Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications – a review

Prediction of the fate of metals in soil requires knowledge of their solid–liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid–liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or K_d, which gives the ratio of the concentration in the solid phase to that in the solution phase. The K_d value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The K_d value can be derived from the solid–liquid distribution of added metal or that of the soil‐borne metal. Only part of the solid‐phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ‘labile’ phase, and K_d values based on this phase often correlate better with soil properties than K_d values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for K_d determinations. Alternatively, water or dilute‐salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal‐complexing ligands such as dissolved organic matter. Multi‐surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, K_d values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The K_d coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non‐labile pools in the solid phase may invalidate this equilibrium assumption.     

Transfer functions for solid-solution partitioning of cadmium, copper, nickel, lead and zinc in soils: derivation of relationships for free metal ion activities and validation with independent data

Models to predict the solid-solution partitioning of trace metals are important tools in risk assessment, providing information on the biological availability of metals and their leaching. Empirically based models, or transfer functions, published to date differ with respect to the mathematical model used, the optimization method, the methods used to determine metal concentrations in the solid and solution phases and the soil properties accounted for. Here we review these methodological aspects before deriving our own transfer functions that relate free metal ion activities to reactive metal contents in the solid phase. One single function was able to predict free-metal ion activities estimated by a variety of soil solution extraction methods. Evaluation of the mathematical formulation showed that transfer functions derived to optimize the Freundlich adsorption constant ( K _f ), in contrast to functions derived to optimize either the solid or solution concentration, were most suitable for predicting concentrations in solution from solid phase concentrations and vice versa . The model was shown to be generally applicable on the basis of a large number of independent data, for which predicted free metal activities were within one order of magnitude of the observations. The model only over-estimated free-metal ion activities at alkaline pH (>7). The use of the reactive metal content measured by 0.43 m HNO₃ rather than the total metal content resulted in a close correlation with measured data, particularly for nickel and zinc.     

Retention of Cd, Cu, Pb and Zn by Wood Ash, Lime and Fume Dust

Heavy metals are of interest due to their deleterious impacts on both human and ecosystem health. This study investigated the effectiveness of wood ash in immobilizing the heavy metals Pb, Cd, Cu and Zn from aqueous solutions. The effects of initial metal concentrations, solution pH, ash dose and reaction time on metal sorption, as well as the metal sorption mechanisms were studied. To investigate the effect of initial metal concentrations, solutions containing Cd, Zn (25, 50, 75, 100 or 125 mg L^?1), Cu (25, 50, 75, 100, 125, 150 or 175 mg L^?1) or Pb (250, 500, 750, 1000, 1250, or 1500 mg L^?1) were reacted with 10 g L^?1 ash for two hours. For the effect of pH, solutions containing 100 mg L^?1 of Cd, Cu or Zn or 1500 mg L^?1 of Pb were reacted with 15 g L^?1 ash over a pH range of 4 to 7. The wood ash was effective in immobilizing the four metals with a sorption range of 41–100 %. The amounts of metals retained by the ash followed the order of Pb > Cu > Cd > Zn. As expected, absolute metal retention increased with increasing initial metal concentrations, solution pH and ash dose. Metal retention by the ash exhibited a two-phase step: an initial rapid uptake of the metal followed by a period of relatively slow removal of metal from solution. Metal retention by the ash could be described by the Langmuir and Freundlich isotherms, with the latter providing a better fit for the data. Dissolution of calcite /gypsum minerals and precipitation of metal carbonate/sulfate like minerals were probably responsible for metal immobilization by the ash in addition to adsorption.     

Effects of Soil Sample Storage Treatment on the Composition and Fe,Al, and Mn Speciation of Soil Solutions Obtained by Centrifugation

Soil solution chemistry is a powerful tool for studying many aspects of soil science. Among several isolation techniques, centrifugation appears most promising as a method of extracting the soil solution in the laboratory. However, some operational conditions must be defined. The present work reports the influence of sample storage on the observed composition of the soil solution of two Brazilian soils submitted to different managements. Since metal speciation in soil solution significantly influences metal bioavailability, a second experiment was conducted to evaluate the effects of storage on Fe, Al, and Mn speciation by size exclusion chromatography (HPLC-SEC). The results showed that the effects of soil handling prior to solution extraction had a significant effect on soil solution composition, mainly when the sample was dried and rewetted. Only the samples that were kept refrigerated (4 °C) for 15 days led to results comparable to those obtained from fresh soils. However, considering the patterns of the UV detection chromatograms and metal distribution, only field moist samples should be used in studies related to Al, Mn, and Fe speciation in the studied soils.     

