•Zn formed edge-sharing tetrahedral surface complexes on γ-alumina at pH 5.5.•Zn formed Zn-Al LDH-like (layered double hydroxide) precipitates on γ-alumina at pH 7.0.•Zn-Al LDH-like and adamite-like precipitates formed on γ-alumina when Zn and As coexisted at pH 7.0.Contaminants zinc (Zn) and arsenate (As) often coexist in soils. However, little is known concerning the impacts of coexisting As on Zn adsorption and precipitation on soil minerals. In the present study, adsorption and precipitation of Zn on γ-alumina in the absence and presence of arsenate was investigated employing batch experiments and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Results indicated that Zn formed edge-sharing tetrahedral surface complexes at pH 5.5 and Zn-Al LDH-like (layered double hydroxide) precipitates at pH 7.0 on the surface of γ-alumina. The presence of arsenate significantly enhanced Zn sorption densities, and remarkably changed its bonding environment. At pH 5.5, SR-XRD (Synchrotron Radiation-based X-ray Diffraction) and EXAFS showed that koettigite-like precipitate were formed in the cosorption of Zn and As on γ-alumina regardless of the addition sequence of As and Zn. At pH 7.0, when Zn was preequilibrated with γ-alumina prior to the As introduction, mixed Zn-Al LDH-like and amorphous adamite-like precipitates formed. However, when Zn and As were added simultaneously, only amorphous adamite-like precipitate was observed. Zn inner-sphere complexes and surface ternary complexes γ-alumina-As-Zn were the main outcome when As was preequilibrated firstly. Zn-arsenate precipitates could significantly decrease the concentration of Zn in aqueous solution and decrease the bioavailability and mobilization of Zn in soils.Download high-res image (226KB)Download full-size image

Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn in soil, as affected by SOM, were determined by employing Wien effect measurements. The presence of SOM markedly decreased the Zn binding energy in soils in the following order: Top (5.86 kJ mol–1) < Bottom (8.66 kJ mol–1) < Top OM-free (9.44 kJ mol–1) ≈ Bottom OM-free (9.50 kJ mol–1). The SOM also significantly decreased the adsorption energy of Zn on black soil particles by reducing nonspecific adsorption of Zn on their surfaces. The speciation of Zn in soils was elucidated by extended X-ray absorption fine structure spectroscopy and microfocus X-ray fluorescence. The results obtained by linear combination fitting of EXAFS spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite, and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total immobilized Zn increased, and the proportion of nonspecific adsorbed Zn decreased. The present results implied that SOM is an important controlling factor for the environmental behavior of Zn in soils.

For agricultural production and environment protection, it is cations loosely bound to the soil particles that have a great significance in short-term processes of adsorption–desorption, exchange, and transport. It is beneficial to be able to evaluate the fractions of these cations in order to correctly predict potential pollution of soils by heavy metals and availability of plant nutrients.The homionic suspensions of yellow-brown soil (YB) and black soil I (BI) saturated with Na+ and Ca2+ and three subsamples of black soil II (BII) saturated with Ca2+ and Cd2+ were prepared to determine the electrical conductivity (EC) of the suspensions. On the basis of electrical conductivity vs. field strength (EC-E) curve, the fraction of electrically associated cations on surfaces of soil particles was evaluated by extrapolation of strong-field Wien effect measurements in dilute suspensions.The maximum dissociation degree (αmax) of Na+ adsorbed on surfaces of yellow-brown soil and black soil I was about 0.21, which is approximately twice as much as those of Ca2+ (0.07–0.10) adsorbed on surfaces of two soils. The soil type was not the main factor in evaluating αmax, and the valence of the cations was. For divalent cations, αmax of Ca2+ and Cd2+ adsorbed on soil particles with different contents of organic matter descended in the order: top black soil II > bottom black soil II > OM-free bottom black soil II.The relatively small fractions of electrically adsorbed cations—about 0.2 for Na+ and 0.1 for Ca2+ on yellow-brown and black soils particles indicated that even for the more loosely adsorbed Na+ ions, most of the cations in the double layers of soil particles were adsorbed strongly by other, more specific mechanisms and cannot be stripped off into the solution, which would increase its electrical conductivity in a strong applied field.

