Variable Charge Soils Research Articles

Nitrate and Ammonium Leaching in Variable- and Permanent-Charge Paddy Soils

A variable-charge (VC) and a permanent-charge paddy soil (PC) were selected to study nitrate-N (NO − 3-N) and ammonium-N (NH + 4-N) leaching with N isotopes for one consecutive year. An irrigation and intermittent drainage pattern was adopted to mimic natural occurrence of rainfall during the upland crop season and drainage management during the flooded rice season. Treatments to each soil type were no-N controls (CK), 15N-labeled (NH 4) 2SO 4 (NS), and milk vetch (NV) applied at a rate equivalent to 238 kg N ha −1 to unplanted lysimeters, totaling six treatments in triplicates. Results indicated that the soil type dominated N leaching characteristics. In the case of PC, NO − 3-N accounted for 78% of the total leached inorganic N; NS was prone to leach three times more than the NV, being 8.2% and 2.4% of added 15N respectively; and > 85% of leached NO − 3-N came from native N in the soil. In the case of VC, NH + 4-N made up to 92% of the total inorganic N in leachate. Moreover, NH + 4-N particularly high during the flooded season. NO − 3-N leaching in VC occurred later at a lower rate compared to that in PC. The findings of this study indicate that NO − 3-N leaching during the drained season in permanent-charge paddy soils and NH + 4-N leaching in variable-charge soils deserve more attention for regional environmental control.

Surface charge characteristics of variable charge soils in Thailand

Surface charge characteristics were investigated using a charge fingerprinting procedure for 90 samples from 32 profiles of highly weathered Oxisols and Ultisols derived from shale/limestone, basalt, granite, local alluvium, sedimentary, and metasedimentary rocks under tropical savanna and tropical monsoonal climates in Thailand. The charge fingerprints of 5 whole soils, after removal of organic matter and of kaolin and ion oxides from these soils, were also measured to clarify factors contributing to the variable charge behaviour. Phosphate sorption was determined and related to charge properties. Kaolin was the dominant mineral of the clay fraction with moderate amounts of goethite and hematite. Gibbsite was present in Oxisols formed on limestone and basalt under more humid conditions. All soils exhibited variable charge behaviour but the magnitude and rate of change in variable charge with pH varied greatly between Oxisols and Ultisols. Oxisols had higher amounts of variable charge than Ultisols, reflecting the differences in amounts of clay and extractable Fe and Al. Oxisols formed on basalt and limestone under more humid conditions had high values of anion exchange capacity (AEC) due to the contributions of goethite, gibbsite, and boehmite. The relationships of base cation exchange capacity (CECB) and AEC with pH were linear. Rates of change with pH of both negative and positive variable charge (Ac, Aa) were closely related to CEC, specific surface area (SSA), pH(NaF), and various forms of extractable Fe, Al, and Mn. The charge fingerprints of soil after removal of organic matter indicated that organic matter exerted a strong influence on both the magnitude of negative charge and rate of change with pH. Kaolin had permanent and variable charge, with SSA and crystal size (MCD001) being highly predictive of the rate of change in variable negative charge with pH. Iron oxide concentrates exhibited strongly pH-dependent charge and the mean coherently diffracting length (MCD110) of hematite was highly predictive of both the magnitude and rate of change in variable positive charge with pH. Charge coefficients (Ac, Ba) had highly significant (P = 0.005) positive relationships with Langmuir and Freundlich phosphate sorption maximum coefficients (Xm, k), indicating that the surfaces of amorphous, poorly ordered, and crystalline Fe and Al oxides are prime sites for both P sorption and variable charge.

Kinetic and thermodynamic study of chlorophenol sorption in an allophanic soil

The sorption of 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol by a allophanic soil was studied in a series of batch experiments. Chlorophenol sorption behavior was evaluated as a function of reaction time (0–96 h) and input concentration at a fixed ionic strength (0.1 mol L −1 KCl) at 25, 35, and 45 °C. Sorption results for the various reaction temperatures were used in calculating thermodynamic parameters. Chlorophenol sorption increased with temperature, suggesting an endothermic process. The Elovich equation was used to describe the kinetic data. Data from the isotherm experiments were described by the Triple-Layer Model in which monodentate outer- and inner-sphere complexes were formed between deprotonated organic molecules and active sites on the variable-charge soil. The calculated thermodynamic parameters suggest that chlorophenol sorption is a spontaneous (Δ G < 0), endothermic (Δ H > 0) and entropy-driven reaction (Δ S > 0).

