Environmental risk assessment of metals depends to a great extent on modeling the fate and the mobility of metals based on soil−liquid partitioning coefficients. A large variability is observed among the reported values that could be used to predict metal mobility and bioavailability. To evaluate this, soil−liquid partitioning coefficients (K d) for many elements but especially for the metals cadmium, copper, lead, nickel, and zinc were compiled from over 70 studies of various origins collected from the literature. The relationships between the reported values are explored relative to variations in soil solution pH, soil organic matter (SOM), and concentrations of total soil metal. The results of multiple linear regressions show that K d values are best predicted using empirical linear regressions with pH (with R  2 values of 0.29−0.58) or with pH and either the log of SOM or the log of total metal and with resulting R  2 values of 0.42−0.76. A semi-mechanistic model based on the competitive adsorption of metal and H+ [dependent on solution pH, total metal content, and log(SOM)] was a better tool to predict dissolved metal concentrations (with R  2 values of 0.61−0.88), with the exception of Pb (at 0.35).

We developed a semiempirical equation from metal complexation theory which relates the metal activity of soil solutions to the soil's pH, organic matter content (OM) and total metal content (MT). The equation has the general form:where pM is the negative logarithm (to base 10) of the metal activity, and a , b and c are constants. The equation successfully predicted free Cu2+ activity in soils with a wide range of properties, including soils previously treated with sewage sludge. The significant correlation of pCu to these measured soil properties in long-contaminated soils suggests that copper activity is controlled by adsorption on organic matter under steady state conditions. An attempt was made from separate published data to correlate total soluble Cu, Zn, Cd and Pb in soils to soil pH, organic matter content and total metal content. For Cu, the total Cu content of the soil was most highly correlated with total soluble Cu. Similarly, total soluble Zn and Cd were correlated with total metal content, but were more strongly related to soil pH than was soluble Cu. Smaller metal solubility in response to higher soil pH was most marked for Zn and Cd, metals that tend not to complex strongly with soluble organics. The organic matter content was often, but not always, a statistically significant variable in predicting metal solubility from soil properties. The solubility of Pb was less satisfactorily predicted from measured soil properties than solubility of the other metals. It seems that for Cu at least, solid organic matter limits free metal activity, whilst dissolved organic matter promotes metal solubility, in soils well-aged with respect to the metal pollutant. Although total metal content alone is not generally a good predictor of metal solubility or activity, it assumes great importance when comparing metal solubility in soils having similar pH and organic matter content.

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Seventeen soil samples from acidic forest soils (Typic and Pergelic Cryorthods) and near‐neutral agricultural soils (Typic Haplaquepts) were used in an experiment to compare three methods of determining cation exchange capacity (CEC). The methods tested are the sum of cations displaced with 0.1 M BaCl2, and exchangeable Ba after saturation with BaCl2 and replacement with either MgSO4 (compulsive exchange) or MgCl2. The results show that when Al, Mn, and Fe are included with Ca, Mg, Na, and K in the sum of cations, this method gives the same result as do the other methods. It is recommended that this simple one‐step method be used in routine analysis of acidic soils, especially when nonagricultural lands are under investigation.

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We report the soil solution speciation of Cd in 64 fieldcollected contaminated soils containing between 0.1 and 38 mg Cd kg -1 . The soils were analyzed for pH (3.5-8.1), soil organic matter (8.0-108 g C kg -1 ), total dissolved Cd (0.03-182 µg Cd L -1 ), dissolved organic carbon (1.5-12 mg C L -1 ), and free Cd 2+ (10 -10 -2 × 10 -7 M). Free Cd 2+ activity in solution was determined using differential pulse anodic stripping voltammetry (DPASV), assuming DPASV is sensitive to easily dissociated inorganic ion-pairs and free Cd 2+ while excluding organic complexes. The solid/liquid partition coefficient (K d ) varied over a range from 10 to 100 000, and the fraction of the dissolved Cd present in solution as the estimated free Cd 2+ species varied between 0 and 60% but averaged about 20%. The dissolved concentrations of Cd and the free Cd 2+ activity in the soil solutions of contaminated soils of different origins can be predicted with reasonable accuracy using a simple competitive adsorption model dependent on pH and total metal loading.

A review is made of soil toxicological data that report soil total lead or copper in combination with soil pH. Using these data, free metal activities in the soil solution of those tests are estimated using published regressions that describe the relationships between metal activities and the total metal content, pH, and soil organic matter content. The toxicological significance of the predicted free metal activities is compared to that of total metal contents. In a majority of cases, estimated free metal activity improves the prediction for toxic effects on crops, soil organisms, or soil microbial processes. The free metal activity level corresponding to a defined level of risk can be used to derive a pH-dependent soil criterion for total metal content that should generate a soil solution-free metal activity below the determined critical level.

The effects of adjusting the levels of soil organic matter in a Pbcontaminated soil on the solubility and free Pb 2+ speciation were studied within the pH range 3 to 8. A contaminated orchard soil containing 284 mg Pb kg" 1 was treated with leaf compost to increase soil organic matter and with H 2 O 2 to decrease it, yielding six soil organic matter levels between 25.6 and 83.7 g C kg" 1. The equilibrated solutions were then analyzed for total dissolved Pb by graphite furnace atomic absorption spectrometry and for labile Pb by differential pulse anodic stripping voltammetry (DPASV). The labile Pb values were used to calculate the free Pb 2 * activity based on the assumption that organo-Pb complexes are not DPASV-labile. The data showed that 30 to 50% of dissolved Pb is present as soluble OM complexes at low pH and up to 80 to 99% at near-neutral pH. The solubility of Pb shows a linear decrease from pH 3 to 6.5 and is independent of soil organic matter in that pH range. From pH 6.5 to 8, higher pH promotes the formation and dissolution of organo-Pb complexes, which increase Pb solubility. In this pH range, higher organic matter content results in higher concentrations of dissolved and labile Pb.