The effects of pH and chloride concentration on mercury sorption. II. By a soil

The pH of a soil was altered by incubation with either acid or lime, and the sorption of mercury was measured in the absence of chloride and at three chloride concentrations. In the absence of chloride there were only small effects of pH on sorption between pH 4 and 6; sorption decreased at higher pH. Addition of chloride decreased sorption at low pH but had little effect on sorption at high pH. Consequently, in the presence of chloride, sorption increased with increasing pH between pH 4 and 6 and then decreased. Many of the mercury sorption curves were sigmoid. This was explained by assuming that a small amount of complexing material was present in the solution after mixing with the soil. Calculation of the mercury species present in solution was made difficult by uncertainties about the postulated complexing material. Nevertheless, between pH 4 and pH 5.8, it was possible to explain the effects of pH and of chloride concentration on sorption as entirely due to changes in the HgOH⁺ concentration.     

Comparing the effects of pH on the sorption of metals by soil and by goethite, and on uptake by plants

The effects of soil pH on sorption of cadmium, zinc, nickel and cobalt were studied by changing the pH of a soil and measuring sorption. Results were compared with published results for effects of pH on sorption of cadmium, zinc and nickel by goethite. In a further experiment, the effects of pH on the uptake of zinc and cobalt by subterranean clover were measured. Effects of pH on sorption were described in terms of the concentration of metal ions required to produce equal sorption. Where the metal ions were incubated with the soil, unit increase in pH decreased the concentration of metal ions required about 10-fold for zinc, about 7-fold for nickel, about 6-fold for cobalt, and about 4-fold for cadmium. When the soil was mixed with a large volume of solution, the effects were similar for zinc and cadmium but slightly smaller for cobalt and slightly larger for nickel. In all cases, the magnitude of the effect varied somewhat with pH. Sorption was greater with a dilute background electroiyte than with a concentrated one and the effects of pH were greater. The effects for soil were smaller than effects of pH on sorption by iron oxides for which unit increase in pH can decrease the required concentration of Zinc 35-fold and cadmium 11-fold. These results are consistent with adsorption of divalent ions on a variable charge surface that is negatively charged. They are not consistent with the adsorption of monovalent metal ions on a variable charge surface. This mechanism requires at least a 10-fold effect of pH. They show that the change in electric potential with change in pH is smaller for reacting surfaces in soil than for goethite. The effects of changing pH on the amounts of zinc and cobalt fertilizer required for equal uptake by plants was even smaller with unit increase in pH, causing a 1.4-fold increase in the amount of fertilizer required, that is, a 1.4-fold decrease in fertilizer effectiveness.     

Irreversibility of sulfate sorption on goethite and hematite

Sulfate ion adsorption and desorption experiments carried out on synthetic goethite and hematite and natural hematite show adsorption to be a highly irreversible reaction. All oxides showed an increase in sulfate ion adsorption with decrease in pH. Only a small fraction of sorbed sulfate was desorbable after 48 hr, and only at a pH of 3. Extreme irreversibility of sulfate sorption on these common soil minerals suggests that adsorbed sulfate is more immobile in watersheds than previously considered and that recovery models which inherently assume reversibility may need to be modified.     

Testing a mechanistic model. X. The effect of pH and electrolyte concentration on borate sorption by a soil

Borate sorption by a soil was measured with 0.01, 0.1, or 1.0 M sodium chloride as background electrolyte and samples of soil with a range of pH values achieved by incubating with either calcium carbonate or hydrochloric acid for 24 h at 60°C.
Borate sorption generally increased with increasing pH. The more concentrated the electrolyte, the steeper the increase. At low pH, increasing the salt concentration decreased borate sorption; at high pH, it increased sorption. There was an intermediate pH at which salt had no effect on borate sorption. The point of zero salt effect on borate sorption was at a higher pH than the point of zero salt effect on pH. This result was explained by a mechanism in which borate ions react with variable charge surfaces which are heterogeneous and for which part of the heterogeneity is in the electric potential of the surfaces. It cannot be explained by mechanisms which do not take into account the effects of the electric potential of the reacting surfaces on the reaction with borate ions. Although the behaviour of borate was broadly consistent with that of other anions, it differed in that about half of the heterogeneity had to be allocated to the binding constant for borate ions. It was suggested that this was because reaction with organic matter was more important for borate than for other anions.     