The effect of pH on the retention of Cu,Pb, Cd,and Zn by clay-fulvic acid mixtures

The addition of fulvic acid to clay suspensions (kaolinite, illite or montmorrillonite) resulted in increased uptake of Cu, Pb, Cd, and Zn ions over the pH range 3 to 6, due to the limited solubility of one of the metal-fulvate species formed. At higher pH values, residual metal ion was retained in solution, instead of precipitating as hydroxy species. The amount of total metal ion found in solution at equilibrium was determined by the quantity and type of clay added; the amount of organic acid present; and to a lesser extent, pH. The behavior of the clay-fulvic acid systems differed from that observed using other organic materials such as gelatine, tannic acid or a humic acid.     

Dissolution kinetics of iron-, manganese-, and copper-containing synthetic hydroxyapatites

Micronutrient-substituted synthetic hydroxyapatite (SHA) is being evaluated by the National Aeronautics and Space Administration's (NASA) Advanced Life Support (ALS) Program for crop production on long-duration human missions to the International Space Station or for future Lunar or Martian outposts. The stirred-flow technique was utilized to characterize Ca, P, Fe, Mn, and Cu release characteristics from Fe-, Mn-, and Cu-containing SHA in deionized (DI) water, citric acid, and diethylene-triamine-pentaacetic acid (DTPA). Initially, Ca and P release rates decreased rapidly with time and were controlled by a non-SHA calcium phosphate phase(s) with low Ca/P solution molar ratios (0.91-1.51) relative to solid SHA ratios (1.56-1.64). At later times, Ca/P solution molar ratios (1.47-1.79) were near solid SHA ratios and release rates decreased slowly indicating that SHA controlled Ca and P release. Substituted SHA materials had faster dissolution rates relative to unsubstituted SHA. The initial metal release rate order was Mn > Cu > Fe which followed metal-oxide/phosphate solubility suggesting that poorly crystalline metal-oxides/phosphates were dominating metal release. Similar metal release rates for all substituted SHA (approximately 0.01 cmol kg-1 min-1) at the end of the DTPA experiment indicated that SHA dissolution was supplying the metals into solution and that poorly crystalline metal-oxide/phosphates were not controlling metal release. Results indicate that non-SHA Ca-phosphate phases and poorly crystalline metal-oxide/phosphates will contribute Ca, P, and metals. After these phases have dissolved, substituted SHA will be the source of Ca, P, and metals for plants.     

Adsorption density,mobility and limit values of Cd,Zn, Cu and Pb in acid forest soils

To investigate Cd, Zn, Cu and Pb adsorption in acidified forest soils, six soil samples of the aluminium buffer range were selected and analyzed for their physical and chemical properties. Determination of the specific surface area using ethylene glycol monoethyl ether (EGME) adsorption yielded a characteristic value of the solid phases, which can parameterize the major properties of the various soil constituents with sufficient accuracy.

Traditional adsorption isotherms reveal the relation between the amount of a heavy metal adsorbed and the heavy metal concentration in the soil solution only for the soil under study and can therefore not be applied to other soils. To meet the aim of modelling heavy metal adsorption and mobility also for soils differing greatly in their properties, it was attempted to establish a generalizing adsorption isotherm for soils of entirely different composition of the solid phase. The generalizing adsorption density isotherms introduced in the following provide a useful mathematical model for the quantity/intensity relation of heavy metals in soils that differ greatly in their specific surface area and their composition.

It is also shown that limit values which take into account the major quantities influencing heavy metal adsorption and mobility in acid soils can be established from the regression equation between the adsorption density of a heavy metal (ions/m² specific surface area) and its concentration in the soil solution. In particular in view of the groundwater contamination to be expected if acid rain and, as a result, soil acidification continues, these limit values seem to provide considerably more information than the European limit values, given in mg heavy metal /kg soil, which are presently valid for any soil condition and property.     

