土壤中可变电荷表面磷的解吸特性

DESORPTION CHARACTERISTICS OF PHOSPHATE ON VARIABLE-CHARGE SURFACE IN SOILS

本文研究了土壤中可变电荷表面磷的解吸特性。结果表明,磷解吸量与吸附量成正相关,并与直线方程和指数方程拟合。其平均解吸百分数依次为:高岭石(66%)>无定形硅酸铝(60%)>砖红壤粘粒(55%)》无定形氧化铁(29%)>三水铝石(23%)》火山灰粘粒(9%)。解吸体系的pH是借对磷酸根质子的解离和表面电荷的影响而制约解吸量。F^-和OH^-离子对磷酸盐化的三水铝石连续解吸的结果表明,磷酸根和氟离子在三水铝石表面的吸附量或剩余量之和均较为接近,这揭示了H_2PO_4^-和F^-离子之间既竞争又相互补充表面空位。它可作为一种区分磷吸附形态和沉淀的定量方法来进一步研究。

The desorption of phosphate determines the transformation and translocation of phosphorus in soils,and affects the P nutrition of soils and the environmental effect of phosphorus. Accordingly,it is of great importance to study the desorption characteristics of phosphate. In the present study laterite (L),volcanic ash soil (V),kaolinite (KI) and synthetic amorphous aluminum silicate (A_(Al-Si),amorphous iron oxide (A_(Fe)) and gibbsite (G) were used to investigate the desorption characteristics of phosphate on the variable-charge surface in soils.The results showed that the phosphate desorbed by 0.01 molL^1 NaF(pH 7.08) was determined by phosphate adsorbed preyiously,and the relationship between them could be well described by using lillear and exponential equations.The average desorption percentages were shown as follows: Kl(66%)>A_(Al-Si)(60%)>L(55%)》A_(Fe)(29%)>G(23%)》V(9%).The variation degtee of desorption percentage was related to the constituents of samples.The phosphate desorption was determined by the pH of desorption solution which could affect the proton dissociation of phosphate and the stirface charges of colloids.The phosphate desorption increased coilsistelitly with the initial pH(pHi) of desorbent except that for the kaolinite and gibbsite it decreased at first and then increased.The pH of equilibrium desotption solution.(pHe) firstly showed an increase and then followed by a decrease.When the △pH,the difference of pHe and pHi,was equal to zero,the pH of desorption solution was affected by the stirface characteristics of minerals.The critical pH for the samples was V(7.7)>A_(Al-Si)>(6.9)>L(6.8)>A_(Fe)(6.7)>Kl(6.1)>G(4.2).Using F^- and OH^- to desorb continuously the phosphate adsorbed by gibbsite,we foundtbat the desorption percentage of phosphate by F^- and OH^- was 62%—92%.The phosphate desorbed by F^-,the phosphate desorbed by OH^- and the residHal phosphate adsorption was significantly positively correlated to phosphate adsotption. The phosphate desorbed by OH^- was more than that by F^-,and their average desorption percentages were 58%±8% and 23%±3%respectively, At the same time when F^- desorbed the phosphate adgorbed,F^- was adsoybed to the gibbsite stirface, and the F^- adsorbed was negatively correlated with the phosphate adsorbed.The sum of adsorption or residual of phosphate and F^- on the gibbsite surface varied unconspicuously,which indicated that phosphate and F^- could compete compensate each other’s surface adsorption sites.The undesorbed phosphate and F^- might exist in a form of aluminum phosphate precipirate or cryolite on the gibbsite surface.Therefore,it seems important to distinguish the adsorption.from the precipitation of the phosphate retained on the vatiable-charge surface in soils by using F^- and OH^- to desorb the adsotbed phosphate contintiously.