The binding energy of ions on soils is one of the most important parameters to quantify the sorption strength of ions on soils. The aim of this study was to determine the mean free binding energies of four divalent cations Ca2+, Cd2+, Cu2+, and Pb2+ in suspensions of yellow-brown soil and black soils clay particles through ion activity and Wien effect methods, respectively, and to justify which method will be positive.The homoionic suspensions of yellow-brown soil (YB) and two black soils (BSI, BSII), which were saturated with divalent cations Ca2+, Cd2+, Cu2+, and Pb2+, were allowed to stand for 7 to 10 days and to achieve sufficient equilibration of ion reactions in the suspensions. The same sample was used to prepare two suspensions: one for the Wien effect measurement and another for the ion activity measurement, and the binding energies of divalent cations on yellow-brown soil and black soils that were determined by the two methods were calculated and compared.The mean Gibbs free binding energies values (ΔGbi) of Ca2+, Cu2+, and Pb2+ on YB and BSI determined by ion activity method are obviously greater than ones by Wien effect method. The ΔGbi values (I) of Cd2+, Cu2+, and Pb2+ to subsamples of black soilII determined by ion activity method are also larger than ones (II) by Wien effect method, the values (I) of the three cations for top black soilII were in the order Pb (14 kJ mol−1) > Cu (9 kJ mol−1) > Cd (7 kJ mol−1), and the values for bottom black soilII and OM-free black soilII were in the order Pb (14–16 kJ mol−1) > Cu (11–13 kJ mol−1) ≈ Cd (11–13 kJ mol−1). The ratios (I/II) of ΔGbi are in the range of 1.3–2.4 except the suspensions of Cd-top black soilII in which I/II value is equal to about 1.The mean Gibbs free binding energies between divalent cations and soil particles determined by ion activity method were larger than those determined by Wien effect method. The Wien effect method supported by the results acquired from atom absorption spectrum is the better method to determine the binding energies of divalent cations to soil particles because the activities of divalent cations in suspensions determined by ion activity method were underestimated.

In this research, the effects of glyphosate (GPS) on Zn sorption/precipitation on γ-alumina were investigated using a batch technique, Zn K-edge EXAFS, and 31P NMR spectroscopy. The EXAFS analysis revealed that, in the absence of glyphosate, Zn adsorbed on the aluminum oxide surface mainly as bidentate mononuclear surface complexes at pH 5.5, whereas Zn–Al layered double hydroxide (LDH) precipitates formed at pH 8.0. In the presence of glyphosate, the EXAFS spectra of Zn sorption samples at pH 5.5 and 8.0 were very similar, both of which demonstrated that Zn did not directly bind to the mineral surface but bonded with the carboxyl group of glyphosate. Formation of γ-alumina-GPS-Zn ternary surface complexes was further suggested by 31P solid state NMR data which indicated the glyphosate binds to γ-alumina via a phosphonate group, bridging the mineral surface and Zn. Additionally, we showed the sequence of additional glyphosate and Zn can influence the sorption mechanism. At pH 8, Zn–Al LDH precipitates formed if Zn was added first, and no precipitates formed if glyphosate was added first or simultaneously with Zn. In contrast, at pH 5.5, only γ-alumina-GPS-Zn ternary surface complexes formed regardless of whether glyphosate or Zn was added first or both were added simultaneously.

Gibbs free binding energy and adsorption energy between cations and charged soil particles were used to evaluate the interactions between ions and soil particles. The distribution of Gibbs free adsorption energies could not be determined experimentally before the development of Wien effect measurements in dilute soil suspensions. In the current study, energy relationships between heavy metal ions and particles of Hapli-Udic Argosol (Alfisol) and Ferri-Udic Argosol were inferred from Wien effect measurements in dilute suspensions of homoionic soil particles (< 2μm) of the two soils, which were saturated with ions of five heavy metals, in deionized water. The mean Gibbs free binding energies of the heavy metal ions with Hapli-Udic Argosol and Ferri-Udic Argosol particles diminished in the order of Pb2+ > Cd2+ > Cu2+ > Zn2+ > Cr3+, where the range of binding energies for Hapli-Udic Argosol (7.25-9.32 kJ mol−1) was similar to that for Ferri-Udic Argosol (7.43-9.35 kJ mol−1). The electrical field-dependent mean Gibbs free adsorption energies of these heavy metal ions for Hapli-Udic Argosol and for Ferri-Udic Argosol descended in the order: Cu 2+≥Cd 2+≥Pb2+ > Zn2+ > Cr3+, and Cd2+ > Cu2+ > Pb2+ > Zn2+ > Cr3+, respectively. The mean Gibbs free adsorption energies of Cu2+, Zn2+, Cd2+, Pb2+, and Cr3+ at a field strength of 200 kV cm−1, for example, were in the range of 0.8-3.2 kJ mol−1 for the two soils.