Influence of cation building blocks of metal hydroxide precipitates on their adsorption and desorption capacity for phosphate in wastewater - A screening study
Thirteen metal hydroxide adsorbers were synthesized via precipitation by systematically varying different two-, three- and four-valent metal precursors, namely Mg2+, Ca2+, Zn2+, Fe3+, Al3+ and Zr4+. The resulting materials were classified in four groups based on their structure, morphology and BET surface area. Two of the groups could be classified as layered double hydroxides (LDH). The rest of the materials could be either only partially related to a LDH-like structure or formed non-layered precipitates. The phosphate removal performance of each adsorber (dose 200 mg/L) was tested in spiked distilled water and municipal wastewater (10 mg P/L) at pH 7-8. The phosphate adsorption capacity of the materials after 1 h varied between 32 mg P/g (for a rod-like-sample, lacking a layered structure) and 51 mg P/g (for a sample appearing as a mixture of particulate matter covered with a web-like structure). Longer contact time (24 h) did not increase significantly the adsorption efficiency. None of the materials was ideally selective for phosphate, especially the ones with a clear LDH structure and highest surface area (110 m2/g) which adsorbed up to 25 mg/g Cl− and other competing anions. The adsorbed phosphate could be desorbed (>90% in most of the cases) by treatment with an alkaline solution (0.5 M NaOH or 1 M NaOH + 1 M NaCl) which turned out to be the best option for regeneration among many other tested combinations.