Modelling the effects of pH on phosphate sorption by soils

Samples of six soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Phosphate sorption was then measured on sub-samples of the treated soils over 24 h at 25°C. In one set of measurements on all soils, 0.01 M calcium chloride was used as the background electrolyte. In another set, on two soils, 0.01 M sodium chloride was used. An interpolation method was used to give points on the three-dimensional surface relating the final pH of the suspensions to sorption of phosphate at specified solution concentrations of phosphate. The effects of pH on phosphate sorption differed between soils. For unfertilized soils, increases in pH up to about pH 5.5 decreased sorption. Further increases in pH decreased sorption further in one soil and increased it in three others. For fertilized soils, measured sorption increased with pH. When sodium chloride was used instead of calcium chloride, there was a more marked trend for sorption to decrease as pH increased. Differences between the soils were ascribed to differences in two soil properties. One was the rate at which the electrostatic potential in the plane of adsorption decreased as pH increased. Only small differences in the rate of change of potential were needed to reproduce the observed differences between soils. The electrostatic potential would decrease more quickly in solutions of a sodium salt than in solutions of a calcium salt and this explains the observed differences between these media. The other soil property that affected observed sorption was the release of phosphate from the soil. The amount released was largest at low pH. Consequently, for fertilized soils, measured sorption increased with pH.     

不同pH、电解质浓度条件下草酸对针铁矿吸附Cd(II)的影响及机制

用吸附平衡法研究了不同草酸浓度、体系pH对针铁矿 (G)吸附Cd2+的影响与机制以及电解质 (KNO3)浓度对针铁矿、草酸化针铁矿 (G+40 )吸附Cd2+的影响差别及原因。结果表明 ,低浓度草酸 (1mmolL-1 )促进Cd2+的吸附 ;高浓度草酸 (1mmolL-1/sup )抑制Cd2+的吸附。已吸附在针铁矿表面的草酸对Cd2+ 吸附的影响与液相中草酸的影响不同 ,这主要与草酸引起的针铁矿表面电荷性质的变化、草酸在固液两相间的分配、草酸与Cd2+的配合作用和竞争作用有关。电解质 (KNO3)浓度对针铁矿和草酸化针铁矿吸附Cd2+的影响明显不同 ,随KNO3 浓度的提高 ,针铁矿的Cd吸附率由 44.5%增至 95%以上 ,而草酸化针铁矿吸附率由 29%降至6.2% ,这主要决定于二者的电荷零点 (PZC)和体系pH变化的不同。     

The sorption of mercury species by clay minerals

The amount of Hg sorbed by kaolinite and illite, in the absence of ligands, changes little with pH; with montmorillonite, Hg uptake decreases with increasing pH. An overall decrease in the amounts sorbed by these clays occurs in solutions which contain ligands such as cyanide (2:1); acetate (5:1); and nitrilotriacetate (6:5). [Values in parentheses are mole ratio of ligand: Hg.] In the presence of chloride (2:1), the order for the uptake of Hg by the three clays is illite > montmorillonite > kaolin. The addition of thiourea (2:1) results in total precipitation of mercury at pH > 4; in the presence of sulfate or phosphate (> 1:1) Hg is lost from solution by precipitation/sorption at pH 4 but the amount decreases to near zero at pH > 8. Solution processes (i.e. complex formation, precipitation) appear to have a dominating influence on mercury distribution.     

