Chengbao Li

Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien Effect Measurements and EXAFS Spectroscopy

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.

Wien Effect in Suspensions and Its Application in Soil Science: A Review

Abstract The exchange and adsorption of ions in soils depend on their interactions with the charged mineral and organic soil particles. In the last several decades, these interactions were quantified in terms of adsorption energies, but the few reported attempts to evaluate adsorption energies between ions and soil particles were based on indirect deduction, rather than on direct measurement, because of lack of reliable and practical experimental methods. This chapter presents and reviews a recently suggested Wien Effect method; it offers an opportunity for more reliable and direct characterization and quantification of interactions between soil particles and ions, in terms of several quantifiers that range in nature from phenomenological to mechanistic to thermodynamic. The term “Wien Effect” refers to the increase of electrical conductivity (EC) with increasing applied electrical field ( E ). After introducing the subject of ion adsorption on charged soil particles, we describe our first experimental findings and discuss the mechanisms of the Wien Effect in simple electrolyte solutions and in suspensions. We then review a few methods of interpreting Wien Effect measurements in suspensions, along with demonstrating their application to quantifying the particle–ion interactions for several systems of soil, clay, and oxide particles interacting with various cations and anions. The examples of EC(E) measurements of various soil particle–ion systems and their interpretations by several quantifiers presented in this chapter demonstrate the merits of the Wien Effect method for quantifying particle–ion interactions and adsorption energies. Obviously, the Wien Effect method can also be used to characterize the adsorption of ions on other charged, nonsoil colloidal particles.

Wien effect of Cd/Zn on soil clay fraction and their interaction

BackgroundThe coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. While many studies have been done on the mutual toxicity of Cd2+ and Zn2+, few studies can be found in the literature focused on the interaction of Cd2+ and Zn2+ on soil clay fractions especially in terms of energy relationship.ResultsThe binding energies of Cd2+ on boggy soil (Histosols) particles and Zn2+ on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd2+ and Zn2+ on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd2+ and Zn2+ have a strong capacity to adsorb in the solid phase at the soil–water interface of boggy soil and yellow brown soil, respectively. However, both Cd2+ and Zn2+ adsorbed on paddy soil particles easily release into the solution of the soil suspension. Unlike the binding energy, the higher adsorption energies of ions in boggy and yellow brown soils showed a weak binding force of ions in boggy soil and yellow brown soil. A 1:1 ratio of Cd2+ to Zn2+ promotes the mutual inhibition of their retentions. Cd2+ and Zn2+ have high mobility and bioavailability in paddy soil and yellow drab soil (Ustalfs), whereas they have high potential mobility and bioavailability in boggy soil and yellow brown soil.ConclusionIn the combined system, Zn2+ had preferential adsorption than Cd2+ on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd2+ and Zn2+ while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd2+ and Zn2+ restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+–Zn2+.

Evaluating the fraction of electrically associated cations on surfaces of soil particles by extrapolation of strong-field Wien effect measurements in dilute suspensions

PurposeFor 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.Materials and methodsThe 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.Results and discussionThe 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.ConclusionsThe 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.

Wien Effect Measurements in Soil Colloidal Suspensions: A Novel Method for Characterizing the Interactions between Charged Particles and Counter Ions

Adsorption is one of the most important chemical processes at the interface between soil particles and water. It determines the quantity of plant nutrients and pollutants which are retained on the surfaces of soil particles, and therefore, is one of the primary processes that affect transport of nutrients and contaminants in soils. The Wien effect, i.e., the dependence of the electrical conductivity of soil suspensions on electrical field strength, was proposed as the basis of a new method to characterize energy relationships between cations and soil particles. The results showed that the mean Gibbs free binding energies of the heavy metal ions with yellow-brown, black and brown soil particles decreased in the order of Pb2+>Zn2+>Cu2+>Cd2+, Pb2+>Cu2+>Zn2+>Cd2+ and Pb2+ >Cd2+>Cu2+>Zn2+, respectively, where the range of binding energies for yellow-brown soil (7.16∼8.54 kJ⋅mol−1) was less than that for black soil (9.05∼9.88 kJ⋅mol−1). The electrical field-dependent mean Gibbs free adsorption energies of these heavy metal ions for yellow-brown, black and brown soils descended in the order of Cu2+>Cd2+>Pb2+>Zn2+, Cu2+>Zn2+>Pb2+>Cd2+, and Cu2+>Pb2+>Cd2+>Zn2+, respectively. The mean Gibbs free adsorption energies of Cu2+, Zn2+, Cd2+ and Pb2+ at a field strength of 150 kV⋅cm−, for example, were in the range of 1.23 to 2.15 kJ⋅mol−1 for the three soils.