Variable Charge Soils Research Articles

Mobilization of phosphate in variable-charge soils amended with biochars derived from crop straws

The content of available phosphorus in variable-charge soils is often low due to the strong adsorption of phosphate by the soils. The application of crop straw-derived biochars can change surface chemical properties of variable-charge soils, and thus, should affect phosphate adsorption and phosphorus availability in the soils. However, little information is available on the effect of biochars on the mobility and bioavailability of phosphate in variable-charge soils. The objectives of this study were to investigate the effect of crop straw-derived biochars incorporated on phosphate adsorption by variable-charge soils, and then gain insight into the mechanisms for the effect of biochars on phosphate adsorption. Results showed that crop straw-derived biochars are characterized by large specific surface area, highly negative charged surface, ample of anionic functional groups, and moderate concentrations of divalent Ca2+ and Mg2+. The incorporation of the biochars increased soil CEC (cation-exchange capacity), thus, increased the repulsion of soil surfaces to phosphate, and subsequently decreased the phosphate adsorption by the soils. The ample anionic functional groups on added biochars competed for the adsorption sites on the variable-charge soils with phosphate, and thus, inhibited phosphate adsorption by the soils. The Ca2+ and Mg2+ from the biochars formed precipitates with phosphate in the soils and have the potential to increase apparent phosphate adsorption. Rice straw biochar with the higher CEC and the lowest contents of Ca2+ and Mg2+ showed the greatest inhibition on the phosphate adsorption, and thus, could likely be the best choice as amendment to mobilize phosphate in the variable-charge soils. The phosphate adsorption by both control soil and biochars-amended soils decreased with rising pH. Incorporation of the biochars increased the pH of the amended soils, thereby, further mobilizing phosphate in the soils.

Interactions Between Escherchia coli and the Colloids of Three Variable Charge Soils and Their Effects on Soil Surface Charge Properties

The adhesion of Escherchia coli (E. coli) to the colloids of three variable charge soils and its effect on surface charge properties and potassium adsorption of these soil colloids were investigated. The adhesion isotherms of E. coli by soil colloids can be described using the Langmuir equation. The amount of E. coli adhered by the soil colloids varied with soil type and followed the order: Ultisol from Guangxi > Oxisol from Yunnan > Ultisol from Jiangxi. The iron and aluminum oxide contents and CECs of the soils are the important factors affecting the adhesion of E. coli to soil colloids. The relatively lower iron and aluminum oxide contents and higher CEC of the Ultisol from Jiangxi led to the lower adhesion of E. coli to the soil colloids compared to the Ultisol from Guangxi and the Oxisol from Yunnan. The amount of E. coli adhered to the soil colloids decreased with increasing pH, which was consistent with the results predicted from the DLVO theory. E. coli adhesion made the zeta potential of the soil colloids more negative and reduced the isoelectric point of the soil colloids, suggesting that E. coli decreased the surface positive charge and increased negative charge of the soil colloids. In addition, E. coli adhesion increased K+ adsorption by the soil colloids. Therefore, bacterial adhesion improves the fertility of variable charge soils by increasing soil CEC because the CECs of variable charge soils are usually low.

Effect of Crop-Straw Derived Biochars on Pb(II) Adsorption in Two Variable Charge Soils

Two variable charge soils were incubated with biochars derived from straws of peanut, soybean, canola, and rice to investigate the effect of the biochars on their chemical properties and Pb(II) adsorption using batch experiments. The results showed soil cation exchange capacity (CEC) and pH significantly increased after 30 d of incubation with the biochars added. The incorporation of the biochars markedly increased the adsorption of Pb(II), and both the electrostatic and non-electrostatic adsorption mechanisms contributed to Pb(II) adsorption by the variable charge soils. Adsorption isotherms illustrated legume-straw derived biochars more greatly increased Pb(II) adsorption on soils through the non-electrostatic mechanism via the formation of surface complexes between Pb(II) and acid functional groups of the biochars than did non-legume straw biochars. The adsorption capacity of Pb(II) increased, while the desorption amount slightly decreased with the increasing suspension pH for the studied soils, especially in a high suspension pH, indicating that precipitation also plays an important role in immobilizing Pb(II) to the soils.