Sorption and lability of cadmium and lead in different soils from Egypt and Greece

Reactions of heavy metals with soils are important in determining their availability and fate in the environment. Mono-metal sorption and lability of sorbed cadmium (Cd) and lead (Pb) in different representative soils from Egypt and Greece as influenced by their properties were investigated in this study. For this purpose eleven surface soil samples varying widely in their origin and properties were selected. Four of them were from Egypt representing the main soil orders i.e., Entisols and Aridisols and the rest, seven, from different sites of Greece belonging to the orders Entisols, Alfisols, Vertisols, Mollisols, and Histosols. In these samples sorption isotherms were developed from which sorption parameters, and distribution coefficient ( K d) of Cd and Pb were determined. In addition lability of these metals was estimated by DTPA extraction following their sorption. The results showed that Freundlich model described satisfactorily sorption of both metals. In all the soils studied K d values of Pb were higher than that of Cd indicating that this was retained by the soils stronger than Cd. Sorption parameters ( k f , n) and K d values of Egyptian Entisol developed on lacustrine deposits showed higher affinity for Pb, Greek Histosol for Cd while acidic Alfisols showed the lowest affinity for both metals studied. Permanent charge clayey soils with relatively low Fe, Al and Mn oxides content sorbed more Cd and Pb than the variable charge red soils with higher content of these oxides. In variable charge red soils with similar sesquioxides content, Pb and Cd sorption was pH dependent. However, in variable charge soils with similar soil pH, no significant differences were recorded for Pb sorption, while Cd sorption capacities differed significantly depending on the active ratios of Fe and Al oxides. Sorption parameters of Cd were correlated to clay content, cation exchange capacity, organic matter content, total free and amorphous aluminum oxides, amorphous iron oxides and CaCO 3 content while Pb sorption parameters were correlated with clay content, total free and amorphous silica oxides, and amorphous aluminum oxides content as well as cation exchange capacity (CEC). Lability of the adsorbed Cd was higher than Pb in all the studied soils and may pose more threats to the ground water and plants. The Greek acidic Alfisols (Rhodoxeralf) showed the lowest lability of sorbed Cd, while the alkaline one exhibited the highest Cd lability. The lowest lability of sorbed Pb was in Greek Histosols and the highest in acidic Alfisol (Typic Haploxeralf). Labile Cd was negatively correlated only with CaCO 3 content in all the soils studied while labile Pb was negatively correlated with CEC, clay, organic matter, total free aluminum oxides and the amorphous iron and aluminum oxides content.

Wien Effect Characterization of Interactions Between Ions and Charged Sites on Clay Surfaces of Variable-Charge Soils

To investigate the interactions of oppositely charged sites on the surfaces of variable-charge soil particles with cations and anions, and to evaluate the mean Gibbs free binding and adsorption energies of various cations on particles of red soil and latosol, clay fractions smaller than 2 μm were separated from samples of the two variable-charge soils. Ferric oxides were removed from part of the clay fractions, which were then saturated with various chlorides (NaCl, KCl, CaCl 2, CdCl 2 or LaCl 3). Electrical conductivities (EC) of dilute suspensions of the original and of the iron oxides-free clay fractions in deionized water were measured with the SHP-2 short high-voltage pulse apparatus, which enables measurement of the Wien effect at field strengths ( E) from 14 to 250 kV cm −1. The Wien effect (EC- E) curves revealed EC increases of red soil suspensions between 14 and 200 kV cm −1, of 8.3, 8.4, 12.1, 5.9, and 1.2 μScm −1 for NaCl, KCl, CaCl 2, CdCl 2, and LaCl 3, respectively, reflecting the differing interactions with the various cations and chloride. The EC increments with the iron-free red soil suspensions were higher, being 29.7, 17.2, and 15.3 μScm −1 for NaCl, CaCl 2, and CdCl 2, respectively. In the natural latosol suspensions the EC increments were practically zero, whereas in the iron-free fractions there were significant EC increments of 10.3, 5.7, 5.0, and 1.6 μScm −1 for NaCl, CaCl 2, CdCl 2, and LaCl 3, respectively.

