Adsorbents from rice husk and shrimp shell for effective removal of heavy metals and reactive dyes in water

Widespread contamination by heavy metals (HMs) and dyes poses a major health risk to people and ecosystems requiring effective treatment. In this work, rice husk (RH) and shrimp shells were extracted to obtain amorphous silica and chitosan, respectively, which were utilized to produce nano-chitosan-coated silica (NCCS). To ensure the stability of the nanoparticles, silica was freeze-dried after being coated with nano-chitosan. Functional groups (–NH₂, –OH, P]O) from chitosan nanoparticles (CNPs) were introduced to the surface of silica during this process. Dyes such as brilliant green (BG), methylene blue (MB) and reactive brown (RB) as well as HMs (Cr⁶⁺, Pb²⁺, Cd²⁺, Ni²⁺) were removed by adsorbents. CNPs showed the highest adsorption capacity for RB (59.52 mg/g) among dyes and Cr⁶⁺ (42.55 mg/g) among HMs. CNPs showed the highest adsorption capacity for HMs among different adsorbents. Although NCCS and CNPs showed similar adsorption capabilities for HMs and dyes, NCCS showed the best stability. The adsorption performance decreased as RB > Cr⁶⁺ > MB > BG > Pb²⁺ > Cd²⁺ > Ni²⁺. The adsorption reactions followed both pseudo-first-order and second-order kinetics, and was spontaneous from thermodynamic analysis. In summary, the waste-derived adsorbents demonstrated excellent potential for removing HMs and dyes from water, while supporting effective management solid waste.