Modification of sludge from tap water production by iron for arsenic ions adsorption followed by solidification using cement

Abstract

Sludge of tap water production from the Metropolitan Waterworks Authority (Bangkhen, Thailand) was modified by iron via borohydride reduction of ferric chloride. The modified sludge with 10% (w/w) iron was used as an adsorbent for arsenic removal from arsenic-contaminated water. The modified sludge was characterized by inductively coupled plasma-optical emission spectrometry (ICP-OES), x-ray fluorescence spectrometry (XRF), x-ray diffraction spectrometry (XRD), scanning electron microscopy with energy dispersive x-ray spectrometry (SEM-EDS), transmission electron microscopy (TEM) and surface area analysis. Then, factors affecting the adsorption of arsenite and arsenate were studied. The optimal pH was 3.0. The equilibrium adsorption of both arsenite and arsenate was reached within 1 hour. The maximum adsorption capacity was found to be 24.15 and 35.71 mg/g for arsenite and arsenate, respectively. High concentrations of phosphate, sulfate and humic acid caused a decrease in arsenite and arsenate removal efficiency. Moreover, the iron(0)-modified sludge was applied to remove arsenic from wastewater, surface water and ground water samples. All results showed that the iron(0)-modified sludge was effective for arsenite and arsenate removal from water. Furthermore, the sludge obtained after arsenic removal process was stabilized/solidified using a mix proportion of 60% (w/w) cement. The leaching characteristic of arsenic studied by dynamic monolithic leaching test (DMLT) showed that arsenic was hardly leached from the solidified waste. Its leaching mechanism was controlled by diffusion at the beginning and the depletion had occurred at the end of leaching process.