Kinetics, equilibrium and thermodynamics of the sorption of p-nitrophenol on two variable charge soils of Southern China

The sorption of p-nitrophenol (PNP) on two variable charge soils was studied under a series of batch experimental conditions of solution pH, contact time, and temperature. Their kinetic and equilibrium parameters were assessed. The optimum pH for sorption of PNP was 5.0 for Xuwen soil and 5.4 for Guangzhou soil, respectively. Langmuir and Freundlich models were successfully used to describe the sorption isotherms. Results implied that monolayer sorption and heterogeneous energetic distribution of active sites on the surface of the soils were possible. The kinetic data were fitted with pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. The sorption process was well described by pseudo-second-order kinetic model, and the intra-particle diffusion was not the sole rate-controlling step. The negative standard Gibbs free energy (ΔrGm0) values of the sorption implied that the sorption of PNP on soils was spontaneous, and the changes of the standard enthalpy (ΔrHm0) showed that the sorption as endothermic. In addition, the soils were characterized by surface area, X-ray diffraction (XRD) spectrum and Fourier transform infrared (FTIR) spectroscopy.

Heavy metal accumulation in balsam pear and cowpea related to the geochemical factors of variable-charge soils in the Pearl River Delta, South China.

Variable-charge (v-c) soils in subtropical areas contain considerable amounts of iron/aluminum (Fe/Al) oxides that can strongly influence the fate of heavy metals in agricultural ecosystems. However, the relationship between heavy metal accumulation in vegetables and the geochemical factors associated with v-c soils in subtropical regions remains unknown. The present study investigated heavy metal accumulation under field conditions in the Pearl River Delta (PRD) by measuring the content of 8 heavy metals (zinc (Zn), arsenic (As), copper (Cu), mercury (Hg), lead (Pb), chromium (Cr), nickel (Ni) and cadmium (Cd)) in 43 pairs of v-c soil and vegetable (balsam pear and cowpea) samples. Soil physicochemical properties including pH, texture, organic matter and oxide minerals (Fe2O3, SiO2, Al2O3, CaO, MgO, K2O and Na2O) were also analyzed. Heavy metal accumulation from soil to vegetables was assessed based on bioconcentration factors (BCFs). The results showed that soil extractable Fe, oxide minerals and chemical weathering indices of v-c soils strongly affected heavy metal accumulation, whereas the content of Zn, Cu, Cr and Ni in vegetables was strongly affected by the soil clay content. Significant correlations were found between the BCFs of heavy metals and oxide minerals. However, no significant relationship was found between pH and heavy metal accumulation (except for Cu) in balsam pear and cowpea. Correlation analyses showed that a lower oxalate/DCB- extractable Fe content might indicate greater heavy metal (Zn, Cu, Hg, Cr and Ni) accumulation in vegetables. Therefore, it can be deduced that oxalate/DCB- extractable Fe content is a critical geochemical factor that determines the bioavailability of heavy metals and that iron biogeochemical cycles play vital roles in the fate of heavy metals in vegetable fields in this area. These findings provide new insights into the behaviors and fate of heavy metals in subtropical v-c soils and can be used to develop possible guidelines for vegetable safety management.

