Linking Treatment to Performance: A Review of How Modification Governs Metal and Dye Adsorption on Rice Husk

Abstract

Water pollution by heavy metals and synthetic dyes is one of the more stubborn environmental challenges affecting both industrialised and emerging nations. Conventional treatment approaches – precipitation, ion exchange, membrane filtration and the like – work well enough, but are often too costly or operationally demanding for small and medium firms. Thus agricultural by-products have continued to be the subject of interest as inexpensive, abundant and mostly biodegradable alternatives. Rice husk is the protective outer layer detached from the grain during milling and is generated in huge amounts every year and is usually thrown or burnt. This review summarises and discusses the literature on the use of rice husk, in its raw and modified forms, for the removal of metal ions and dyes from aqueous solutions. We summarise the physicochemical features that make the material attractive, i.e. its lignocellulosic backbone, its abundant silica and its surface functional groups, and review the main preparation strategies, among them alkali and acid treatment, thermal conversion to biochar, activation into carbons, extraction of silica and assembly of composites. Reported adsorption capacities are compiled for a range of metals, notably Pb, Cd, Cu and Cr, and for both cationic and anionic dyes. The influence of solution pH, adsorbent dose, contact time, initial concentration and temperature is considered, together with the isotherm, kinetic and thermodynamic models most often applied. Regeneration, the practical limitations of the material, and some directions for further study are addressed in closing.