Effects of crystallinity of goethite: II. Rates of sorption and desorption of phosphate

Eight samples of goethite ranging in surface area from 18 to 132 m² g^-1 were mixed with phosphate at a range of pH values for periods which ranged from 0·5 h to 6 weeks. The sample with a surface area of 18 m² g^-1 had been hydrothermally treated to improve its crystallinity. Its rate of reaction with phosphate depended on pH but was complete within a day. Its maximum observed reaction was close to the theoretical maximum for surface adsorption of 2·5 μmol m^?2. For the other samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2·5 μmole m^?2. The duration and extent of the reaction depended on the crystallinity of the goethite. The results were closely described by a model in which the phosphate ions were initially adsorbed on to charged external surfaces. The phosphate ions then diffused into the particles. This was closely described using equations for diffusion into a cylinder. Samples of goethite which had been loaded with phosphate dissolved more slowly in HCl, and had a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HCl removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it was necessary to dissolve much more of the iron to obtain a similar removal of phosphate. Brief treatment with NaOH removed most of the phosphate from the hydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite. These results are consistent with the idea that phosphate ions were not only bound on external surface sites but had also penetrated into meso- and micro-pores between the domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sites for phosphate adsorption are therefore caused mainly by their location on either external or internal sites. Models that ignore this are incomplete.     

Ionic-strength and pH effects on the sorption of cadmium and the surface charge of soils

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp^?3 Ca(NO₃)₂, and various concentrations of NaNO₃ (0.01–1.5 mol dm^?3) as background solutions at a range of pH values (3–8). In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaNO₃ increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.     

Effect of soil microorganisms on the sorption of zinc and lead compounds by goethite

The objectives of this study were (1) to determine the effect of microorganisms during in‐vitro incubation on the amount of Zn and Pb from solution retained on goethite precipitated as coatings on a sand matrix and (2) to evaluate accumulation of heavy metals in the biomass of soil microorganisms in the fresh soil samples using an extractive approach. A mixture of colonies of cultivated microorganisms extracted from a Haplic Luvisol (Russia) and an Antropi‐urbic Regosol (Germany) were used to prepare the cell and the microbial‐debris suspensions. The concentrations of Zn and Pb in the studied solutions supplied with microbial suspensions and/or goethite coated sand were 0.1 mM (130.8 and 414 mg kg^–1 of sand, respectively). Exchangeable forms of metals were determined by extraction with 10 mL of 1.0 M KNO₃. Nonexchangeable forms of Zn and Pb were recovered using 40 mL of 0.3 M NH₂OH‐HCl in 1 M HNO₃. Concentrations of Pb increased in the solutions and decreased on the surface of the Fe‐mineral due to living microorganisms. In comparison to incubation of heavy‐metal solutions with goethite only, the absolute concentrations of nonexchangeable forms of metal were reduced by microbial suspension to a greater extent than those of the exchangeable forms, whereas the relative content of both fractions decreased by a factor of almost two. Sorption of Pb by goethite was inversely correlated with the concentration of organic C in the solution. Microorganisms clearly influenced the Zn sorption by goethite at concentrations of C_org > 400 mg L^–1. The amount of Zn retained was decreased primarily due to decreasing Zn portions in the exchangeable fraction. Microbial debris prepared by autoclaving reduced the Pb sorption by goethite similar to the results for living cells. Living microorganisms accumulated more Zn than did microbial debris. The data of this paper show that a direct determination of heavy‐metal accumulation in soil microorganisms by extraction with 2.0 M KCl as well as by extraction with 1 M CH₃COONH₄ at the natural pH of the soils after chloroform fumigation of fresh soils samples with different concentrations of organic C was not possible.     

Effect of porosity of goethite on the sorption of six heavy metal ions

Seven different samples of goethite with differing crystallinity were prepared and the rate of reaction of six metal ions was measured for up to 4 weeks. The ions used were: Cu²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺ and Cr³⁺. For Cu, decreasing goethite crystallinity, and therefore increasing porosity, increased the rate of reaction. In contrast, for Pb there was no effect. Hydrothermal treatment, which was used to induce ‘healing’ in one of the goethites, did not prevent the continuing reaction. It decreased the rate of reaction for Cu, Mn and Cd, but had no effect on Ni and Cr. This suggests that the main route for Ni and Cr is not via pores, but by lattice diffusion. The presence of silicate in the goethite decreased the reaction of Mn and Cd. We suggest that silicate decreased the spaces between goethite domains and that these spaces are the pathways largely used by these two ions. These observations show that metallic cations differ amongst themselves, and differ from anions such as phosphate, in the pathways they follow when they penetrate materials such as goethite subsequent to an initial adsorption reaction.     