Adsorption of Glyphosate on Variable‐Charge, Volcanic Ash–Derived Soils

Glyphosate (N-phosphonometylglycine) is widely used due to its broad spectrum of activity and nonselective mode of action. In Chile it is the most used herbicide, but its adsorption behavior in the abundant and widespread variable charge soils is not well understood. In this study, three volcanic ash-derived soils were selected, including Andisols (Nueva Braunau and Diguillin) and Ultisols (Collipulli), to evaluate the adsorption kinetics, equilibrium isotherms, and the effect of pH in glyphosate adsorption. The influence of glyphosate on soil phosphorus retention was also studied. Glyphosate was rapidly and strongly adsorbed on the selected soils, and adsorption isotherms were well described by the Freundlich relationship with strong nonlinearity (n(fads) < 0.5). The n(fads) values were consistently higher than n(fdes) values, suggesting strong hysteresis. Adsorption (K(ads)) increased strongly when pH decreased. The presence of glyphosate (3200 mug mL(-1)) changed the adsorption behavior of phosphate at its maximum adsorption capacity. Andisol soils without the addition of glyphosate had similar mean K(ads) values for Nueva Braunau (5.68) and Diguillin (7.38). Collipulli had a mean K(ads) value of 31.58. During the successive desorption steps, glyphosate at the highest level increased K(ads) values for phosphate in the Andisol soils but had little effect in the Ultisol soil. This different behavior was probably due to the irreversible occupation of some adsorption sites by glyphosate in the Ultisol soil attributed to the dominant Kaolinite mineral. Results from this study suggest that in the two types of volcanic soils, different mechanisms are involved in glyphosate and phosphate adsorption and that long-term use of glyphosate may impose different effects on the retention and availability of phosphorus. Volcanic ash-derived soils have a particular environmental behavior in relation to the retention of organic contaminants, representing an environmental substrate that may become highly polluted over time due to intensive agronomic uses.

Effect of ionic strength on adsorption of As(III) and As(V) on variable charge soils

The study was to investigate the adsorption behavior of arsenite (As(III)) and arsenate (As(V)) on two variable charge soils, i.e., Haplic Acrisol and Rhodic Ferralsol at different ionic strengths and pH with batch methods. Results indicated that the amount of As(III) adsorbed by these two soils increased with increasing solution pH, whereas it decreased with increasing ionic strength under the acidic condition. This suggested that As(III) was mainly adsorbed on soil positive charge sites through electrostatic attraction under the acidic condition. Moreover, intersects of As(V) adsorption-pH curves at different ionic strengths (a characteristic pH) are obtained for both soils. It was noted that above this pH, the adsorption of As(V) was increased with increasing ionic strength, whereas below it the reverse trend was true. Precisely the intersect pH was 3.6 for Haplic Acrisol and 4.5 for Rhodic Ferralsol, which was near the values of PZSE (soil point of zero salt effect) of these soils. The effects of ionic strength and pH on arsenate adsorption by these soils were interpreted by the adsorption model. The results of zeta potential suggested that the potential in adsorption plane becomes less negative with increasing ionic strength above soil PZSE and decreases with increasing ionic strength below soil PZSE. These results further supported the hypothesis of the adsorption model that the potential in the adsorption plane changes with ionic strength with an opposite trend to surface charge of the soils. Therefore, the change of the potential in the adsorption plane was mainly responsible for the change of arsenate adsorption induced by ionic strength on variable charge soils.

