Enhanced adsorption of molybdenum(VI) onto drinking water treatment residues modified by thermal treatment and acid activation

All known drinking water treatments have produced a large amount of sludge until now, and a significant environmental issue has been left over, which would be how to effectively utilize the “waste” economically and environmentally. This paper attempts to modify drinking water treatment residuals by an approach of consecutive thermal and acid activation and use them to enhance molybdenum(VI) adsorption capability in water solution. The surface structure characteristics of drinking water treatment residuals before and after activation were investigated. Moreover, the effects of different factors on molybdenum(VI) adsorption and the adsorption mechanism were also analyzed. The optimal condition of the activation was ascertained as thermal activation at 600 °C for 4 h followed by activation of hydrochloric acid at 4.0 mol/L with a solid-to-liquid ratio of 1:1. X-ray diffraction and Fourier-transform infrared spectroscopy indicated that there was a resemblance between the structure of drinking water treatment residuals before and after the activation. Nevertheless, the average pore size was reduced because of the loss of activated carbon during the heating process, and the surface areas and average diameter were increased after activation. The pseudo-second-order model could better fit the kinetic data, and the maximum capacity of molybdenum(VI) adsorption was improved from 18.44 mg/g before modification to 39.52 mg/g at 25 °C at pH 4.0, as indicated by the Langmuir model. Thermodynamic parameters indicated that the process of molybdenum(VI) adsorption was endothermal, entropy increasing, and spontaneous. The adsorption efficiency of molybdenum(VI) by the modified drinking water treatment residuals was dependent on pH values and low pH was beneficial for molybdenum(VI) adsorption. The effect of pH, as well as Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy measurements indicated that electrostatic interaction and ion exchange were the main mechanisms of molybdenum(VI) adsorption. Regeneration experiments revealed that molybdenum(VI) was removed efficiently and the modified drinking water treatment residuals could be recyclable for a long term. These results suggested that the modified drinking water treatment residuals was an green and recyclable adsorbent and could be used for the repair of molybdenum(VI) sewage.