pH对针铁矿表面磷酸盐吸附的化学形态的影响

Chemical forms of the phosphate adsorbed on goethite surfaces and characteristics of the coordinate groups which exchange with P on goethite surfaces in solutions with different pll values were investigated.Results showed that the chemical forms of P on goethite surfaces changed from the dominance of monodentate corrdination to that of bidentate one with increasing pH of the solution.By influencing types of phosphate ions in solutions,pH affected the chemical forms of P on goethite surfaces,The amount of OH^- displaced by phosphae on goethite surfaces was the most at pH 7.0,the second at pH 9.0,and the least at pH 4.5.     

通过微量量热法研究Cd2+在针铁矿上的吸附机制

A study on energy changes and mechanisms of Cd sorbed on goethite was performed using the technique of microcalorimetry.The data of the amount of Cd sorpion(Aq) and concentration of Cd in equilibrium solution(Ce),and the data of Aq and the heat effect(AH) caused by Cd^2 sorption on goethite all fitted Langmuir isotherm.The amount of heat released from Cd sorption on goethite increased with the amount of Ce or Aq.The reaction process of Cd sorption on goethite may be divided into five stages and three plateaus,depending on the variation of enthalpy change(ΔaHm) of Cd sorption with Aq,which implied three mechanisms of interaction between Cd and goethite.The experimental results showed that the microcalorimetry may be useful for determination of microcalorie variation in soil.     

Models relating soil pH measurements in water and calcium chloride that incorporate electrolyte concentration

Soil pH is the most routinely measured soil property for assessing plant nutrient availability. Nevertheless, there are various techniques for soil pH measurement, which vary with regard to the solution used and the soil‐to‐solution ratio. Soil pH is commonly measured in water or 0.01 m CaC1₂. Soil pH in CaCl₂ is usually preferred as it is less affected by soil electrolyte concentration and provides a more consistent measurement. Therefore there is a need to convert measurement values between the two methods. Previous models reported linear and curvilinear relationships between the two measurements. However, the pH difference between measurements in water and CaCl₂ is related to the soil solution electrolyte concentration. We observed that the pH difference between the two methods became smaller with increasing soil electrical conductivity (EC). We therefore developed models that relate pH in CaCl₂ and water and incorporate EC values. We calibrated a linear and a non‐linear model (artificial neural networks, ANN) using 9817 soil samples from Queensland, Australia. Soil pH in water and CaCl₂ and EC were measured with a 1:5 soil‐to‐solution ratio. The results show that incorporating EC in the prediction model improves the prediction of pH in CaCl₂ significantly. We validated these models using 4576 independent samples obtained from a diverse range of soils across Australia. Although the linear and ANN models performed similarly, the ANN (which has a curvilinear relationship) provided a better prediction and aligns with the theory that for acid and alkaline pH values, the difference between pH in water and CaCl₂ is less than that for pHs between 4.5 and 7.     

Temperature,pH and photoperiod effects on mercury bioaccumulation by nymphs of the burrowing mayflyHexagenia rigida

Accumulation of HgCl₂ and CH₃HgCl byHexagenia rigida nymphs from contaminated sediment and water column was investigated experimentally, taking into account 3 abiotic factors (temperature, pH and photoperiod). When the contamination of the experimental units was based on sediment compartment, Hg concentrations at the whole organism level revealed very high bioaccumulation differences between the two chemical forms of Hg (ratio close to 20 in favour of MeHg). When Hg compounds were added to the water column, the highest Hg accumulation rates were observed for MeHg, but with a small difference between the 2 compounds (ratio close to 2.0–3.0). These bioaccumulation processes were very dependent on the 3 abiotic factors taken into account, especially temperature and water column pH.     