Interaction of Electrical Double Layers Between Oppositely Charge Particles in Variable-Charge Soils as Related to Source to Salt Adsorption

The interaction of electrical double layers between oppositely charged particles of variable-charge soils collected from southern China was investigated by means of column leaching experiments. The results showed that cations and anions of indifferent electrolytes such as NaCl and NaNO3 were simultaneously adsorbed by some of the soils during leaching experiments. The number of the ions released was shown to be less than the number of the ions adsorbed. This probably led to the decrease in the electrical conductivity (EC) of the leachate at the initial stage of the leaching experiments. When the soil with adsorbed salt was washed with distilled water, both cations and anions adsorbed were immediately removed from the soil particle surfaces. This phenomenon is attributed to the overlapping of the diffuse layers of the electrical double layers between positively charged iron (Fe)/aluminum (Al) oxides and negatively charged phyllosilicates of the variable-charge soils. The extent of interaction between oppositely charged particles was shown to be directly related to the EC and the content of free Fe/Al oxides in the soils. The lower soil EC and higher content of soil free Fe/Al oxides appeared to cause a stronger interaction between oppositely charged particles.

Phosphate adsorption at variable charge soil/water interfaces as influenced by ionic strength

The effect of phosphate adsorption on zeta potential of the colloids of variable charge soils and the effect of ionic strength on phosphate adsorption by the soils were investigated using batch experimental method. The presence of phosphate resulted in the decrease in zeta potential and isoelectric point (IEP) of the colloids of the soils, which further suggested that the phosphate was adsorbed specifically by these soils. The effect of phosphate adsorption on zeta potential was correlated with the content of free Fe/Al oxides in the soils; the higher the content of Fe/Al oxides in a soil the greater was the decrease in zeta potential and IEP of the soil colloids. The intersection of phosphate adsorption–pH curves at different ionic strengths (a characteristic pH) was obtained for 2 Oxisols. Above this pH, the adsorption of phosphate increased with increasing ionic strength, whereas below it the reverse trend occurred. The intersect pH was 4.60 for the Oxisol from Guangdong and 4.55 for the Oxisol from Yunnan, which was lower than the values of PZSE (point of zero salt effect) of these soils, but near the PZNC (point of zero net charge) of the soils. The effects of ionic strength and pH on phosphate adsorption by these soils were interpreted with the help of an adsorption model developed previously by Bowden et al. The results of zeta potential suggested that the potential in an adsorption plane became less negative with increasing ionic strength above the soil PZNC and decreased with increasing ionic strength below the soil PZNC. These results support the hypothesis of the adsorption model that the potential in the adsorption plane changed with ionic strength with an opposite trend to the surface charge of these soils. The phosphate adsorption by these soils was related not only to the ionic strength but also to the types of electrolytes present. K+ induced a greater increase in phosphate adsorption than Na+ due to the greater affinity of the soils to K+ than Na+.

Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength

Phyllosilicates with net negative surface charge and Fe/Al oxides with net positive surface charge co-exist in variable charge soils and the interaction between these oppositely charged particles affects the stability of mixed colloids, aggregation, and even the surface chemical properties of variable charge soils. The overlapping of the diffuse layers of electrical double layers between kaolinite and goethite, hematite, gibbsite and γ-Al 2O 3 was investigated in this article based on the change of zeta potential and electrokinetic charge density induced by the addition of Fe/Al oxides. The results indicated that in the range of pH from 3 to 7, the kaolinite is negatively charged and the Fe/Al oxides are positively charged, and the overlapping of the diffuse layers led to the increase in zeta potential and the decrease in surface charge density of binary-system containing kaolinite and Fe/Al oxides compared with the single kaolinite system. The presence of gibbsite resulted in the strongest interaction of electrical double layers between this and kaolinite, followed by goethite and hematite. The interaction extent of electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added. The interaction also resulted in the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. The increase in ionic strength led to the decrease in the thickness of colloid diffuse layer and thus resulted in the reduction of overlapping of the diffuse layers between kaolinite and Fe/Al oxides.

