Evaluation of natural quartz and zeolitic tuffs for As(V) removal from aqueous solutions: a mechanistic approach

Naturally occurring pyroclastic materials, quartz-rich tuffs (Qz) and stellerite tuffs, were evaluated for their capacities to remove As(V) from aqueous solutions. The mechanism of As(V) uptake was evaluated using sequential extraction of As(V)-loaded adsorbent which entailed that the plausible removal mechanism is sorption to short-range ordered hydrous oxides of iron and aluminum. In addition, buffering effects of adsorbents could form favorable charges upon them through hydrolysis of amphoteric oxides, enhancing the performance of sorption. The influence of anions co-existing with As(V) in water such as carbonate, bicarbonate, nitrate, chloride, phosphate, and sulfate was studied in a batch sorption process. The impact of most anions on As(V) removal was found to be negligible except phosphate. The sorption behavior well fitted to Langmuir and Freundlich models. Estimated maximum sorption capacities of 0.42 and 0.23 mg/L were observed using quartz-rich tuffs and stellerite tuffs, respectively. As(V) concentration of 0.8 mg/L was easily decreased to below the drinking water standard of 0.01 mg/L using Qz adsorbent, whereas 0.1 mg/L As(V) decreased to below this limit upon the use of stellerite tuffs under similar conditions. The buffering capacity of quartz-rich tuffs and stellerite tuffs induced a pH increase to 5.76 and 5.40, respectively, from initial pH of 3.50, which will incur an important asset in real applications.