Cadmium soil sorption at low concentrations: I. Effect of time,cadmium load,pH, and calcium

Sorption of Cd at low concentrations onto two Danish soils (loamy sand, sandy loam) was examined in terms of kinetics and governing factors. From an environmental point of view soil sorption of Cd is a fast process: More than 95% of the sorption takes place within 10 min, equilibrium is reached in 1 hr, and exposures up to 67 wk did not reveal any long term changes in Cd sorption capacities. The soils have very high affinity for Cd at pH = 6.00 (10^?3 M CaCl₂) exhibiting distribution coefficients in the order of 200 to 250 (soil Cd concentration/solute Cd concentration). However, the sorption isotherms describing the distribution of Cd between soil and solute are slightly curvelinear. In the pH-interval 4 to 7.7, the sorption capacity of the soil approximately increases 3 times for a pH increase of one unit. Increasing the Ca concentration from 10^?3 to 10^?2 M reduces the sorption capacity of the sandy loam to one third.     

Effect of phosphate status and pH on sulphate sorption and desorption

For soils from tea estates in northern India, sulphate sorption was of a similar magnitude to, and sometimes exceeded, phosphate sorption. Only a small part of this relatively large sulphate sorption was caused by the low pH of these soils. Most was caused by increased negative charge as a result of prior reaction over many decades with phosphate fertilizers. This decreased sorption of both phosphate and sulphate, but the effect on phosphate was larger. This is compatible with a model in which the mean location of the charge on the adsorbed phosphate ions is closer to the surface than for sulphate. On soils of low phosphate status, sulphate desorption curves showed hysteresis; on soils of high phosphate status, they did not. Further, on soils of high phosphate status, displacement of sulphate by phosphate solutions was faster. We interpret these observations as showing that, for low phosphate status soils, sulphate ions penetrated the surface, but for high phosphate status soils it did not because the pathways by which sulphate diffuses into the adsorbing material were blocked. We also show that, with increasing soil phosphate status, phosphate solutions were less effective in displacing sorbed sulphate. We think this also occurred because reaction with phosphate had decreased the affinity for phosphate more than it decreased the affinity for sulphate.     

Sorptive stabilization of organic matter by microporous goethite: sorption into small pores vs. surface complexation

Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (Ø < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α‐FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X‐ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N₂ adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N₂‐accessible SSA of the goethite, mostly because micropores (Ø < 2 nm) were rendered inaccessible to N₂. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (Ø < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze‐dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes.     

Simultaneous nitrification and diffusion in soil. I. The effects of the addition of a low level of ammonium chloride

The nitrification of NH₄⁺ and the simultaneous diffusion of NH₄⁺, NO₃^? and H⁺ following the addition of ammonium chloride to a fine sandy loam soil was analysed experimentally and theoretically. Experimentally, the concentration profiles of mineral N and pH were analysed 140h and 284 h after the homogeneous addition of 11 μmoles NH₄Cl_cm^?3 of soil to one part of a composite soil column. The mathematical model presented includes a kinetic model of nitrifier growth and activity, the adsorption equilibria of NH₄⁺ and soil acidity with the soil solid phase and the influence of other ions on the diffusion characteristics of each diffusing ion. The predictions of the model were generated using parameters derived from independent experiments so that the predictions did not depend on data derived from the experimental concentration profiles. Good agreement was found between experimental and predicted profiles. The use of the model for predicting the penetration of NH₄⁺ and NO₃^? into the soil is demonstrated.     

Influence of organic matter and pH on bentazone sorption in soils

Bentazone (3-isopropyl-1H-2,1,3-benzonthiadiazain-(4)3H-one 2,2-dioxide) is a postemergence herbicide which is used extensively worldwide, especially in China. The sorption of bentazone in various types of soils and extracted humic acids was investigated using a batch equilibration technique. Significant linearity was observed in sorption isotherms in five different types of soil, with distribution coefficients (K(d)) that varied between 0.140 and 0.321 mL g(-1). The distribution coefficient was determined to be a function of organic matter and pH in the soil. A model based on distribution coefficients was developed to predict bentazone sorption in soils. The organic matter-normalized partition coefficients for the neutral and anionic forms of bentazone were 370.3 and 2.40 mL g(-1), respectively. Hence, more attention should be given to the potential leaching problem when bentazone is applied in soils containing low organic matter and high pH.