Facile method to granulate drinking water treatment residues as a potential media for phosphate removal

The reuse of drinking water treatment residues (DWTRs) to remove phosphate has been proposed as an effective strategy for P pollution control. The application of DWTRs as a filter media requires the control of their shape; thus, DWTR powder is unsuitable for use in filter systems due to clogging. In this work, a facile method is proposed to granulate DWTRs by incorporating DWTR powder in a poly (vinyl alcohol) matrix through the repeated freeze-thaw method. The physicochemical characteristics, phosphate adsorption performance, and structural stability of the DWTR granules were investigated and further compared with those of the DWTR powder. The results showed that both the granules and the powder had similar functional groups and crystal structures, but granulation reduced the leaching and bioaccessibility of most metals within the DWTRs. Batch adsorption tests showed that the granules presented a slower adsorption process at a low P concentration (10 mg-P/L) due to a reduction in the accessible surface area. The P adsorption capacity of the granules was 23.34 mg/g, slightly lower than that of the powder (26.55 mg/g). Higher phosphate removal rates were observed at acidic (3-6) pH compared to neutral and basic conditions. The granules exhibited excellent selectivity for phosphate in the presence of competing ions (e.g. Cl-, SO42-, and NO3-) even at high ionic strength (0.1 mol/L); nevertheless, the removal of phosphate was inhibited by HCO3- with a decrease of similar to 25.29% and 36.42% at ionic strengths of 0.01 and 0.1 mol/L, respectively. Further mechanistic studies indicated that the micro- and mesoporous structures were beneficial to phosphate adsorption, and the inner-sphere complexation involving the possible formation of Fe-P and Al-P complexes was mainly responsible for the phosphate removal. Also, granular DWTRs presented a high phosphate removal efficiency and a good mechanical stability in column studies, adapting a filter system.