Study on the effectiveness of activated alumina in removing arsenic from groundwater

Abstract

Elevated level of arsenic in ground water is a major concern for many communities in the world, because long term exposure to arsenic via drinking and cooking water leads to a wide range of health problem. The public health sector of Bangladesh is currently under serious threat from widespread arsenic contamination of shallow groundwater aquifers, as water abstracted from these aquifers is the prime source of drinking and cooking water. An estimated 35 million people in Bangladesh are at risk of arsenic toxicity. Providing safe water to the affected people is a viable solution of the arsenic related health problems and treatment of arsenic contaminated water is one of the options to do this. Among the methods available for removing arsenic from water, adsorption on activated alumina is a promising one for implementing on a small-scale rural community or household levels. Various factors influence the removal efficiency of activated alumina. In this thesis work the effects of a number of parameters including oxidation state of arsenic, pH, iron, chloride, sulfate; phosphate, activated alumina size, empty bed contact time and the regeneration of spent alumina in removing arsenic by activated alumina were evaluated. Laboratory column tests were conducted using synthetic water with different compositions and operating conditions. A method was also developed for easy and effective regeneration of spent alumina. In this method, spent alumina was taken out of adsorption column and acid wash was applied after washing with normal tap water. Then the usual base regeneration and neutralization of the alumina were carried out. The efficiency/effectiveness of activated alumina in removing arsenic were determined in terms of Empty Bed Volume (BV), quantity of adsorbed arsenic and average arsenic removal efficiency up-to 50 ppb arsenic level in the effluent. The results of the study show that pentavalent arsenic is removed efficiently than trivalent arsenic. The bed volume of treated water decreases from 2115 to 375 with the increase of arsenic concentration from lOa ppb to 500 ppb for As(V), whereas, the bed volume decreases from 372 to lOa as the As(III) concentration increases from 100 ppb to 500 ppb. The study reveals that the presence of iron has a significant negative effect on As(V) removal efficiency and some positive effect on As(III) removal efficiency. The bed volume of treated water for As(V) changes from 5287 to 160 as the iron concentration increases from 0.0 mgIL to 5.0 mglL, whereas, the variation for As(III) is from 25 to 316 as the iron concentration increases from 0.15 mglL to 5.0 mg/L. As(V) removal efficiency is better within the pH range of 5.0 to 6.0, whereas, As(III) removal increases at higher pH values. Chloride has no effect on the arsenic removal efficiency but the presence of sulfate and phosphate has significant effect. The results show that the increase of phosphate from 0.68 mg/L to 345 mglL decreases the bed volume oftreated water from 2100 to 70. Arsenic removal efficiency is found better for activated alumina size 28x48 as compared with the size l4x28. The arsenic removal efficiency increases significantly with the increase of the empty bed contact time. The study also shows that the modified method of regeneration of spent alumina is very effective in removing arsenic from groundwater.