R.N.J. Comans’s research while affiliated with Wageningen University & Research and other places

The adsorption and desorption of boric acid onto reactive materials such as metal (hydr)oxides and natural organic matter are generally considered to be controlling processes for the leaching and bioavailability of boron (B). We studied the interaction of B with ferrihydrite, a nano-sized iron (hydr)oxide omnipresent in soil systems, using batch adsorption experiments at different pH values and in the presence of phosphate as competing anion. Surface speciation of B was described with a recently developed MUSIC (Multi-site ion complexation) and CD (Charge Distribution) approach. To gain insight into the B adsorption behavior in whole-soil systems, and in the relative contribution of Fh in particular, the pH-dependent B speciation has been evaluated for soils with representative amounts of ferrihydrite, goethite, and organic matter. The pH-dependent B adsorption envelope of ferrihydrite is bell-shaped with a maximum around pH 8–9. In agreement with spectroscopy, modeling suggests formation of a trigonal bidentate complex, and an additional outer-sphere complex at low to neutral pH values. At high pH, a tetrahedral bidentate surface species becomes important. In the presence of phosphate, B adsorption decreases strongly and only formation of the outer-sphere surface complex is relevant. The pH-dependent B adsorption to Fh is rather similar to that of goethite. Multi-surface modeling predicts that ferrihydrite may dominate the B binding in soils at low to neutral pH, and that the relative contribution of humic material increases significantly at neutral and alkaline pH conditions. This study identifies ferrihydrite and natural organic matter (i.e. humic substances) as the major constituents that control the B adsorption in top soils.

Knowledge on organic matter (OM) concentration and composition is of major importance for predicting Zn speciation and bioavailability in soils, especially for low-Zn soils. However, comprehensive knowledge on the effect of soil-like organic amendments such as compost on metal speciation is limited. For the first time multi-surface modeling is applied on compost to study the effect of solid and dissolved OM composition on the speciation of reactive Zn, as influenced by conditions applied in frequently used extractions to estimate Zn bioavailability. First, compost OM composition was determined by fractionation in operationally defined humic, fulvic and hydrophilic acid pools under various extraction conditions, and subsequently Zn speciation was modeled using the generic non-ideal competitive adsorption-Donnan (NICA-Donnan) model, in addition to adsorption to hydrous ferric oxide (HFO) and clay. The results show a strong effect of extraction conditions on OM concentration and composition, and related dissolved Zn speciation. Model predictions show that Zn in solution is mainly bound to dissolved humic acids. Analysis of deviations between measured and modeled Zn concentrations reveal specific limitations of the current generic parameters, particularly with regard to Zn binding to OM at low concentrations and Ca-Zn competition, i.e. typical conditions that occur in low-Zn soils.

Insights into the influence of redox conditions, that is the availability of electron acceptors, and dissolved organic matter (DOM) on pesticide biodegradation in groundwater are key to understanding the environmental fate of pesticides in natural groundwater systems. Here, the influence of redox conditions and supplemental DOM addition on biodegradation of pesticides, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,6-dichlorobenzamide (BAM), mecoprop-p (MCPP) and bentazone, was tested in microcosm and subsequent column experiments. Pesticide degradation, functional genes and changes in specific fractions and quantity of DOM were systematically quantified. In aerobic microcosm experiments, the highest 2,4-D degradation rate was obtained with the presence of more assimilable DOM. In column experiments, minimal pesticide degradation (≤33.77%) in any anaerobic redox conditions was observed in the absence of DOM. However, in the presence of DOM, 2,4-D biodegradation was considerably enhanced under nitrate-reducing conditions (from 23.5 ± 10.2% to 82.3 ± 11.6%) and in a column without external electron acceptor amendment (from −6.3 ± 12.6% to 31.1 ± 36.3%). Observed preferential depletion of the fulvic acid fraction of DOM provides indications for specific functional DOM properties. The qPCR results show an increase in microbial biomass and functional genes (tfdA) in liquid phase after DOM addition. The results of this work provide insights into the interplays among DOM, redox geochemistry, and pesticide biodegradation, and show the potential of a novel approach – DOM addition to groundwater systems – for in situ biostimulation technology to remove pesticides from groundwater systems.