Effect of organic acids on the adsorption of copper,lead, and zinc by goethite

The adsorption of Cu, Pb, and Zn by synthetic goethite was studied in the absence and presence of oxalic, citric, and glutamic acids at different pH values. It was shown that, in the absence of an acid, the content of adsorbed metals increased with the increasing pH. The content of adsorbed cations at constant pH values decreased in the sequence: Cu > Pb > Zn. The simultaneous addition of metal cations and organic acids to the goethite suspension increased the content of the adsorbed elements. The oxalic and citric acids had similar effects on the adsorption of copper and lead in the studied pH range. The metal: acid concentration ratios significantly affected the adsorption of the heavy metals by goethite. An increase in the metal adsorption was observed to a certain metal: acid ratio, which was followed by a gradual decrease. The adsorption of the metals by goethite also depended on the properties of the metal cations and the organic ligands. The observed tendencies were attributed to the complexation of heavy metals with organic acid anions and the simultaneous sorption of acids at positively charged sites on the goethite surface with the formation of mineral-organic compounds, which significantly modified the surface properties of the mineral. The study of the effect of increasing lead concentrations in the solution on the copper adsorption by goethite in the absence, in the presence, and at the addition of an oxalic acid solution to the goethite suspension one hour before the beginning of the experiment showed that lead decreased the adsorption of copper in all the treatments. The possible mechanisms of the processes occurring in the system were considered.     

Toxicity interaction of metals (Ag, Cu, Hg, Zn) to urease and dehydrogenase activities in soils

A study of the individual and mixture toxicities of the trace metals Ag, Cu, Hg, Zn to the soil enzymes dehydrogenase and urease was undertaken. An agricultural soil and a sandy forest soil were spiked with metal salts, individually and in combinations. The anion additions to the various treatments and controls were normalized for added anions using Ca salts. The soils were then left to equilibrate and leached to reduce the excess metals and anions. Total and dissolved metal concentrations were measured concurrently in order to consider the effect of soil chemistry on the enzyme activities. Dose-response relationships for total soil metals and soil solution metals were estimated for each metal separately following a log-logistic curve fitting. Ag and Hg were the most efficient metals to reduce soil enzyme activities. The Bliss independence model was used to predict the toxicity of metal combinations. The enzyme responses in relation to the total soil metal combinations were synergistic for the agricultural soil and antagonistic for the forest soil; possibly as a result of a higher organic matter content and higher pH in the latter soil. Enzyme activities expressed in relation to the dissolved metal concentrations were more variable than against the total metal contents and consequently we observed both synergistic and antagonistic interactions.     

Comparative effectiveness of selected adsorbant materials as potential amendments for the remediation of lead-, copper- and zinc-contaminated soil

Bonemeal, coir, compost, green waste compost, peat and wood bark all potentially could be used as amendments to remediate heavy metal contamination in soils. Their ability to sorb Pb, Cu and Zn was evaluated in the laboratory, using metal solutions ranging from 0 to 5 mmol/L as contaminants. The effects on sorption of metal concentration, background salt concentration and metal competition were evaluated. Single metal sorption by the six amendments was significantly different at metal concentrations of 1.5–5 mmol/L, with green waste compost, coir, compost and wood bark having the highest capacities to adsorb Pb, Cu and Zn. Langmuir sorption maxima were approximately 87 mg Pb/g (coir and green waste compost), 30 mg Cu/g (compost and green waste compost) and 13 mg Zn/g (compost and green waste compost) (equivalent to approx. 0.5 mmol/g of Pb and Cu, and 0.2 mmol/g Zn), all in a background solution of 0.001 M Ca(NO₃)₂. A higher background salt concentration and a combination of all three metals led to significant reduction in the amounts of Pb, Cu and Zn sorbed by all the amendments tested. Competing heavy metal cations in solution decreased Pb sorption to about 50–60% of that from a solution containing Pb alone; Cu sorption was reduced to about 30–40%; the effect of competition on Zn sorption was variable. Overall, in both single metal and competitive sorption, the order of strength of binding was Pb>Cu>>Zn.     

