Construction of a novel lanthanum carbonate-grafted ZSM-5 zeolite for effective highly selective phosphate removal from wastewater

Eutrophication caused by excessive phosphate has become a global environmental issue that urgently needs to be solved due to its destructive effect on the ecosystem and adverse effect on human health. In this study, a novel lanthanum carbonate-grafted ZSM-5 zeolite (LC-ZSM-5) was developed to effectively achieve highly selective phosphate removal from wastewater. The LC-ZSM-5 adsorbent shows high adsorption capacity (highest at over 45 mg-PO43-/g) in the pH range of 3–6 with a fast adsorption rate (reaching 80% of the maximum adsorption capacity within 150 min and equilibrium at 350 min). High stability can be observed with less than 0.3 mg/L La leakage during the adsorption process. Four kinetic models are used to describe the adsorption process. The pseudo second order model and the Elovich model are found to fit the data best with a high correlation coefficient (r2 = 0.99), showing that the adsorption process is influenced simultaneously by the phosphate concentration and interaction between adsorbent and adsorbate. The change of curve slope from 4.0 to 0.89 and intercept from 0.76 to 27.6 in intra-particle model indicates that the adsorption process is controlled by a combination of intra-particle diffusion and film surface diffusion. The maximum phosphate adsorption capacity calculated from the Langmuir model is 47.7 mg-PO43-/g, and the adsorption isotherm data are fitted well by the Freundlich model (r2 = 0.95) and the Koble-Corrigan model (r2 = 0.95), suggesting that the phosphate adsorption process is dominated by nonregular multilayer adsorption with the coexistence of monolayer adsorption. The ionic strength and co-existing anions have little impact on the adsorption capacity of LC-ZSM-5, proving the adsorption selectivity of phosphate onto LC-ZSM-5. Characterization results show that spindle-like lanthanum carbonate particles are fabricated on ZSM-5. Combining the adsorption process and the characterization, it can be derived that phosphate is adsorbed by LC-ZSM-5 mainly through electric attraction and ligand exchange. Our study demonstrates the great potential application value of LC-ZSM-5 in commonly seen phosphate-containing wastewater.