Effect of kaolinite on removal of Pb in simulated wastewater by oxalic acid–activated phosphate rock

Purpose Oxalic acid–activated phosphate rock (APR) can efficiently immobilize Pb in soil and water. However, the application of APR could cause acidification in environment. This study aims to investigate the joint effects of APR and kaolinite on the Pb(II) removal in simulated wastewater. Materials and methods A series of experiments combined with X-ray diffraction (XRD) analysis was conducted to investigate the joint effects of APR and kaolinite on Pb removal, P concentration, and acidity. Results and discussion The results indicated that the data of isothermal adsorption of Pb by kaolinite was well-fitted with the Langmuir adsorption isotherm equation, while the Pb removal rates using APR and the mixture of kaolinite and APR (kaolinite + APR) increased with the increasing initial Pb concentration ranging from 0 to 400 mg/L. The Pb removal rates with kaolinite decreased from 51.85 to 6.91% and increased from 97.15 to 99.35% with APR. With kaolinite + APR, the rates changed between 81.74 and 98.18%. Conclusion The X-ray diffraction patterns and the various desorption results with two different desorption agents further indicated that the removal of Pb(II) using kaolinite is an adsorption process, while Pb phosphate precipitates are formed during the removal of Pb(II) both with APR and kaolinite + APR. There are two advantages with the application of kaolinite + APR in Pb removal: (i) Compared to the use of kaolinite only, Pb is bound tightly in kaolinite + APR treatment with a slight decrease of solution pH; (ii) phosphorus effectiveness is still great in an equilibrium solution. This study serves as a scientific evidence for the comprehensive utilization of kaolinite + APR in the removal of Pb from water and soil.