General purpose Freundlich isotherms for cadmium,copper and zinc in soils

Assessing the accumulation and transport of trace metals in soils and the associated toxicological risks on a national scale requires generally applicable sorption equations. Therefore Freundlich equations were derived for Cd, Zn and Cu using multiple linear regression on batch sorption data from the literature with a wide variety of soil and experimental characteristics, and metal concentrations ranging over five orders of magnitude. Equations were derived based on both total dissolved metal concentrations and free metal activities in solution. Free metal activities were calculated from total metal concentrations taking into account ionic activity, and inorganic (all metals) and organic complexation (Cu only). Cadmium and Zn were present in solution predominantly as free ions, while Cu was present as organic complexes. Since actual dissolved organic carbon (DOC) concentrations were not available they were estimated using an empirical field relation between DOC and organic matter content. The logarithmic transformation of the Freundlich constant for Cd was regressed on the logarithmic transformations of cation exchange capacity (CEC) (H⁺) and dissolved Ca, and for Zn with CEC and (H⁺). For Cu the log–log regression model of the Freundlich constant included the solid:solution ratio of the batch to account for dilution of DOC in the batch as compared with the field. The explained variance for the fitted Freundlich equations was 79% for Cd, 65% for Cu and 83% for Zn, using log-transformed adsorbed concentrations and soil solution activities. The Freundlich adsorption models underestimated metal contents determined from 1 m HNO₃ digestion on field samples, up to a factor of 6 (Cd and Cu) or 10 (Zn).     

Mobilization of dissolved organic matter, aluminium and iron in podzol eluvial horizons as affected by formation of metal-organic complexes and interactions with solid soil material

Interactions with dissolved organic matter (DOM) are generally believed to play a crucial role in the translocation of Al and Fe in acid sandy soils. Binding of Al and Fe to DOM affects their mobility in soils by altering sorption equilibria of charged sites on solid soil material, inducing precipitation of organo‐metallic complexes and preventing the formation of inorganic Al and Fe phases. The relative importance of the different processes, especially with respect to the translocation of Al, Fe and organic matter in podzols, remains unresolved. We determined the effect of the presence of solid soil material from the eluvial (AhE and AE, respectively) horizons of a Fimic Anthrosol and a Haplic Podzol on the metal‐to‐organic carbon (M/C) ratio in solution and the formation of dissolved organic Al and Fe complexes. Furthermore, we assessed the resulting influence on the mobilization of Al, Fe and DOM. Even under considerable metal loading, the M/C ratios and ‘free’ metal fractions in solution remained low and relatively constant, due to an apparent buffering by the solid phase and the formation of organo‐metal complexes in solution. The M/C ratios remained so low that significant precipitation of organo‐metal complexes due to saturation with metals was not found. The apparent buffering by the solid phase can be explained by a strong release of organic matter from solid soil material and adsorption of non‐complexed Al and Fe on solid organic matter upon metal addition. Adsorption of organo‐metal complexes most likely played only a minor role. The observations confirm the expected mobilization of Al, Fe and DOM in eluvial horizons and seem to indicate that even under fluctuating input of Al, Fe and DOM the soil solution will have a constant composition with respect to M/C ratios and percentage of Al and Fe present in dissolved organo‐metal complexes.     

THE SPECIFIC ADSORPTION OF DIVALENT Cd, Co, Cu, Pb, AND Zn ON GOETHITE

The specific adsorption of divalent Cd, Co, Cu, Pb, and Zn on goethite is measured as a function of pH. For each mole of heavy metal adsorbed approximately two moles of H+ ions are displaced from the interface. Using these results the heavy metal adsorption data are expressed as functions of the solution concentrations of both H+ and metal ions, and the interfacial reaction is described by the equation, ₂SH+M²⁺= S₂M+₂H⁺. The adsorption data are consistent with an electrochemical model of the simultaneous adsorption of H⁺ ions and divalent metal ions on to the oxide. The intrinsic affinities of the metal ions for the oxide surface increase in the order, Cd < Co < Zn < Pb < Cu. However, besides the affinity of the metal ion for the surface, the adsorption curves are considered to be influenced by surface charge, the adsorption density of the metal ions and their size. The analysis of the data in terms of H⁺ and M²⁺ ion adsorption is considered to be complementary to the hydrolysis model for heavy metal adsorption.     

