Arsenic oxyanion binding to NOM from dung and aquaculture pond sediments in Bangladesh: Importance of site-specific binding constants

Groundwater contamination by arsenic (As) is a serious public health concern in many different areas worldwide, particularly in the Bengal region. Mobilization and fate of As in natural waters is controlled by a variety of factors including the presence of natural organic matter (NOM). This study experimentally determined conditional distribution coefficients (with KD defined as the ratio between As bound to NOM and truly dissolved As) and apparent stability constants between AsIII oxyanions and NOM from cow dung, chicken dung, and Bangladeshi aquaculture pond sediment prior to and after one year of operation. As-sorption experiments with cow dung as the source of NOM resulted in the highest range for log KD, from 4.7 to 6.3. Pond sediment from Bangladesh after a year of operation for fish production showed greater affinity for binding As oxyanions than fresh sediment prior to fish production. PHREEQCI modeling using constants derived from the experiments along with water chemistry parameters typical for the site supports the dominance of AsIII oxyanion-NOM complexation in this system. Models employing constants from previous studies using purified NOM considerably underestimate observed complexation by environmental NOM; thus, applying site-specific constants to geochemical models will better predict As speciation for the field site.