Natural dissolved organic matter (DOM) and nitrification can play an important role in biodegradation of pharmaceutically active compounds (PhACs) in aerobic zones of constructed wetlands (CWs). This study used an enrichment culture originating from CW sediment to study the effect of DOM and nitrification on aerobic biodegradation of seven PhACs. The enriched culture degraded caffeine (CAF), metoprolol (MET), naproxen (NAP), and ibuprofen (IBP) with a consistent biodegradability order of CAF > MET > NAP > IBP. Biodegradation of propranolol, carbamazepine, and diclofenac was insignificant (<15%). CAF biodegradation was inhibited by the easily biodegradable DOM. Conversely, DOM enhanced biodegradation of MET, NAP, and IBP, potentially by contributing more biomass capable of degrading PhACs. Nitrification enhanced biodegradation of NAP and IBP and mineralization of the PhAC mixture as well as less biodegradable DOM, which may result from co-metabolism of ammonia oxidizing bacteria or enhanced heterotrophic microbial activity under nitrification. MET biodegradation was inhibited in the presence of nitrification. DOM and nitrification effects on PhAC biodegradation in CWs gained from this study can be used in strategies to improve CW operation, namely: designing hydraulic retention times based on the biodegradability order of specific PhACs; applying DOM amendment; and introducing consistent ammonium streams to increase removal of PhACs of interest.

The present study describes a possible way of establishing limit values for pollutants in waste-derived aggregates with a view of using such aggregates in a wide variety of construction projects. More specifically, the study focuses on aggregate substances that are subject to leaching and/or release through wear.

The present study describes a possible way of establishing limit values for pollutants in waste-derived aggregates with a view of using such aggregates in a wide variety of construction projects. More specifically, the study focuses on aggregate substances that are subject to leaching and/or release through wear.

Uptake of trace metals by crops is determined by the solubility of trace metals. In paddy soils, flooding and drainage influence redox chemistry and consequently trace metal solubility and thus uptake by rice plants. Current knowledge on how the dynamics in redox chemistry affect the solubility of trace metals in contaminated paddy soils is still limited. The objectives of our study were to investigate (i) the effects of flooding and drainage on trace metal solubility in paddy soils and (ii) to what extent a multi-surface modeling approach can predict trace metal solubility under changing redox conditions. We performed a column experiment with two contaminated paddy soils with similar soil properties but contrasting pH. During two successive flooding and drainage cycles, dynamics in Eh, pH and dissolved organic matter concentrations greatly affected trace metal solubility for both soils. Multi-surface model predictions indicate that under aerobic conditions, the higher pH of the alkaline soil leads to a stronger complexation of trace metals by reactive surfaces of the soil and, consequently, to lower dissolved concentrations than in the acidic soil. Under anaerobic conditions, predictions shows that sulfide precipitates control trace metal solubility in both soils, but still the higher pH of the alkaline soil leads to lower trace metal concentrations in soil solution at equilibrium. Furthermore, model calculations showed that stoichiometry and solubility of copper sulfide minerals can substantially affect solubility of other trace metals especially when trace element concentrations exceed soil sulfate concentrations. This stoichiometry and solubility should be considered when predicting the solubility of trace metals under anaerobic conditions.

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.

In deze verkennende studie is de uitloging van een aantal anorganische stoffen vanuit de bodem naar het grondwater onderzocht door gebruik te maken van een geochemisch rekenmodel. Het betreft de stoffen antimoon, arseen, barium, cadmium, chroom, kobalt, koper, kwik, lood, molybdeen, nikkel, tin, vanadium en zink. In dit onderzoek is de uitloging berekend uit de drie belangrijkste bodemtypen zand, klei en veen