Adsorptive Mechanism And Competitive Adsorption Behavior Of Phosphate On HyFeAl-Mt Complexes

Phosphate geochemical behaviors in soil are closely related to many factors such as soil components, pH. In acid soils and aqueous environments (pH =4.0~5.5), soluble Fe and Al can interact with OH-, forming polymeric hydroxyaluminum (HyAl), polymeric hydroxyiron (HyFe), and polymeric hydroxyironaluminum (HyFeAl) with various OH/Al ratios. These cations can adsorb on external and/or internal surfaces of the montmorillonite (Mt), forming HyAl-Mt, HyFe-Mt, and HyFeAl-Mt complexes after aging. Mt complexes are ubiquitous in acidic to slightly acidic soils and waters, so they may seriously affect adsorption/desorption of nutriments and contaminants. At present, adsorption of phosphate on many minerals and soil have been widely reported. However, phosphate adsorption on HyFeAl-Mt has not been investigated in detail.In this study, one HyAl-Mt complex, two HyFe-Mt complexes with different iron contents, and three HyFeAl-Mt complexes with different Fe:Al molar ratios (0.1, 0.2, 0.5) were synthesized and characterized. These complexes were used as model materials to study phosphate adsorption/desorption in natural acidic soil condition. The main works of this paper include: (1) effects of initial pH and the dosage of Mt complexes on P adsorption; (2) adsorption kinetics, isotherm of P adsorption; (3) the effects of aging on phosphate adsorption; (4) competitive adsorption of low molecule weight acids (LMWA) with phosphate on Mt-complexes; and (5) evaluation on the stability of phosphate fixation on HyFeAl-Mt using desorption experiments. The main results of this research are as follows:(1) Montmorillonite-complexes were prepared and characterized by powder X-ray diffraction (XRD), BET, points of zero charge (pHzpc), cation exchange capacity (CEC) and specific surface area. The results show that HyAl, HyFe, HyFeAl ions have been intercalated in the interlayers of the Na-Mt. The crystallinity of Mt-complexes decreased after intercalation, and the amount of micropores increased compared to Na-Mt. The contents of Fe and Al in HyFeAl-Mt have great effects on the layer structure and surface chemical properties of the Mt-complexes.(2) Adsorption behavior of phosphate on Mt-complexes was investigated in acidic to slight acidified conditions. The results indicate that the amount of phosphate sorption increases with a decrease in pH in the range of 3.0~6.5. However, the effects of pH became weaker with increasing Fe:Al molar ratio of Mt-complexes. The effect of pH on phosphate adsorption on HyFe-Mt is slight. The sorption capacities of Mt-complexes for phosphate mainly depended on the amounts of external and internal sites available. The adsorption data for phosphate adsorption can be well fit by the Langmuir isotherm. The process of phosphate adsorption on HyFeA1-Mt0.2 and HyFeA1-Mt0.5 is characterized of the initial fast step and later slow one and both of them can be described by pseudo second-order equations. The fast process on HyFeAl-Mt0.2 can also be fit to the Elovich equation. The kinetic data of fast phosphate adsorption on HyFeAl-Mt0.5 can also be fit to the particle diffusion equation and the Elovich equation, and slow step can also be well fit to the pseudo first-order equation and Elovich equation.(3) Effects of aging on phosphate adsorption on various Mt-complexes in acid and slightly acidic condition were investigated. The main contents include the effects of aging time and acidity on phosphate adsorption/desorption. The findings show that: the more iron Mt complexes contained, the smaller pH changed when the complexes adsorbed phosphate; the longer aging time is, the more the phosphate was absorbed. At initial eight days, the rate of phosphate adsorption is rapid, then decreased gradually, even no change in the amount of adsorption with increasing time. The aging can more significantly affect P adsorption at low pH in comparison with at higher pH, especially on the high aluminum Mt-complexes. Because the phosphate can strongly adsorb on HyFe/Al-Mt0.2, the aging has no measurable effect on P desorption.(4) The effects of low molecular weight (LMW) organic acids, citric acid, oxalic acid and malic acid, as example, on phosphate adsorption on Mt-complexes were studied. The conclusions are that the effects of LMW organic acids on P adsorption were more effective in low pH than in high pH, and the effect is increasing with enhancing concentration of LMW organic acid. For different LMW organic acids, the effects are different. P Sorption in the presence of LMWA follows the Freundlich isotherms. Maximum adsorption of phosphate on the HyFe/Al-Mt0.2 is slightly affected by the presence of LMW organic acids. Different orders of LMW organic acid and P addition into the solution containing HyFe/Al-Mt0.2 have appreciable effects on P sorption.