水分及干燥过程对土壤重金属有效性的影响

土壤水分变化可显著改变土壤性质进而影响土壤重金属有效性。本试验通过测定土壤溶液和采用薄层凝胶梯度法(DGT)表征的Zn、Cd、Cu、Ni浓度,研究土壤含水量变化对重金属有效性的影响。结果表明,不同水分处理显著影响土壤溶液中可溶性有机碳(DOC)含量和土壤中重金属的有效性;随土壤水分降低DGT表征的Zn、Cd、Cu和Ni浓度和土壤溶液中Cu和Ni浓度呈下降趋势,且随干湿交替次数增加而降低;与长期风干土壤相比,经干湿交替后风干土壤重金属有效性降低或显著降低;与土壤溶液法相比,DGT法能反映水分变化对土壤固相金属缓冲补给能力的影响,能更好地表征土壤金属有效性的变化。在农业生产中可通过适当水分管理措施降低重金属的有效性,从而缓解重金属的毒害作用。     

Trivalent metal (Cr, Y, Rh, La, Pr, Gd) sorption in two acid soils and its consequences for bioremediation

The recent addition of trivalent metals to soil and their subsequent movement within the biosphere are of concern. For this reason, the sorption of chromium (Cr), yttrium (Y). rhodium (Rh), lanthanum (La), praseodymium (Pr) and gadolinium (Gd) in two contrasting acid soils has been determined. Except for Rh, the sorption of the other trivalent metals conformed well to the Langmuir equation with derived sorption parameters similar for all the trivalent species tested. Calculation of the buffer powers indicated that under both small (0·01 mmol kg^?1) and large (1 mmol kg^?1) trivalent metal soil loadings > 99·5% of the metals will be associated with the exchange phase with small quantities present in the bulk soil solution (<0·5%). It seems that the slight availability of metals within the bulk soil solution will slow the rate of trivalent metal bioremediation of contaminated sites.     

Predicting the solubility of Cd,Cu, Pb and Zn in uncontaminated Croatian soils under different land uses by applying established regression models

Soil pH, soil organic matter (SOM), dissolved organic carbon (DOC) and total trace metal concentration (M_(tot)) control the solubility of metals in the soil. Several regression models have included these soil chemical variables for the prediction of metal solubility and free metal ion (FMI) concentrations in contaminated soils. We hypothesize that models developed on contaminated soils (after optimization of the coefficients) can be used on samples from uncontaminated sites. Soil samples were collected from unpolluted agricultural and forest soils located in Eastern Croatia and extracted with water to determine the concentrations of Cd, Cu, Pb and Zn. We used these data to test the applicability of three regression models on existing conditions under different land uses. The same predictors issued in the three models and the same regression coefficients were utilized in the present study. The results showed a good correlation between the observed and predicted values of metal solubility. However, the models overestimate the total solution concentration (M_(sol)) and the concentrations of free metal ions (FMI) in solution, and therefore the same regression coefficients were optimized to fit our own observations. This was found to be very successful. The results showed that pH and DOC played a very important role in controlling metal solubility, while SOM and CEC were somewhat less significant. The impact of total soil concentration of metals (M_(tot)) was rather minor. However, we feel that to carry out good predictions of M_(sol) and FMI, the M_(tot) is needed in such regression models.     

Simultaneous Sorption of Cd,Cu, Ni,Zn, Pb,and Cr on Soils Treated with Sewage Sludge Supernatant

Disposal of sewage sludge creates the potential for heavy metal accumulation in theenvironment. This study assessed nine soils currently used as Dedicated Land Disposal units(DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento,California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studiedby simultaneously mixing these elements in the range of 0-50 µmol L-1 with sludgesupernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH = 4.5 or 6.5, andconstant ionic strength (0.01 M Na-acetate). The concentration of metals in the supernatant wasdetermined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorptionwas linearly related to its concentration in the supernatant solution. The distribution coefficientKd (Kd = concentration on solid phase/concentration in solution phase) was computed as theslope of the sorption isotherm. The distribution coefficients were significantly correlated to soilorganic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5.There was also a correlation between Kd and soil specific surface area but no relationship to othersoil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soilorganic carbon and surface area appear to be the most important soil properties influencing metaladsorption through formation of organo-metal complexes. The Kd values for all elements werehigher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinitiesbased on their Kd values: Pb>Cu>Zn>Ni>Cd≈Cr at pH 4.5 andPb>Cu≈Zn>Cd>Ni>Cr at pH 6.5.     

