Identification of Arsenic Complex Species in Groundwater of the Chianan Plain, SW Taiwan, Using Ultrafiltration Method

Prior studies on the groundwater geochemistry of Chianan Plain, SW Taiwan, showed that the arsenic concentration was positively correlated with the content of organic matters in groundwater. Furthermore, it was proposed that arsenic complexing with humic substances in groundwater is the major arsenic species. However, little evidence has presented to confirm the existence of the complex in groundwater. On the other hand, ultrafiltration is a technique capable of separating molecules based on molecular sizes. Therefore, ultrafiltration is suitable for confirming whether the arsenic-humic substances complex exists in groundwater. In order to confirm if the arsenic-humic substances complex is predominant in the groundwater of Chianan Plain and to identify the dominant arsenic species, the ultrafiltration experiments are conducted on arsenic aqueous solutions and six groundwater samples. In addition, the relationship between arsenic and organic matters are discussed and the roles of organic matters to arsenic mobilization in the groundwater are explored.
The ultrafiltration experiments recruit two kinds of membrane materials for comparison, including cellulose acetate (CA) and regenerated cellulose (RC). The molecular weight cut-off (MWCO) of the membranes is 1000 Daltons. The mass balance experiments were conducted to identify whether arsenic or natural organic matters are adsorbed onto the membranes. Arsenic and dissolved organic carbon (DOC) concentrations in permeates were analyzed and compared with the concentrations in groundwater. Ultrafiltration were also applied to As(III) and As(V) aqueous solutions and the arsenic-spiked Shefeng-2 (SHF-2) samples to figure out the effects of oxidation states of arsenic and the impacts of the matrix. The permeates of SHF-2 sample was further analyzed the As(III) and As(V) oxyanions by LC-HG-AFS to evaluate the contents of inorganic arsenic species.
The results of mass balance experiments showed that neither arsenic nor organic matters were adsorbed on both of the membranes. The results of groundwater ultrafiltration demonstrated that the DOC concentrations in permeates while using different membranes were comparable to each other; however, the arsenic concentrations in permeates differ significantly between the two membranes. Ultrafiltration with CA membranes rejects more arsenic than ultrafiltration with RC membranes. The difference in arsenic concentrations in permeates suggests that arsenic-humic substances complex is not predominant in the groundwater of Chianan Plain. Ultrafiltration of As(III) and As(V) aqueous solutions respectively showed that, in the ultrafiltration conditions used in this study, there was no rejection to As(III) but strong rejection to As(V) by both CA and RC membranes. Moreover, CA membranes rejected more As(V) than RC membranes. The difference in rejection between As(III) and As(V) results from the influence of Donnan exclusion. The ultrafiltration of the spike sample showed that there is no matrix effect on As(III) but on As(V). The less rejection of As(V) in groundwater samples results from neutralization of membrane charges by matrix. The arsenic speciation of the SHF-2 sample confirmed that As(V) is the predominant arsenic species in the groundwater.
As revealed by the ultrafiltration of groundwater and arsenic aqueous solutions, it is thus suggested that arsenic-humic substances complex is not predominant in the groundwater of the Chianan Plain. The presence of high amount of As(V) may derived from oxidation of As(III) by natural organic matters or by sulfide ions to form thioarsenates. The roles of organic matters to arsenic mobilization and enrichment in groundwater are acting as substrates for microbes to reductively dissolve Fe(III) oxyhydroxide and make arsenic released, desorbing arsenic directly from sediments, and impeding arsenic from re-adsorbing onto the surfaces of sediments.