Field-based partition coefficients for trace elements in soil solutions

A total of 48 elements was detected in the soil solutions centrifuged from two acid sandy (humus-iron podzol) profiles from southern England. Concentrations ranged from mm for the major ions to mm for trace metals such as U and the rare earth elements. Field-based solid/solution partition coefficients, K_d, were determined by calculating the ratio of the amount of an element extracted by 0.43 m HNO₃ or a neutral salt (0.01 m CaCl₂ or 0.1 m Ba(NO₃)₂) to the concentration in the soil solution. These partition coefficients did not show the expected trend in selectivity. For example, Cd consistently had one of the highest K_d values, higher even than Cu. This was thought to be due in part to the nature of the K_d which reflects a balance between binding to the soil solids and to the dissolved organic carbon (DOC), which is present at relatively high concentrations (1–20 mm ) in the soil solutions. Because of the underlying functional similarity between metal binding by the solid and dissolved organic matter, the partition coefficient (and hence element mobility) will be relatively insensitive to changes in pH and metal-ion activity in the soil solution.     

Modelling phosphate-sorption kinetics in acid soils

A model of P-sorption kinetics was developed, that accounts for the dependency of reaction rate on concentration, sorbed P content, sorption maximum and time. The sorption phenomena predicted by the model agree with available observations in the literature. The model was tested with batch experiments for 17 acid top– and subsoils. The experiments revealed a significant correlation between the sorption maximum, F_m, and the sum of amorphous iron and aluminium in soils. Plotting the dimensionless P–sorption ratio, F(c, t)/F_m, which may be interpreted as the fractional saturation of the sorption capacity of a soil, against the natural logarithm of the exposure variable, I, gave S–shaped curves. Different parts of the S–shaped curve can be experimentally assessed, depending on the initial fractional saturation of the P–sorption capacity of soils. Apart from this dependency, one set of parameter values sufficed to describe the sorption kinetics of 10 different sandy top– and subsoils. For non–sandy soils, the parameter values differed and depended on the initial P content. The model enables extrapolation to long times, which is necessary for applications to field conditions.     

Interactions between citrate and montmorillonite-A1 hydroxide polymer systems

Citrate forms strong complexes with A1 ions and may thus influence the stability and formation of Clay-A1 hydroxide polymer systems (CAIHO). We studied the adsorption of citrate to CAIHO and the influence of citrate on the stability and formation of CAIHO at different A1:clay and A1:citrate ratios and pH values and at a total salt concentration of 0·01 M monovalent anions. The amount of citrate sorbed to the aged CAIHO was independent of the A1 fixed to the clay as A1 hydroxide polymers (AIHO) at 5 < pH < 6·6. The added citrate seemed not to sorb to the AIHO but only to the edges of the clay. As the citrate: Al ratio increased from 15–1:l at pH 6, more of the AIHO of the aged CAMO systems dissolved. The change in the CEC of the clay indicated that the nature of the remaining AIHO is independent of the amount of A1 dissolved. Citrate influenced the formation of CAIHO systems as measured at pH 6·6, to an extent which depended on the citrate: Al ratio. At a small ratio (1:5), AIHO formed and all citrate was incorporated in the AIHO, probably leading to a coprecipitate. The amount of citrate incorporated depended linearly on the amount of AIHO present. Co-precipitation of AIHO and citrate probably led to the formation of a separate phase, which was only weakly bound to the clay particles. At a large citrate: Al ratio (1:1) soluble Al-citrate complexes became dominant, and only a small part of the added A1 was present as AIHO.     

Availability to plants of phosphate adsorbed on goethite: experiment and simulation

A model has been developed to predict the availability of sorbed phosphate to plants. It is based on uptake by a growing root system. Phosphate sorption is assumed to be reversible and the rate of desorption is assumed not to limit uptake. The model was tested against observed availability of sorbed phosphate estimated from uptake by plants growing in phosphorus-deficient conditions. The plants were grown on quartz sand coated with goethite on which phosphate was adsorbed, and which was mixed with nutrient solution. The influence of the phosphate loading of goethite and the goethite content of the sand on available phosphate was examined. Phosphate uptake was predicted well by the model for treatments with high phosphate loading and different goethite content of the sand. However, for a treatment with low phosphate loading, uptake was underestimated by the model. In this treatment the pH of the 'soil’ solution decreased from 5·5 to 4·2 during the experiment. The phosphate concentration in solution increased with a lowering of the pH in this pH range, resulting in increased phosphate uptake.     

Comparison of different models for phosphate sorption as a function of the iron and aluminium oxides of soils

Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into zccount. Oxalate-extractable P, (P_ox), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Fe_ox) and aluminium (Al_ox). A significant relation between crystalline iron (Fe_d– Fe_ox) and total P sorption was not found. Reversibly adsorbed phosphate (P_i), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.