Effect of pH, Ionic Strength and Heavy Metal Ions on p-Nitrophenol Adsorption by Variable Charge Soil of South China

Effect of pH, Ionic Strength and Heavy Metal Ions on p-Nitrophenol Adsorption by Variable Charge Soil of South China

Adsorption and desorption characteristics of diphenylarsenicals in two contrasting soils

Diphenylarsinic acid (DPAA) is formed during the leakage of aromatic arsenic chemical weapons in soils, is persistent in nature, and results in arsenic contamination in the field. The adsorption and desorption characteristics of DPAA were investigated in two typical Chinese soils, an Acrisol (a variable-charge soil) and a Phaeozem (a constant-charge soil). Their thermodynamics and some of the factors influencing them (i.e., initial pH value, ionic strength and phosphate) were also evaluated using the batch method in order to understand the environmental fate of DPAA in soils. The results indicate that Acrisol had a stronger adsorption capacity for DPAA than Phaeozem. Soil DPAA adsorption was a spontaneous and endothermic process and the amount of DPAA adsorbed was affected significantly by variation in soil pH and phosphate. In contrast, soil organic matter and ionic strength had no significant effect on adsorption. This suggests that DPAA adsorption may be due to specific adsorption on soil mineral surfaces. Therefore, monitoring the fate of DPAA in soils is recommended in areas contaminated by leakage from chemical weapons.

Impacts of inorganic ions and temperature on lead adsorption onto variable charge soils

Abstract The effect of inorganic ions and temperature on lead (Pb2 +) adsorption was evaluated on three acidic soils: yellow-brown soil (YBS), latosol soil (LS), and lateritic red soil (LRS). The results showed that the adsorption of Pb2 + in the presence of anions decreased in the order: SO42 − > NO3− = Cl−, while in the presence of cations, it reduced in the order: K+ > Ca2 +. Accordingly, the contamination of YBS, LS and LRS soils with Pb2 + was more noticeable when they contained more NO3−, Cl− and Ca2 + than SO42 − and K+. The distribution coefficient (Kd) of Pb2 + declined in the order: YBS > LS > LRS in the presence of anions. Adsorption and Kd values of Pb2 + were mainly dependent on ion types and altered surface properties caused by the adsorption and/or the formation of ion complexes. The Kd values enhanced slightly with increasing temperature up to 35 °C and then decreased at 45 °C. Based on the thermodynamic parameters, including the heat of reaction (ΔH), entropy change (ΔS) and the free energy change (ΔG), the adsorption process of Pb2 + on YBS, LS and LRS soils was endothermic and impetuous at low temperatures, whereas it was exothermic and non-spontaneous at high temperatures.

Effect of biochars on adsorption of Cu(II), Pb(II) and Cd(II) by three variable charge soils from southern China

The purpose of this study is to compare the relative contribution of different mechanisms to the enhanced adsorption of Cu(II), Pb(II) and Cd(II) by variable charge soils due to incorporation of biochars derived from crop straws. The biochars were prepared from the straws of canola and peanut using an oxygen-limited pyrolysis method at 350 °C. The effect of biochars on adsorption and desorption of Cu(II), Pb(II) and Cd(II) by and from three variable charge soils from southern China was investigated with batch experiments. Based on the desorption of pre-adsorbed heavy metals, the electrostatic and non-electrostatic adsorptions were separated. EDTA was used to replace the heavy metals complexed with biochars and to evaluate the complexing ability of the biochars with the metals. The incorporation of biochars increased the adsorption of Cu(II), Pb(II) and Cd(II) by the soil; peanut straw char induced a greater increase in the adsorption of the three metals. The increased percentage of Cd(II) adsorption induced by biochars was much greater than that for the adsorption of Cu(II) and Pb(II). Cu(II) adsorption on three variable charge soils was enhanced by the two biochars mainly through a non-electrostatic mechanism, while both electrostatic and non-electrostatic mechanisms contributed to the enhanced adsorption of Pb(II) and Cd(II) due to the biochars. Peanut straw char had a greater specific adsorption capacity than canola straw char and thus induced more non-electrostatic adsorption of Cu(II), Pb(II) and Cd(II) by the soils than did the canola straw char. The complexing ability of the biochars with Cu(II) and Pb(II) was much stronger than that with Cd(II) and thus induced more specific adsorption of Cu(II) and Pb(II) by the soils than that of Cd(II). Biochars increased heavy metal adsorption by the variable charge soils through electrostatic and non-electrostatic mechanisms, and the relative contribution of the two mechanisms varied with metals and biochars.

