Advances and Challenges in Adsorbent Materials for Rainwater Treatment

Rainwater harvesting is increasingly recognized as a sustainable supplement to conventional water supplies and a strategy to improve water resilience. However, harvested rainwater may contain organic and inorganic contaminants introduced by atmospheric deposition and contact with collection surfaces, requiring adequate treatment before safe use. This review critically evaluates the application of adsorbent materials for harvested rainwater treatment, namely carbon-based materials, surface-modified mineral matrices, and biosorbents. The available literature was analyzed considering adsorbent characteristics, treatment conditions, removal performance, and the mechanisms potentially governing contaminant removal. The reviewed studies demonstrate that activated carbon is the most investigated adsorbent class, while surface-modified minerals and biosorbents remain less explored. Direct comparison between studies is limited by differences in experimental conditions and incomplete characterization of adsorbent properties. Adsorption performance depends on both adsorbent characteristics and rainwater chemistry, particularly pH, which affects surface charge and the aqueous speciation of many contaminants. Reported removal efficiencies do not always reflect adsorption alone, as treatment configuration and operating conditions may also promote precipitation, physical retention, and in some cases, biologically mediated degradation. Future research should prioritize standardized material characterization, mechanistic evaluation, regeneration strategies, and long-term validation to support the development of more predictable and reproducible rainwater treatment technologies.