Adsorption of myo-inositol hexakisphosphate in water using recycled water treatment residual

Dissolved organic phosphorus (DOP) in rainwater runoff or other contaminated waters can cause or aggravate eutrophication of water bodies. Water treatment residual (WTR) containing spent coagulant has been shown to provide excellent adsorption capacity for inorganic phosphorus such as orthophosphate, but little information has been available on adsorption of DOPs by WTR. In this study, the adsorption characteristics of myo-inositol-1,2,3,4,5,6-hexakisphosphate (IHP), a prototype DOP in soil and stormwater, by WTR were investigated through batch adsorption equilibrium and kinetic experiments. The influences of pH and various size fractions of WTR on the adsorption capacity were tested and analyzed, and the adsorption mechanism was elucidated based on Fourier-transform infrared spectroscopy (FTIR) analysis. The experimental results showed that WTR can effectively adsorb IHP from simulated rainwater, and the IHP uptake was favored under neutral and acidic conditions. Moreover, the 1.0–2.0-mm fraction of the WTR particles was most suitable for practical application because of the well-balanced adsorption rate and capacity. The classical Langmuir isotherm model well described the equilibrium adsorption data and the pseudo-second-order kinetic model adequately interpreted the rate data. Thermodynamic analysis revealed that the adsorption is a spontaneous, endothermic, and entropy-driven reaction. The FTIR analysis indicated that adsorption of IHP on WTR is associated with the formation of ≡Al–PO₃⁻ groups and the release of –OH from WTR. A comparison of the adsorption capacities of orthophosphate and IHP on WTR suggested that binding one IHP may take two times more sites than for orthophosphate, indicating that two of the six phosphate groups in IHP were bound to WTR. This work shows that recycled WTR may be used as a low-cost adsorbent for effective removal of organic phosphate in gray water and wastewater.