Determination of main influence mechanism of fulvic acid on arsenic removal by ferric chloride

In this study, synthetic wastewater containing 110 µg/L arsenate (As(V)), 0-20 mg/L fulvic acid (FA), and 0-12.3 mg/L phosphate was treated with 3 mg/L Fe³⁺. The mechanisms of FA and phosphate effects on As(V) removal by ferric chloride were determined using 0.22-10 µm pore-size filtration, Zetasizer analysis, and in situ flow through cell ATR-FTIR. The results showed that up to 20 mg/L FA had almost no effect on the solubility of ferric hydroxide precipitates and adsorption of As(V) by the precipitates. When FA concentration increased from 0 to 20 mg/L, the adsorption of FA led to higher negative zeta potential of the precipitates and the strong electrostatic repulsion between the precipitates decreased the particle size of ferric hydroxide flocs from larger than 10 µm to smaller than 1 µm. In the presence of 5-20 mg/L FA, 46%-63% As(V) was adsorbed onto the flocs with particle size in the range of 0.45-1 µm. On the other hand, phosphate did not affect the size of ferric hydroxide flocs and significantly increased the dissolved As(V) concentration because it competed with As(V) for adsorption sites on ferric hydroxide precipitates. The addition of 5 mg/L cationic organic flocculant significantly reduced the effect of FA on As(V) removal, but did not reduce the effect of phosphate on As(V) removal. The findings of this study will help develop effective arsenic treatment techniques and predict the mobility of arsenic in the environment.