Arsenic Removal from Drinking Water Using Silica Based Catalytic Media

Arsenic is a common environmental contaminant found naturally in surface and groundwater resources used for potable supply. Typical values in the Waikato River in New Zealand are around 20 parts per billion (ppb) whereas the New Zealand drinking water standard is 10 ppb. It has become a major concern and challenge in water supply due to its toxicity and carcinogenic properties. Sources of arsenic contamination in surface and groundwater originate from both natural and anthropogenic sources. In oxygen-rich environments where aerobic conditions persist and under natural pH conditions, As (V) is predominant, whereas As (III) dominates in a moderately reducing environment with anoxic conditions such as are found in ground waters. A silica based catalytic media was assessed for arsenic removal. Bench top column and batch experiments were carried out to study the performance of the catalytic media under a range of operating conditions. The flow rate through the test columns was varied from 5 ml/min to 15 ml/min equating to a filtration rate of 0.57 to 1.7 m/hr. The pH of the water was varied from 6 to 8.5. In the batch experiments, As (V) removal was highest for low concentrations with 84 – 90 % removal at pH 8.5 while in the bench top column test, greater than 95% As (V) removal was achieved at pH 6 and 5 ml/min flowrate. Adsorption kinetics and isotherms were determined at different pH (5, 6, 7 and 8.5) and the effect of contact time was evaluated. Adsorption isotherms of As (V) were determined using the Langmuir, Freundlich, Langmuir-Freundlich and Dubinin-Radushkevich models. The results are encouraging and further studies are planned to examine the potential to separate the arsenic from water treatment plant sludge.