Phosphorus–arsenic interactions in variable-charge soils in relation to arsenic mobility and bioavailability

Phosphorus (P) influences arsenic (As) mobility and bioavailability which depends on the charge components of soil. The objective of this study was to examine P–As interaction in variable-charge allophanic soils in relation to P-induced As mobilization and bioavailability. In this work, the effect of P on arsenate [As(V)] adsorption and desorption was examined using a number of allophanic and non-allophanic soils which vary in their anion adsorption capacity. The effect of P on As uptake by Indian mustard (Brassica juncea L.) plants was examined using a solution culture, and a soil plant growth experiment involving two As-spiked allophanic and non-allophanic soils which vary in their anion adsorption capacity, and a field As-contaminated sheep dip soil. Arsenate adsorption increased with an increase in the anion adsorption capacity of soils. The addition of P resulted in an increase in As desorption, and the effect was more pronounced in the case of allophanic soil. In the case of both As-spiked soils and field contaminated sheep-dip soil, application of P increased the desorption of As, thereby increasing its bioavailability. The effect of P on As uptake was more pronounced in the high anion adsorbing allophanic than low adsorbing non-allophanic soil. In the case of solution culture, As phytoavailability decreased with increasing concentration of P which is attributed to the competition of P for As uptake by roots. While increasing P concentration in solution decreased the uptake of As, it facilitated the translocation of As from root to shoot. The net effect of P on As phytoavailability in soils depends on the extent of P-induced As mobilization in soils and P-induced competition for As uptake by roots. The P-induced mobilization of As could be employed in the phytoremediation of As-contaminated sites. However, care must be taken to minimize the leaching of As mobilized through the P-induced desorption, thereby resulting in groundwater and off site contamination.

Effect of Fe/Al Hydroxides on Desorption of K+ and NH+4 from Two Soils and Kaolinite

Potassium (K) and nitrogen (N) are essential nutrients for plants. Adsorption and desorption in soils affect K+ and NH+4 availabilities to plants and can be affected by the interaction between the electrical double layers on oppositely charged particles because the interaction can decrease the surface charge density of the particles by neutralization of positive and negative charges. We studied the effect of iron (Fe)/aluminum (Al) hydroxides on desorption of K+ and NH+4 from soils and kaolinite and proposed desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and the positively charged Fe/Al hydroxide particles. Our results indicated that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al hydroxides, as amorphous Al(OH)3 or Fe(OH)3, and negatively charged surfaces from an Ultisol, an Alfisol, and a kaolinite standard caused the effective negative surface charge density on the soils and kaolinite to become less negative. Thus the adsorption affinity of these negatively charged surfaces for K+ and NH+4 declined as a result of the incorporation of the Fe/Al hydroxides. Consequently, the release of exchangeable K+ and NH+4 from the surfaces of the soils and kaolinite increased with the amount of the Fe/Al hydroxides added. The greater the positive charge on the surfaces of Fe/Al hydroxides, the stronger was the interactive effect between the hydroxides and soils or kaolinite, and thus the more release of K+ and NH+4. A decrease in pH led to increased positive surface charge on the Fe/Al hydroxides and enhanced interactive effects between the hydroxides and soils/kaolinite. As a result, more K+ and NH+4 were desorbed from the soils and kaolinite. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of K+ and NH+4 in the soils and thus increase their leaching lsos.

