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The arsenate (AsO4) and phosphate (PO4) mobility in aerobic soil is affected by soil organic matter (OM). This study was set up to quantify the interaction between OM and AsO4 with an observational, experimental and computational approach. The adsorption of AsO4 was measured with the radiotracer 73AsO4 in samples taken from different horizons of two soil profiles. In four samples, the OM concentration was increased experimentally. The AsO4 adsorption data were analysed with the CD-MUSIC model using ferrihydrite, with OM as competitor and isotopically exchangeable phosphorus of the soil as the total PO4 bound on the reactive surface. The solid–liquid distribution coefficient (KD) of 73AsO4 increased by more than two orders of magnitude with a decrease in total organic carbon (OC) concentration. The addition of Suwannee River OM (∼ 1 g OC kg-1) to samples with small OC (∼ 2 g kg-1) decreased the KD values 15-fold, whereas the effect was less (two-fold) in samples with a large OC (∼ 30 g kg-1). Soluble AsO4 and PO4 could be described well and simultaneously by introducing surface reactive OM (RO−) as an adjustable parameter in the geochemical model. The fitted RO− increased with increasing OC concentration, with a slope of 1.3 ± 0.15 mmol RO− g-1 OC. The amount of RO− expressed per mol iron (Fe) and aluminum (Al) hydroxides was maximum at a molar ratio of ∼ 0.34 at > 10 g OC kg-1 soil, which corresponds to earlier published capacities of an organo-mineral association that might affect potential soil C sequestration. Our research showed that OM enhanced the mobility of AsO4 and PO4; however, the surface reactive OM fraction needs experimental quantification. Highlights: Quantification of organic matter–arsenate competition in soil. Include organic matter in geochemical models to predict arsenate and phosphate mobility in soil. Surface reactive organic matter is determined by total organic carbon and Fe and Al hydroxides. Arsenate adsorption is controlled naturally by organic matter concentration.

Ferric ethylene diamine-N,N'-bis(hydroxy phenyl acetic acid) (FeEDDHA)-based iron (Fe) fertilizers are commonly applied to plants grown on calcareous soils and comprise a mixture of FeEDDHA components. Upon application to the soil, the pore water concentrations of the active ingredients racemic and meso o,o-FeEDDHA show a gradual decline unrelated to plant uptake or biodegradation. In the present study, the potential of soil copper (Cu) to reduce the effectiveness of FeEDDHA-based fertilizers in calcareous soils by displacing Fe from o,o-FeEDDHA has been evaluated through modelling and experiments. Predictions with mechanistic multi-surface models show that there is a thermodynamic basis for assuming that under equilibrium conditions a certain fraction of o,o-EDDHA ligands in soil solution can be chelated to Cu, in particular for meso o,o-EDDHA. The large affinity of o,o-CuEDDHA for binding to the soil solid phase, demonstrated in a batch interaction experiment, greatly increases the potential impact of Cu competition on the o,o-FeEDDHA solution concentration; for a given quantity of o,o-CuEDDHA in soil solution, a much larger quantity of o,o-CuEDDHA is adsorbed to the solid phase. Finally, evidence for the actual displacement of Fe from o,o-FeEDDHA by Cu was found in a soil incubation study. With these results, the boundary conditions are met for explaining the observed gradual decline in o,o-FeEDDHA concentration with Cu competition.

Phosphate (PO4) and organic matter (OM) compete for adsorption to metal (hydr)oxides. Our objective was to quantify the effect of OM on PO4 solubility in forest and arable soil by desorption experiments and surface complexation (SC) modelling. We sampled different types of soil along an age gradient (˜50–2500 years) and from different depths (0–80 cm). The soil types are calcareous and cover a range of soil organic carbon (SOC) contents (5.6–43.5 g kg-1), PO4 contents (0.2–5.9 mmol kg-1) and water-soluble PO4 concentrations (0.03–13.4 µm). Assuming that PO4 concentrations are controlled by desorption, PO4 concentrations were expected to correlate with the PO4 loading on metal-(hydr)oxide surfaces. However, we show that the PO4 loading alone is a poor predictor of PO4 solubility because its solubility increases with increasing SOC content. These data were explained by SC modelling, which shows a decrease in the apparent adsorption affinity of PO4 with increasing OM loading on to the metal (hydr)oxides. As a consequence, if the competition with OM is disregarded in SC modelling, it results in underestimation of the PO4 concentration by several orders of magnitude. For forest soil, predicted OM loadings increase slightly with increasing soil age. For arable soil, however, OM loadings were much smaller, which we explain by the replacement of PO4 with OM. Overall, adsorption interactions strongly affect PO4 solubility and levels of OM and PO4 stabilization in soil.

