Sorption Of P-nitrophenol And Cu(Ⅱ) On Two Variable Charge Soils Of Southern China

Large areas of variable charge soils are distributed in the tropical and subtropical regionsof Southern China, which are the important soil resources for sustainable agriculture in theseareas. The mineral compositions and electrochemical properties of these soils are differentfrom those of constant charge soil. These soils are subjected to intensive weathering andeluviation in the formation process. They are rich in iron (Fe) and aluminum (Al) oxides, andthose soils usually carry both positive and negative charges on their surfaces. Thus, they canabsorb both anions and cations, and the specific adsorption of certain ions.Two variable charge soils were collected, respectively, from Xuwen and GuangzhouCounty of Guangdong Province, China. In this study, two soils were characterized byScanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy, X-rayfluorescence (XRF), X-ray diffraction (XRD) spectrum, BET and Zeta potential. Then, thisstudy was to investigate the effect of pH, ionic strength, heavy metals, and organic acids onthe adsorption of PNP onto two variable charge soils, and when PNP and Cu(II) coexist,mutual influence on soil adsorption. Finally, the sorption kinetics, equilibrium andthermodynamics of the sorption of PNP on two soils were studied, and kinetic models andequilibrium adsorption isotherms were established. The main conclusions of this paper weresummarized as follows：(1) The XRD patterns of the two soils demonstrated that they were mainly composed ofkaolin, illite, hydromica and quartz. The surface area was found to be60.05m2/g forGuangzhou soil and12.29m2/g for Xuwen soil, respectively. The XRF patterns of the twosoils demonstrated that they were mainly composed of SiO2,Al2O3, Fe2O3.(2) The amount of PNP adsorbed by the two soils was increased with increasing solutionpH, and then decreased dramatically with increasing solution pH. The uptake of PNPadsorbed by Xuwen and Guangzhou soils was the highest when the values of pH were5.4and5.0, respectively. The equilibrium adsorption amount increased with increasing ionic strength.(3) The uptake of PNP on two soils was inhibited in the presence of Cd(II), Pb(II) andZn(II), and the order of the inhibiting effect of the three heavy metal ions were as follows:Pb(II)> Cd(II)> Zn(II). (4) The uptake of PNP on two soils was inhibited in the presence of Cu(II), and theadsorbed amount of PNP decreased with increasing Cu(II) concentration, while the uptake ofCu(II) on two soils was enhanced in the presence of Cu(II)(4) In the kinetic study, the adsorption of PNP on two soils can reach equilibrium within2h, and the equilibrium sorption capacity of Xuwen and Guangzhou soil141.3mg/kg and160.0mg/kg, respectively. The pseudo-second-order kinetic model and Weber-Morrisintra-particle diffusion model were fit to experimental data well in the initial stage ofadsorption, and Weber-Morris intra-particle diffusion model was the sole rate-controlling step.The experimental data obeyed well the pseudo-second-order kinetic model throughout thesorption process.(5) The equilibrium adsorption capacity increased with the increase of the temperature,and Langmuir and Freundlich models were fit to experimental well. The values of RLof PNPadsorbed by the two soils were between0and1.0, and the values of n of PNP adsorbed by thetwo soils were more than1.0, which revealed the sorption of PNP on the two soils werefavorable.(6) In thermodynamic studies, the values of ΔG0were negative for the two soils,indicating that the sorption of PNP onto the soils was spontaneous and thermodynamicallyfavorable. The values of ΔH0were estimated as4.41kJ/mol for Xuwen soil, and4.81kJ/molfor Guangzhou soil. The positive values of ΔH0were indicative of the fact that the sorptionreaction was endothermic, and high temperature favor adsorption of PNP. The values of ΔS0were estimated as26.19kJ/mol for Xuwen soil, and27.69kJ/mol for Guangzhou soil, whichindicated that ΔS0of PNP in the solution increased than ΔS0of PNP in the soil reduce. Thetotal effectΔS0was positive, and the degree of disorder increased in adsorption system.