Sorption–bioavailability nexus of arsenic and cadmium in variable-charge soils

In this work, the nexus between sorption and bioavailability of arsenic (As) and cadmium (Cd) as affected by soil type, soil pH, ageing, and mobilizing agents were examined. The adsorption of As and Cd was examined using a number of allophanic and non-allophanic soils which vary in their charge components. The effect of pH and ageing on the bioavailability of As and Cd was examined using spiked soils in a plant growth experiment. The effect of phosphate (P)-induced mobility of As on its bioavailability was examined using a naturally contaminated sheep dip soil. The results indicated that the adsorption of both As and Cd varied amongst the soils, and the difference in Cd adsorption is attributed to the difference in surface charge. An increase in soil pH increased net negative charge by an average of 45.7mmol/kg/pH thereby increasing cation (Cd) adsorption; whereas, the effect of pH on anion (As) adsorption was inconsistent. The bioavailability of As and Cd decreased by 3.31- and 2.30-fold, respectively, with ageing which may be attributed to increased immobilization. Phosphate addition increased the mobility and bioavailability of As by 4.34- and 3.35-fold, respectively, in the sheep dip soil. However, the net effect of P on As phytoavailability depends on the extent of P-induced As mobilization in soils and P-induced competition for As uptake by roots. The results demonstrate the nexus between sorption and bioavailability of As and Cd in soils, indicating that the effects of various factors on bioavailability are mediated through their effects on sorption reactions.

Liming Decreases the Vertical Mobility of Potassium in Acidic Soils

Liming increases soil negative charges and thus affects the chemical equilibrium of cations between the solid phase and the soil solution with consequences for the mobility of cations. The effects of lime and potassium chloride (KCl) addition on the vertical movement of potassium (K), calcium (Ca), and magnesium (Mg) were investigated in two Brazilian soils. The experiment was carried out in leaching columns. Treatments included a combination of rates of KCl (0, 50, 100, and 200 mg kg−1 K) and acidity levels (without and with liming). Seven water percolations (400 mL per column) were performed at weekly intervals, totaling an equivalent to 357 mm of rain. Raising the soil pH decreased the leaching of K, in both soils, up to 50% because of the increase of its electrostatic adsorption on created negative charges. Addition of KCl increased the percolation of K, Ca, and Mg proportionally to the rate applied, and it was more efficient than calcium carbonate to percolate Ca in both soils. Liming must be taken into consideration when K leaching or plant needs of K are estimated in predominantly variable-charge soils.

Adsorption of Pb(II) on variable charge soils amended with rice-straw derived biochar

Two Ultisols and one Oxisol from tropical regions of southern China were incubated with rice straw biochar to investigate the effect of biochar on their surface charge and Pb(II) adsorption using batch methods. The incorporation of biochar induced a remarkable increase in soil cation exchange capacity after 30d of incubation. The incorporation of biochar significantly increased the adsorption of Pb(II) by these variable charge soils; the enhancement of adsorption of Pb(II) by these soils increased with the addition level of biochar. Adsorption of Pb(II) involved both electrostatic and non-electrostatic mechanisms; however, biochar mainly increased Pb(II) adsorption through the non-electrostatic mechanism via the formation of surface complexes between Pb2+ and functional groups on biochar. There was greater enhancement of biochar on the non-electrostatic adsorption of Pb(II) by the variable charge soils at relatively low pH. Therefore, the incorporation of biochar decreased the activity and availability of Pb(II) to plants through increased non-electrostatic adsorption of Pb(II) by acidic variable charge soils.