Humic acid is an important component of dissolved organic matter and in two previous papers it has been shown that purified Aldrich humic acid (PAHA) forms strong complexes with the oppositely charged protein lysozyme (LSZ). The complexation and aggregation of enzymes with humic acids may lead to changes in their activity. Therefore, the enzyme activity of LSZ in the PAHA – LSZ complex as a function of mass ratio PAHA:LSZ was investigated at two pH values (pH 5 and pH 8 where PAHA and LSZ are oppositely charged) and two ionic strengths (5 mmol l-1; 50 mmol l-1). The newly prepared complex was characterized and the mass ratios PAHA:LSZ at the isoelectric point (IEP) of the complex at the assayed conditions were obtained with the Mutek particle charge detector. The LSZ activity was measured with an assay specific for LSZ. The activity of LSZ decreased upon complex formation with PAHA and the PAHA effect on the enzyme activity was increased by the subsequent aggregation. A critical PAHA:LSZ mass ratio for the behaviour of the enzyme activity was that at the IEP of the complex. Before this mass ratio was reached the LSZ activity strongly decreased under all conditions. Beyond this mass ratio both pH and ionic strength affected the activity of LSZ in the complex. The results can be explained by the effects that pH and ionic strength have on (i) the electrostatic attraction between LSZ and PAHA in the complex and (ii) the electrostatic repulsion between the complexes. These two factors plus the hydrophobic attraction determine the structure of the complexes and the size of the aggregates and hence the screening of the active site of the protein.

Clay¿Al hydroxide polymers (CAlHO) can bind heavy metals effectively and may play an important role in the adsorption behaviour and metal binding capacity of soils. We studied the dependence of Al loading and pH on the adsorption of Zn on Na-saturated montmorillonite¿Al hydroxide polymer systems. The available binding sites on Al hydroxide polymers (AlHO) had a very strong affinity for Zn ions. Zinc binding on the clay surface became important when the binding sites on the AlHO were nearly all occupied. The pH had a very strong effect on the Zn binding. At pH 6.6 much more Zn could be adsorbed to the AlHO than at pH 5.0. The effect of the Al:clay ratio on Zn binding was influenced by pH. At pH 6.6, Zn binding to the AlHO, expressed per mole AlHO, was independent of the Al:clay ratio, whereas at pH 5.0 this relation was dependent. This is related to the constant charge of the AlHO at pH 6.6, whereas at pH 5.0 the charge decreases with increasing Al:clay ratio. If clay¿Al hydroxide polymers are present in the soil their Zn binding to the AlHO will strongly influence the availability of the Zn.

Al hydroxide polymers (AlHO) can significantly influence the cation exchange behaviour of clays. We have determined the effect of synthesized AlHO on Ca¿Na, Zn¿Na and Pb¿Na exchange for a series of exchanger compositions and two Al loadings at pH 6.0 and an ionic strength of 0.01 m. The preference for Ca on the siloxane surface of the clay¿AlHO system (CAlHO) was greater than for the pure clay, and the average KV (Vanselow selectivity coefficient) was determined to be 2.16 and 1.24, respectively. The selectivity coefficients for the exchange reactions Zn¿Na and Pb¿Na were not directly determined in CAlHO systems, because heavy-metal ions bind as well to the clay surface as to the AlHO over a wide range of pH. We have estimated the effect of the presence of AlHO on the selectivity coefficients of Zn¿Na and Pb¿Na exchange by extrapolation of the experimental results of Ca¿Na, Zn¿Na and Pb¿Na exchange for pure clay and Ca¿Na exchange for CAlHO. The average KV was increased by the presence of the AlHO from 1.23 to 2.16 for Zn¿Na exchange and from 1.59 to 2.77 for Pb¿Na exchange. The increase in the preference for the divalent cations is probably caused by parallel alignment of clay platelets by sorption of AlHO. Increasing the amount of AlHO did not change the selectivity for Ca¿Na exchange, and probably the structure of the system or the arrangement of the clay platelets and AlHO particles was not substantially changed. This was supported by the linear reduction of the cation exchange capacity with amount of AlHO present at pH 6.6. It seems likely that the selectivity coefficients for Ca¿Na, Zn¿Na and Pb¿Na exchange that we found apply in naturally occurring montmorillonite¿AlHO systems.

The coagulation of humic substances and its role in controlling the solubility of organic matter in soils are not well understood. We therefore studied the physico-chemical behaviour of purified humic acid from forest soil coagulated with Na, Ca, Cu, Al at pH 4 and 6, and then modelled the behaviour with the Non-Ideal Consistent Competitive Adsorption Dorman (NICA-Donnan) model. We found that the coagulation of humic acid occurs when the Dorman potential is less negative than -0.08 V. Based on this result, an empirical relation between the Dorman potential of humic acid and its concentration in solution was derived. In addition, the Dorman potential of the dissolved organic matter in the soil solution of six soil profiles from forests was calculated using the NICA-Dorman model under the assumption that all the dissolved organic matter behaves as humic acid. The measured concentration of dissolved organic matter also decreases in a soil profile, as the calculated potential becomes less negative. The results are in many cases in semi-quantitative agreement with the predicted concentration based on the humic acid coagulation experiment. Acid soils contain more dissolved organic matter, which may result from the presence of a fairly large fraction of more soluble organic molecules, such as fulvic acid.