Rice Husk, Sugarcane Bagasse, and Coconut Shell Waste Valorization for Lead and Cadmium Removal: A Systematic Review of Adsorption Performance and Implications for Sustainable Water Treatment

Converting agricultural residues into adsorbents provides a potential strategy for waste valorization and water-pollution control. This systematic review evaluated lead (Pb) and cadmium (Cd) removal using rice husk-, sugarcane bagasse-, and coconut shell-derived adsorbents and examined the implications for sustainable water treatment. Searches of Web of Science Core Collection, Scopus, and Google Scholar identified 1672 records published between 2006 and 2026, of which 86 studies met the final inclusion criteria. These studies yielded 602 extracted records and 324 dependence-reduced study–adsorbent–metal analytical units. Adsorption performance was summarized descriptively, coupled with reporting assessment and exploratory non-parametric comparisons. Median equilibrium adsorption capacity (qe), model-derived maximum adsorption capacity (qmax), and removal efficiency were 25.89 mg/g, 38.22 mg/g, and 85.22%, respectively. Median adsorption capacities were higher for Pb than for Cd, with exploratory differences in qe (BH-adjusted p = 0.008) and qmax (p < 0.001), but not removal efficiency (p = 0.220). Precursor groups differed in qe (p < 0.001), but not qmax (p = 0.168) or removal efficiency (p = 0.951). Most studies used synthetic waters, with limited reporting of analytical quality control and uncertainty. Numeric regeneration cycle entries were available in only 3.4% of analytical units. Experimental heterogeneity and residual within-study dependence further constrained direct comparisons. Although agricultural residue-derived adsorbents show potential for Pb and Cd removal, establishing their sustainability advantages requires evaluation of real-water performance, production inputs, regeneration, safe management of exhausted adsorbents, and life cycle and techno-economic impacts.