The mechanism of chromate sorption by three variable charge soils

Adsorption of chromate and desorption of the pre-adsorbed chromate were studied using three representative variable charge soils from the south of China. The mechanisms of the adsorption were discussed based on the hydroxyl release and the change of ζ potential during the chromate adsorption. The adsorption and desorption of chromate followed the same order: the Hyper-Rhodic Ferralsol > the Rhodic Ferralsol > the Haplic Acrisol. The adsorption and the desorption both increased with elevation of the equilibrium chromate concentration and decreased with increasing of the soil solution pH. The percentage of the specific adsorption of chromate was 54.0–59.4%, 54.3–60.3%, and 43.9–46.2% for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol, and the Haplic Acrisol, respectively; the percentage of the electrostatic adsorption was 40.0–46.6%, 39.7–45.8%, and 50.8–56.5% for the three soils, respectively. These findings suggest that both the specific adsorption and the electrostatic adsorption contributed to the chromate adsorption by the variable charge soils. The hydroxyl release during the chromate adsorption shared the same trend with the adsorption envelopes, and decreased with increasing of pH. This is attributed to the exchange of chromate with the hydroxyl on the soil particle surfaces and the formation of a chemical bond between chromate and the surface. Our results indicate that the adsorption of chromate resulted in a shift of ζ potential-pH curves of the soil colloids to negative values, which suggests that the adsorption increased the negative surface charge and decreased the surface potential of the soil colloids.

Microbial activity and community diversity in a variable charge soil as affected by cadmium exposure levels and time

Effects of cadmium (Cd) on microbial biomass, activity and community diversity were assessed in a representative variable charge soil (Typic Aquult) using an incubation study. Cadmium was added as Cd(NO3)(2) to reach a concentration range of 0-16 mg Cd/kg soil. Soil extractable Cd generally increased with Cd loading rate, but decreased with incubation time. Soil microbial biomass was enhanced at low Cd levels (0.5-1 mg/kg), but was inhibited consistently with increasing Cd rate. The ratio of microbial biomass C/N varied with Cd treatment levels, decreasing at low Cd rate (<0.7 mg/kg available Cd), but increasing progressively with Cd loading. Soil respiration was restrained at low Cd loading (<1 mg/kg), and enhanced at higher Cd levels. Soil microbial metabolic quotient (MMQ) was generally greater at high Cd loading (1-16 mg/kg). However, the MMQ is also affected by other factors. Cd contamination reduces species diversity of soil microbial communities and their ability to metabolize different C substrates. Soils with higher levels of Cd contamination showed decreases in indicator phospholipids fatty acids (PLFAs) for Gram-negative bacteria and actinomycetes, while the indicator PLFAs for Gram-positive bacteria and fungi increased with increasing levels of Cd contamination.

Copper adsorption as a function of solution parameters of variable charge soils

Effects of pH and ionic strength (I) on copper (Cu) adsorption and the driving force of the reaction in variable charge soils were evaluated from batch studies. Experimental results of Cu adsorption fitted Langmuir model. According to adsorption isotherms, Cu affinity (K L) was greater in the subsoil (0.061-0.468 L kg-1) than in the topsoil samples (0.169-0.359 L kg-1). Maximum adsorption (Ads max) ranged from 1114-2422 mg kg-1 (topsoil) to 1002-1334 mg kg-1 (subsoil). Strong dependence of Cu adsorption on the pH was observed in subsoil samples. Adsorption edges showed sharply increase of Cu adsorption (20-90%) in the 4.0-5.0 pH range. Copper adsorption changed with increase in I and indicated different metal retention mechanisms (outer- and inner-spheres). Adsorption reaction was favorable and spontaneous, as indicated by negative values of the free energy variation (ΔG) and the separation factor K R < 1. Soil-solution interface and Cu adsorption were also thermodynamically described by a theoretical approach.

Effects of several amendments on rice growth and uptake of copper and cadmium from a contaminated soil

Heavy metals in variable charge soil are highly bioavailable and easy to transfer into plants. Since it is impossible to completely eliminate rice planting on contaminated soils, some remediation and mitigation techniques are necessary to reduce metal bioavailability and uptake by rice. This pot experiment investigated the effects of seven amendments on the growth of rice and uptake of heavy metals from a paddy soil that was contaminated by copper and cadmium. The best results were from the application of limestone that increased grain yield by 12.5-16.5 fold, and decreased Cu and Cd concentrations in grain by 23.0%-50.4%. Application of calcium magnesium phosphate, calcium silicate, pig manure, and peat also increased the grain yield by 0.3-15.3 fold, and effectively decreased the Cu and Cd concentrations in grain. Cd concentration in grain was slightly reduced in the treatments of Chinese milk vetch and zinc sulfate. Concentrations of Cu and Cd in grain and straw were dependent on the available Cu and Cd in the soils, and soil available Cu and Cd were significantly affected by the soil pH.

