed that BCSiO₂-DAPTMS (4%) consistently achieved the highest removal efficiencies, peaking at 97.14% at 0.3 mg/L. BC-SiO₂-DAPTMS (10%) followed closely, with efficiency stabilizing around 95.53% at higher concentrations. BC-SiO₂-DAPTMS (2%) exhibited lower but improving performance with increasing concentrations. Adsorbent weight dosage experiments demonstrated that increasing weight enhanced removal efficiency, with BC-SiO₂-DAPTMS (10%) achieving optimal performance (95.46%) at 1 g, though benefits plateaued beyond this weight. The impact of contact time showed BC-SiO₂-DAPTMS (10%) achieving equilibrium after 50 minutes, with a maximum removal efficiency of 91.29%. BC-SiO₂-DAPTMS (4%) exhibited a similar trend, but with a slightly lower maximum efficiency of 84.30%. The pH study indicated that acidic conditions (pH 1–4) were most favourable for Cr (VI) removal, with BC-SiO₂- DAPTMS (10%) reaching the highest removal efficiency (89.27% at pH 3) and maintaining superior performance across all pH levels. Overall, BC-SiO₂-DAPTMS (10%) demonstrated the best performance across all conditions, followed by BC-SiO₂-DAPTMS (4%), underscoring the importance of higher DAPTMS functionalization for enhanced Cr (VI) adsorption. These findings offer valuable insights into optimizing composite design and operational parameters for effective Cr(VI) remediation in contaminated water systems. The kinetic modelling followed the pseudosecond order (PSO) model, while the Freundlich and Langmuir isotherms provided insights into the adsorption mechanisms. The overall results demonstrated that the BC-SiO₂-DAPTMS composites, particularly at 4% and 10% DAPTMS concentrations, are effective, scalable, and sustainable adsorbents for Cr (VI) remediation, offering significant potential for practical water treatment applications. The study offered valuable insights into the development of effective adsorbent materials for sustainable heavy metal remediation applications.