Iron oxides serve as natural anti-acidification agents in highly weathered soils

The effect of Fe oxides on the natural acidification of highly weathered soils was investigated to explore the natural acidification process in variable charge soils A variety of highly weathered soils with different Fe oxide contents were collected from the tropical and subtropical regions of southern China to investigate the soil acidity status. Electrodialysis experiments were conducted to simulate natural acidification process and promote accelerated acidification in a variety of systems such as relatively less weathered soils, mixtures of goethite with montmorillonite or kaolinite, an Alfisol, a limed Ultisol, and Fe oxides coated montmorillonite. The objective was to gather evidence for the occurrence of Fe oxide inhibited natural acidification in highly weathered soils. Highly weathered soils with free Fe2O3 100 g/kg (49 soils) had an average pH = 5.25 ± 0.04. A significant linear relationship was found between the soil pH and Fe oxide content of these soils. Similar results were obtained in electrodialysis experiments, i.e., in soils that underwent accelerated acidification. A negative correlation was found between the Fe oxide content and exchangeable acidity or effective cation exchange capacity, respectively. In another set of experiments, goethite slowed down acidification in experiments conducted with this Fe oxide and montmorillonite, or kaolinite, or an Alfisol, or a limed Ultisol. The overlapping of the electrical double layers on the positively charged Fe oxide particles and negatively charged minerals may have caused the release and subsequent leaching of the base cations, but inhibited the production of exchangeable acidity cations. In addition, when montmorillonite or Fe oxide-coated montmorillonite were electrodialyzed in another set of experiments, exchangeable acidity of the former was much greater than that of the latter, suggesting that the positively charged Fe oxide coatings on montmorillonite have partially neutralized the permanent negative charge on montmorillonite surfaces, decreasing exchangeable acidity. Fe oxides may function as natural “anti-acidification” agents through electric double-layer overlapping and coating of phylliosilicates in highly weathered soils.

Adsorption of chromate on variable charge soils as influenced by ionic strength

The adsorption behavior of chromate on two variable charge soils (Oxisol and Ultisol) was investigated through batch experiments at different ionic strengths and pH values. The adsorption of chromate on the variable charge soils was found to be strongly dependent on the pH of the soil solutions. A characteristic pH was observed, which corresponds to the intersection of the chromate adsorption—pH curves at different ionic strengths. The characteristic pH values are 5.50 for Oxisol and 5.04 for Ultisol, close to the point of zero salt effect (PZSE) of these soils. The zeta potentials measured for these soils provide the evidences to support the interpretation of the effect of ionic strength on the adsorption of chromate on these variable charge soils. The adsorption behavior of chromate was interpreted by a schematic representation of chromate distribution at increasing ionic strength. The chromate desorption–pH curves were also found to intersect at pH of 5.15 and 4.89 for the Oxisol and Ultisol, respectively. It is considered that chromate adsorption by the variable charge soils was mainly determined by the electrostatic potential on the adsorption plane, which was controlled by the ionic strength of the soil solutions.

Nickel adsorption by variable charge soils: effect of pH and ionic strength

The effects of pH and ionic strength (I) on Ni adsorption in variable charge soils were evaluated by laboratory batch experiments. Experimental results fitted the Langmuir model. Maximum adsorption (Adsmax) ranged from 260-2818 mg kg-1 (topsoil) to 532-1541 mg kg-1 (subsoil). Nickel affinity (KL) was higher in the subsoil samples (0.022-0.236 L kg-1) than in topsoil (0.003-0.049 L kg-1). Adsorption envelopes showed sharp increase in Ni adsorption (20-90%) in the 4.0-6.0 pH range. Nickel adsorption was affected by I and specific adsorption predominated in the 3.0-5.0 pH range, while above pH 5.0, it was adsorbed by electrostatic mechanisms. Negative values of free energy variation (ΔG) and the separation factor KR<1 indicated that Ni adsorption reaction was favorable and occurred spontaneously, especially when pH increase. Chemical and mineralogical soil attributes should be considered as criteria for selecting the areas for disposal of residues containing Ni to minimize the impact on the environment.