根迹土壤根诱导的化学变化对植物吸收重金属的影响

It is increasingly recognized that metal bioavailability is a better indicator of the potential for phytoremediation than the total metal concentration in soils; therefore, an understanding of the inffuence of phytoremediation plants on metal dynamics at the soil-root interface is increasingly vital for the successful implementation of this remediation technique. In this study, we investigated the heavy metal and soil solution chemical changes at field moisture, after growth of either Indian mustard (Brassica juncea) or sunffower (Helianthus annuus L.), in long-term contaminated soils and the subsequent metal uptake by the selected plants. In addition, the fractions of free metal ions in soil solution were determined using the Donnan membrane technique. After plant growth soil solution pH increased by 0.2-1.4 units and dissolved organic carbon (DOC) increased by 1-99 mg L^-1 in all soils examined. Soluble Cd and Zn decreased after Indian mustard growth in all soils examined, and this was attributed to increases in soil solution pH (by 0.9 units) after plant growth. Concentrations of soluble Cu and Pb decreased in acidic soils but increased in alkaline soils. This discrepancy was likely due to a competitive effect between plant-induced pH and DOC changes on the magnitude of metal solubility. The fractions of free Cd and Zn ranged from 7.2% to 32% and 6.4% to 73%, respectively, and they generally decreased as pH and DOC increased after plant growth. Metal uptake by plants was dependant on the soil solution metal concentration, which was governed by changes in pH and DOC induced by plant exudates, rather than on the total metal concentrations. Although plant uptake also varied with metal and soil types, overall soluble metal concentrations in the rhizosphere were mainly inffuenced by root-induced changes in pH and DOC which subsequently affected the metal uptake by plants.     

Applications of the continuous-flow stirred-cell (CFSC) technique: II. The adsorption behaviour of Na, Cs, Sr, Cu, Ni and Pb on humic acids

The first part of this paper deals with the sorption of Na, Cs, Sr, Ni, Cu and Pb on two humic fractions derived from a soil. The sorption data for individual elements were obtained using the continuous-flow stirred-cell (CFSC) technique, in which the metal sorptive solution is pumped through a cell containing a known mass of the humic acid retained by a filter and the solution emerging from the cell was analysed by either atomic absorption spectrometry (Cu, Ni and Pb) or by the combined use of radioisotope dilution and gamma-ray spectroscopy (Na, Cs and Sr). Sorption isotherms were determined at two flow rates. It was found that, in general, the sorption isotherms for all the metals studied could be described by the Langmuir equation and parameters derived from the fitted line provided information on the metal-humate binding strengths and the maximum amount of metal sorbed per unit mass of humic acid. Caesium showed anomalous behaviour at certain flow rates in both its sorption and desorption behaviour. In the second part of the paper we discuss the sequential sorption and desorption of one metal by another on the same humic fraction, using Na, Cs, Zn and Cd. The aim was to elucidate the nature of the binding process when several metals were competing simultaneously for the functional groups on the humic acids. In addition, at the end of the experiment, the humic material was isolated and analysed by gamma-ray spectroscopy to determine the metals still bound in this fraction. Some tentative conclusions concerning the nature of the metal-humate complexes are made on the basis of these data.     

Simple method for distinguishing maneb, zineb, mancozeb, and selected mixtures

Maneb, zineb, mancozeb, and arbitrarily selected mixtures of those can be differentiated by a simple method. Compounds are differentiated on the basis of colors produced after treatment of saturated solutions of the fungicides in n-propanol-acetone mixture (1 + 1 v/v), first with dithizone and then with monosodium dihydrogen phosphate solution in the same solvent. The color depends on the type and concentration of metal present in the sample.