Zinc adsorption in highly weathered soils

The objective of this work was to assess the effects of pH and ionic strength upon zinc adsorption, in three highly weathered variable charge soils. Adsorption isotherms were elaborated from batch adsorption experiments, with increasing Zn concentrations (0-80 mg L-1), and adsorption envelopes were constructed through soil samples reactions with 0.01, 0.1 and 1 mol L-1 Ca(NO3)2 solutions containing 5 mg L-1 of Zn, with an increasing pH value from 3 to 8. Driving force of reaction was quantified by Gibbs free energy and separation factor. Isotherms were C-, H- and L-type and experimental results were fitted to nonlinear Langmuir model. Maximum adsorption ranged from 59-810 mg kg-1, and Zn affinity was greater in subsoil (0.13-0.81 L kg-1) than in the topsoil samples (0.01-0.34 L kg-1). Zinc adsorption was favorable and spontaneous, and showed sharply increase (20-90%) in the 4-6 pH range. No effect of ionic strength was observed at pH values below 5, because specific adsorption mechanisms predominated in the 3-5 pH range. Above pH 5, and in subsoil samples, Zn was adsorbed by electrostatic mechanisms, since ionic strength effect was observed. Despite depth and ionic strength effects, Zn adsorption depends mainly on the pH.

Effect of low-molecular-weight organic acids on the adsorption of norfloxacin in typical variable charge soils of China

Batch equilibrium experiments were used to study the adsorption of norfloxacin (NOR) onto three kinds of variable charge soils in China, namely Rho-Udic Ferralisols collected from Yunnan, Ali-Perudic Ferrisols collected from Jiangxi and Typ-Hap-Udic Ferralisols collected from Guangdong. Results show that NOR is strongly adsorbed by the soils, with lg K d-values (linear model) of 4.41 ± 0.01, 4.50 ± 0.02, 4.44 ± 0.01 and lg K f-values (Freundlich model) of 4.32, 4.45, 3.08 for the three tested soils, respectively. Both curves of the Freundlich equation and Langmuir equation for the three soils were in excellent linear correlation. Low-molecular-weight (LMW) organic acids, including citric acid, malic acid and salicylic acid were added in the presence of acetate buffer (pH 4.5) to test their effects on NOR adsorption. It was observed that the addition of LMW organic acids inhibited the NOR adsorption process. It was assumed that several effects, including soil pH, solid surface charge and competitive adsorption of co-existing cations, dominated the NOR sorption processes onto variable charge soils. Further studies should be performed to reveal the mechanism of the adsorption.

Pentachlorophenol sorption by variable-charge soils in methanol–water mixture: pH effect at the low solvent volume fraction

Information on how cosolvents affect sorption of ionizable chemicals by soils with heterogeneous variable-charge mineral surface domains is sparse. In this study, the effect of soil–solution pH in methanol/water solutions on sorption of pentachlorophenol (PCP) by variable-charge soils with a range of hydrophilic ( f Hphilic) and hydrophobic ( f Hphobic) sorption domains was characterized. PCP sorption by 10 variable-charge soils was measured as a function of apparent pH (pH app) and methanol volume fraction ( f c ⩽ 0.6). Sorption data of both neutral (pH app < 3) and anionic PCP (pH app > 8) decreased log-linearly with increasing f c, but the slope of the relationship was less for anionic PCP. The empirical solvent–sorbent interaction term for anionic PCP ( α i) was inversely correlated with f Hphilic ( r 2 = 0.82), which is consistent with methanol-induced increases in anion exchange. For neutral PCP, the empirical term ( α n) was positively correlated with f Hphobic ( r 2 = 0.84), supporting methanol-induced increases in solution and sorbent hydrophobicity. Sorption of PCP by two soils with varying f Hphilic in the pH app range from 3 to 8 at f c ⩽ 0.5 was well predicted using a cosolvency model. The model included only pH app - p K a ′ dependent speciation and the different intrinsic hydrophobic partitioning of neutral and anionic PCP without explicitly incorporating the contribution of f Hphilic. Therefore, although the degree to which methanol affects sorptivity of the two PCP species differs with sorption domain types, incorporating both cosolvent-enhanced solubility and cosolvent-induced speciation is sufficient to adequately predict the overall sorption on variable-charge soils.

