Mechanism of arsenate mobilization from goethite by aliphatic carboxylic acid

Arsenic is highly toxic and therefore represents a potential threat to the environment and human health. The mobility and fixation of arsenic in natural environment is significantly affected by co-occurring dissolved natural organic acids, which are widely present in soils and sediments due to the decomposition of natural organic matters and the metabolism of plant roots and microorganisms. It was reported recently that at lower pH, citrate decreased arsenate adsorption on metal oxides as the result of competitive adsorption. This study examined the relationship between citrate-promoted goethite dissolution and citrate-suppressed arsenate adsorption in the equilibrated goethite-arsenate-citrate systems with different contact orders. The results indicated that there is obvious correlation between the suppression of arsenate adsorption and dissolution of goethite. Arsenate adsorption and goethite dissolution in the presence of citrate (1.0 mM), oxalate (1.5 mM) and acetate (3.0 mM) were compared. The results showed that arsenate adsorption was most significantly inhibited by citrate among the three organic acids. Although oxalate dissolved goethite more significantly, it inhibited arsenate adsorption much less than citrate. Hence it is proposed that the competition for binding sites rather than citrate-promoted goethite dissolution played a key role in the reduction of arsenate adsorption by citrate at lower pH.