Removal of arsenite, arsenate and dimethylarsenic acid from wastewater by sludge from tap water production

Abstract

Sludge of tap water production from the Metropolitan Waterworks Authority (Bangkhen, Thailand) was utilized as alternative low-cost adsorbent for the removal of arsenite (As(III)), arsenate (As(V)) and dimethylarsenic acid (DMA) from contaminated water. The elemental analysis of the sludge was done using X-ray fluorescence (XRF) technique and inductively coupled plasma optical emission spectrometry (ICP-OES). The result of ICP-OES indicated that the sludge have high contents of aluminium and iron at 113.8 and 43.7 mg/g, respectively. The XRD result illustrated that the structure of aluminium and iron compounds in sludge might be amorphous, while the pH of point zero charge of the sludge was 6.7. In the equilibrium contact time was 12 hours. For the kinetics study, the adsorption behaviours of As(III), As(V) and DMA showed a good compliance with the pseudo-second order kinetics model. The Langmuir and Freundlich adsorption isotherms were also studied, it was found that the adsorption of As(V) fitted to the Langmuir isotherm, while those of As(III) and DMA preferably obeyed both isotherms. The maximum adsorption amounts calculated from experiments of As(V), As(III) and DMA in the batch system were 8.76, 1.89 and 1.78 mg/g, respectively. After that, the effects of phosphate and sulphate anions were evaluated. It was found that phosphate significantly affected the adsorptive ability for arsenic, but sulphate did not interfere in adsorption. In case of the column study, the percent removal of As(III) and DMA decreased with increasing the flow rate of solution, whereas the flow rates of 0.5-4.0 mL/min did not affect the adsorption efficiency of As(V). Furthermore, the sludge was applied to remove total arsenic in contaminated water containing arsenic of around 74-77 mg/L and used for the removal of arsenic from a surface water sample collected from the Chulalongkorn University pond and spiked with As(V). All results showed that the sludge has a good potential to be used as adsorbent for removal As(V) from water.