An Improved Method for the Extraction of Low Molecular Weight Organic Acids in Variable Charge Soils

Due to specific adsorption to variable charge soils, low molecular weight organic acids (LMWOAs) have not been sufficiently extracted, even if common extractants, such as water and 0.1 M sodium hydroxide (NaOH), were employed. In this work, the method for extracting LMWOAs in soils with 0.1 M NaOH was improved for variable charge soils; e.g. 1.0 M potassium fluoride (KF) with pH 4.0 was applied as an extractant jointed with 0.1 M NaOH based on its stronger ability to change the electrochemical properties of variable charge soils by specific adsorption. With the proposed method, the recoveries of oxalic, tartaric, malic, citric and fumaric acids were increased from 83 +/- 4, 93 +/- 1, 22 +/- 2, 63 +/- 5 and 84 +/- 3% to 98 +/- 2, 100 +/- 2, 85 +/- 2, 90 +/- 2 and 89 +/- 2%, respectively, compared with NaOH alone. Simultaneously, the LMWOAs in Agri-Udic Ferrosol with field moisture were measured with a satisfactory result.

Simultaneous Adsorption of Calcium and Sulfate and Its Effect on Their Movement

Ion retention in variable-charge soils can be enhanced by the presence of certain ions with opposite charge, thereby influencing the movement of these ions through the soil profile. Studies examining these interactions are still incipient, however, especially regarding its modeling. We present results from batch and miscible displacement experiments describing Ca2+ and SO42−movement in a variable-charge soil from New Zealand. Evidence was found for ion-pair adsorption (IPA) of both Ca2+and SO42− The results were modeled using the convection–dispersion equation (CDE), coupled with two different mathematical approaches proposed to account for IPA. The first approach related IPA to the single soil adsorption capacity, which is governed by particle-surface phenomena. For the second approach, IPA was related solely to the soil solution concentration. Both these approaches described the adsorption data from the batch experiment reasonably well, as well as the breakthrough curves from the miscible displacement experiments. The first approach showed better overall agreement. Significant differences were found, however, when the adsorption parameters were identified by fitting models to data from either batch or miscible displacement experiments. Although more studies are needed to better understand IPA, our results showed that the extent of IPA can be large and it should not be ignored when predicting SO42− and Ca2+movement in variable-charge soils.

Hydrolysis of Aluminum Ions in Kaolinite and Oxisol Suspensions as Influenced by Organic Anions

To evaluate the role of kaolinite and variable charge soils on the hydrolytic reaction of Al, the hydrolysis of Al ions in suspensions of a kaolinite and an Oxisol influenced by organic anions was investigated using changes of pH, Al adsorption, and desorption of pre-adsorbed Al. Kaolinite and the Oxisol promoted the hydrolytic reaction of Al above a certain initial Al concentration (0.1 mmol L −1 for kaolinite and 0.3 mmol L −1 for the Oxisol). The Al hydrolysis accelerated by kaolinite and the Oxisol increased with an increase in initial concentration of Al and was observed in the range of pH from 3.7 to 4.7 for kaolinite and 3.9 to 4.9 for the Oxisol. The acceleration of Al hydrolysis also increased with the increase of solution pH, reached a maximum value at pH 4.5, and then decreased sharply. Al hydrolysis was promoted mainly through selective adsorption for hydroxy-Al. Soil free iron oxides compensated a portion of the soil negative charge or masked some soil surface negative sites leading to a decrease in Al adsorption, which retarded acceleration to some extent. For the Oxisol organic anions increased the proportion of adsorbed Al 3+ in total adsorbed Al with the increase in soil negative surface charge and eliminated or reduced the acceleration of Al hydrolysis. Different organic anions inhibited the hydrolysis of Al in the order: citrate > oxalate > acetate (under initial pH of 4.5). The formation of Al-organic complexes in solution also inhibited the hydrolysis of Al.