Abstract
Phosphonate antiscalants are extensively used in reverse osmosis (RO) desalination processes to prevent scaling and improve the recovery of produced water. However, It is desirable to remove them before the disposal of RO concentrate, because the presence of phosphonate antiscalants may impede the removal of hardness from concentrates and affect the ecosystem. In this study, a highly efficient magnetic adsorbent (named magnetic La/Zn/Fe3O4@PAC composite) was prepared by co-precipitating lanthanum(La), zinc(Zn) with Fe3O4 and powdered activated carbon (PAC), and used for removing phosphonate antiscalant 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) from RO concentrates. The characterization results indicated that La, Zn and Fe3O4 were successfully doped into the PAC. The adsorbent possessed superparamagnetic behaviors, which can be effectively separated from RO concentrates within 1 min under an external magnetic field. Batch experiments were carried out to investigate the adsorption capacity and mechanisms of the adsorbent. The obtained experimental data fitted well with the pseudo-second-order kinetic equation and Freundlich isotherm model, indicating that the adsorption process was multilayer and controlled by chemical adsorption. The thermodynamic parameter was analyzed further, which showed the adsorption process was endothermic and spontaneous. The combined results of batch experiments and physiochemical analyses revealed that the adsorption mechanism of the adsorbent was based on ligand exchanges, electrostatic adsorption and hydrogen bonding interactions. Furthermore, Ca2+ played an important role in the adsorption process when pH > pHpzc. The adsorbent displayed high stability for the change of pH, and the maximum adsorption capacity of HEDP was 18.18 mg P/g at 298 K and pH = 6.0, which indicated the adsorbent could serve as a promising material for the removal of phosphonate antiscalant from RO concentrates.
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