Mg(OH)2-modified drinking water treatment residue for simultaneous ammoniacal nitrogen and phosphate removal from wastewater: process and mechanism

To fully exploit the potential of drinking water treatment residue (DWTR) for mitigating excessive nitrogen (N) and phosphorus (P) discharges responsible for water eutrophication, a straightforward precipitation process was used to produce Mg(OH)₂-modified drinking water treatment residue (Mg-DWTR). The resulting material can simultaneously remove N and P and exhibit high adsorption capacity. The effects of solution pH, adsorbent dosage, reaction time, initial concentration and coexisting substances on the simultaneous removal of ammoniacal nitrogen (NH₄⁺-N) and total phosphate (TP) by Mg-DWTR were assessed. Mg-DWTR exhibited high removal capacity across a broad pH range (3-9). The adsorption process applied both pseudo-second-order and Langmuir kinetic models, with predicted maximum adsorption capacities of 43.771-50.295 mg g^-1 (NH₄⁺-N) and 82.050-89.881 mg g^-1 (TP), the adsorption process is exothermic. After five reuse cycles, Mg-DWTR retained adsorption capacities of 8.541 mg g^-1 for NH₄⁺-N and 20.511 mg g^-1 for TP. Additionally, the adsorption capacity of Mg-DWTR was markedly suppressed in the presence of K⁺, SO₄^2-, citric acid, and humic acid. SEM-EDS, XRD, and FTIR analyses before and after adsorption revealed multiple mechanisms governing the adsorption process. Among these, the primary removal pathway for NH₄⁺-N and TP is due to the formation of struvite crystals. Additionally, ligand exchange, electrostatic attraction, and physical adsorption synergistically enhance the removal of nutrients. This work provides fresh insights into N and P removal in aquatic environments and the resource utilisation of DWTR, realising the concept of 'using waste to treat waste'.