The Importance of Reaction Mechanisms in Interpreting the Arsenic Reactive Transport of FeS-coated Sand Column

ABSTRACT FeS, as a natural reduced iron mineral, has been recognized to be a viable reactive material for As(III) sequestration innatural and engineered systems. In this study, FeS-coated sand packed columns were tested to evaluate the As(III) removalcapacities under anaerobic conditions at pH 5, 7 and 9. The column obtained As(III) removal capacity was then comparedwith the capacity result obtained from batch reactors. In the comparison, two different approaches were used. The firstapproach was used the total As(III) removal capacity which method was proved to be useful for interpreting pH 5 system.The second approach was used to consider sorption non-linearity and proved to be useful for interpreting the pH 9. Theresults demonstrated that a mechanistic understanding of the different removal processes at different pH conditions isimportant to interpret the column experimental results. At pH 5, where the precipitation of arsenic sulfide plays the majorrole in the removal of arsenic, the column shows a greater removal efficiency than the batch system due to the continuousdissolution of sulfide and precipitation of arsenic sulfide. At pH 9, where adsorption mainly governs the arsenic removal,the sorption nonlinearity should be considered in the estimation of the column capacity. This study highlighted theimportance of understanding reaction mechanism to predict column performance using batch-obtained experimental results.Key words : FeS-coated sand, reactive transport, precipitation/adsorption mechanism, column and batch comparison