Effect of Fe/Al oxides on desorption of Cd2+ from soils and minerals as related to diffuse layer overlapping

Cadmium is a toxic metal with high reactivity in acid variable charge soils. Adsorption and desorption of Cd2+ in soil and mineral particles can be affected by the interaction between the electrical double layers on oppositely charged particles, because the interaction can decrease the surface-charge density of the particles. We studied the effect of Fe/Al oxides on desorption of Cd2+ from soils and minerals and proposed the desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and positively charged Fe/Al oxide particles. Our results indicate that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al oxides [crystalline and amorphous Al(OH)3 and amorphous Fe(OH)3] and negatively charged Ultisol, Alfisol, kaolinite, and bentonite caused the effective negative surface-charge density on the soils and minerals to become less negative, and thus the adsorption affinity of these negatively charged surfaces for Cd2+ declined as a result of the incorporation of the Fe/Al oxides. Consequently, the release of exchangeable Cd2+ from the surfaces of the soils and minerals increased with the amount of Fe/Al oxides added. The more positive the charge on the surfaces of the Fe/Al oxides, the stronger the interaction of the electrical double layers between the oxides and soils and minerals, and thus the greater the release of Cd2+ from the soils and minerals. A decrease in pH led to an increase in the positive surface charge on the Fe/Al oxides and enhancement of the interaction of the electrical double layers between the oxides and soils and minerals. As a result, more Cd2+ was desorbed from the soils and minerals. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of cadmium in the soils and thus increase its environmental risk.

Chemical composition and Zn bioavailability of the soil solution extracted from Zn amended variable charge soils

Abstract A study on variable charge soils (volcanic Italian and podzolic Scottish soils) was performed to investigate the influence of soil properties on the chemical composition of soil solution. Zinc speciation, bioavailability and toxicity in the soil solution were examined. The soils were spiked with increasing amounts of Zn (0, 100, 200, 400 and 1000 mg/kg) and the soil solutions were extracted using rhizon soil moisture samplers. The pH, total organic carbon (TOC), base cations, anions, total Zn and free Zn 2+ in soil solution were analysed. A rapid bioassay with the luminescent bacterium Escherichia coli HB101 pUCD607 was performed to assess Zn toxicity. The influence of soil type and Zn treatments on the chemical composition of soil solution and on Zn toxicity was considered and discussed. Different trends of total and free Zn concentrations, base cations desorption and luminescence of E. coli HB101 pUCD607 were observed. The soil solution extracted from the volcanic soils had very low total and free Zn concentrations and showed specific Zn 2+/Ca 2+ exchange. The soil solution from the podzolic soil had much higher total and free Zn concentrations and showed no evidence of specific Zn 2+/Ca 2+ exchange. In comparison with the subalkaline volcanic soils, the acidic podzol showed enhanced levels of toxic free Zn 2+ and consequently stronger effects on E. coli viability.

Variable Charge of Ultisols due to Phosphate Application and Incubation Time

Variable Charge Ultisols due to Phosphate Application and Incubation Time (M Mahbub): The laboratory experiment was conducted to study the effect of phosphate (P) application and its incubation time on pHo (pH at the point of zero charge) and variable charge of ultisols . The determined parameters were pHo and variable surface-charges. Soil samples were added by 0, 375 and 1,125 mg P kg-1 (or 0, 50 and 150% of the P sorption maximum, respectively). Then, they were incubated for 2, 4, 6 and 8 weeks. The variable surface-charges and pHo were determined by using the activity of potential determining ions (H+ and OH-) within two salt concentrations as counter ions (0.1N and 0.001N CaCl2) through a potentiometric titration method. The results were indicated that the high P sorption and 766 mg P kg-1 in maximum sorption were due to high contents in clay fractions and aluminum as well as low pH of experimental soil. Application of P and incubation time were able to decrease pHo and to increase negative surface-charges. Additionals of 375 and 1,125 mg P kg-1 incubated for 8 weeks gave the value of pHo 2.86 and 2.69; as well as the magnitude of negative charges 14.48 and 16.76 cmol(-).kg-1, respectively (both for 0.001N CaCl2). For pH > pHo (the characteristic of variable charge soils), the higher the salt (CaCl2) concentration and pH solution, the higher the negative surface-charge.