OPTIMIZATION OF A COLORIMETRIC METHOD TO DETERMINE ARSENIC SPECIATION IN COLUMN EXPERIMENTS TO STUDY THE EFFECTS OF IRON, MANGANESE, AND PHOSPHATE CONCENTRATION IN GROUNDWATER IN ARSENIC REMOVAL PROCESS

Arsenic is a very toxic semi-metal with the highest toxicity in its inorganic forms that, when ingested, can cause severe illnesses. Scientists are increasingly concerned with the importance of studying the behavior of arsenic compounds in terms of their chemical species in the presence of other redox sensitive elements including iron, manganese and phosphate. The project aimed to optimize an existing reliable colorimetric method to characterize inorganic arsenic speciation in environmental samples and the optimized method was used to monitor the arsenic speciation in solid and liquid aliquots in the column experiments.

The optimization includes the scanning of the molybdo-arsenate and molybdo-phosphate blue color complexes from wavelength 1100 nm to 450 nm by using HACH DR 5000 spectrophotometer. Several scans are obtained for 0, 2, 5, 10, 20, 30, 50, 100 μmolel^-1 of phosphate solution spiked with 0, 50, 100, 500, 1000 μgl^-1 of arsenic As (III) and As (V) solution. The results suggest that the position of the most stable and max. peaks are phosphate dependent. The max. peak was obtained at ~900 nm for complexes with low to moderate concentration of phosphate (<20 μM) and for high phosphate (>20 μM) the peak shifted to left at relatively lower wave length at ~880 nm. Therefore, the experiments suggest two different wave lengths for arsenic determination depending on the phosphate concentration in aliquots.

A series of columns experiments were conducted in laboratory ambient condition. The columns were packed with different media such as beach sand (Fisher, S25-10 sea washed), iron coated beach sands, zero valent iron (HePure) and their combinations to assess the sorption capacity of arsenic. Groundwaters from upper glacial aquifers (UGA) in southeast Queens of New York City with the limited phosphate was spiked with 2-3 mg l^-1 of arsenite (As(III)) and arsenate (As (V)) to conduct the all experiments. The project intend to find the optimal condition for the best arsenic removal by investigating the variation in sorption behavior due to presence of other redox sensitive elements such as iron, manganese and phosphate in the aliquots. The arsenic speciation and other elemental measurements will be done by optimized colorimetric methods (HACH DR 5000) and voltammetric methods (MetroOhms 797).