Philippe Cambier

Influence of Reducing Conditions on Solubility of Trace Metals in Contaminated Soils

Dissolved trace metals Cd, Pb, Zn, andother solutes were determined after reducingconditions have been imposed to samples of anagricultural polluted soil. The ploughed layer wassampled as undisturbed blocks for floodingexperiments, and sieved samples were incubated inaqueous suspensions at controlled pH (pH 6.2) underdifferent redox conditions. Redox potential and pH,concentrations of major and trace elements, andorganic and inorganic ligands, were measured in thesolutions. Their chemical speciation was calculated byusing the programme Soilchem.These experiments and calculations have shown that pHvariations definitively influence trace metalsolubility, whatever they are induced by reductivedissolution, organic acid formation, or otherprocesses, and that strong acidification can beobtained with well buffered soil when about 1%available carbon is anaerobically transformed intoorganic acids. The organic acids also intervene bycomplexation, particularly for Pb. On another hand,denitrification can limit these effects by consumingprotons and organic substances. Given a steady pH,reducing conditions enhance the mobility of tracemetals, at first by dissolution of manganic and ferricoxides; Pb appeared more sensitive to these processesthan Zn, and finally Cd. As a general rule,hydromorphy in a well-buffered contaminated soil at afirst step should increase the mobility of divalenttrace metals, by decreasing pH and reducing Mn and Feoxides, but prolonged flooding can lead to fix tracemetals again, rather by re-adsorption or precipitationphenomena than by formation of insoluble sulphides.

Experimental soil acidification

Abstract A laboratory experiment was designed to investigate the interaction of acid mine drainage (AMD) with soils. The study used 3 undisturbed cambisol columns from the mining region of Banska Stiavnica, Sobov (Slovakia). Columns were acidified with AMD emerging from the mine wastes rich in pyrite, with pure H2SO4 solutions of equivalent concentration (0.15 M H2SO4) or with distilled water. The experiment was performed for 30 weeks. The pH of H2SO4 leachates decreased from 4.1 to 3.3 and was accompanied by increasing amounts of dissolved Al. Progressive acidification increased leaching of major cations Ca, Mg, and K. Acidification of the soil with AMD produced leachates with a pH close to 3 and a high content of different cations and sulphates. Almost 100% of the Fe and 25% of the Al added with AMD were retained in the soil column. Leaching of alkali-earth and alkali cations from the soil by AMD was much lower than by H2SO4 solution. Analyses of solid samples from the 3 columns used selective extractions, XRD and analytical electron microscopy. These showed the different formations and transformations of minerals in acidified columns: amorphous Fe-oxyhydroxides were formed in both AMD and H2SO4 leached soils in the form of a characteristic “mohair” structure rich in Al and Si when AMD was applied. Clay minerals were hardly affected in these samples, in contrast with samples from the H2SO4 acidified column where dissolution of vermiculite, transformation of chlorite, and precipitation of Si gel were observed. Thick Fe-coatings on particles and within the pores of soils leached with AMD change their micro-aggregation and so inhibit the dissolution of phyllosilicates.

Impact of long-term organic residue recycling in agriculture on soil solution composition and trace metal leaching in soils.

Recycling composted organic residues in agriculture can reduce the need of mineral fertilizers and improve the physicochemical and biological properties of cultivated soils. However, some trace elements may accumulate in soils following repeated applications and impact other compartments of the agrosystems. This study aims at evaluating the long-term impact of such practices on the composition of soil leaching water, especially on trace metal concentrations. The field experiment QualiAgro started in 1998 on typical loess Luvisol of the Paris Basin, with a maize-wheat crop succession and five modalities: spreading of three different urban waste composts, farmyard manure (FYM), and no organic amendment (CTR). Inputs of trace metals have been close to regulatory limits, but supplies of organic matter and nitrogen overpassed common practices. Soil solutions were collected from wick lysimeters at 45 and 100 cm in one plot for each modality, during two drainage periods after the last spreading. Despite wide temporal variations, a significant effect of treatments on major solutes appears at 45 cm: DOC, Ca, K, Mg, Na, nitrate, sulphate and chloride concentrations were higher in most amended plots compared to CTR. Cu concentrations were also significantly higher in leachates of amended plots compared to CTR, whereas no clear effect emerged for Zn. The influence of amendments on solute concentrations appeared weaker at 1 m than at 45 cm, but still significant and positive for major anions and DOC. Average concentrations of Cu and Zn at 1m depth lied in the ranges [2.5; 3.8] and [2.5; 10.5 μg/L], respectively, with values slightly higher for plots amended with sewage sludge compost or FYM than for CTR. However, leaching of both metals was less than 1% of their respective inputs through organic amendments. For Cd, most values were <0.05 μg/L. So, metals added through spreading of compost or manure during 14 years may have increased metal concentrations in leachates of amended plots, in spite of increased soil organic matter, factor of metal retention. Indeed, DOC, also increased by amendments, favours the mobility of Cu; whereas pH variations, depending on treatments, influence negatively the solubility of Zn. Generic adsorption functions of these variables partly explain the variations of trace metal concentrations and helped to unravel the numerous processes induced by regular amendments with organic waste products.

Simultaneous Simulations of Uptake in Plants and Leaching to Groundwater of Cadmium and Lead for Arable Land Amended with Compost or Farmyard Manure

The water budget of soil, the uptake in plants and the leaching to groundwater of cadmium (Cd) and lead (Pb) were simulated simultaneously using a physiological plant uptake model and a tipping buckets water and solute transport model for soil. Simulations were compared to results from a ten-year experimental field study, where four organic amendments were applied every second year. Predicted concentrations slightly decreased (Cd) or stagnated (Pb) in control soils, but increased in amended soils by about 10% (Cd) and 6% to 18% (Pb). Estimated plant uptake was lower in amended plots, due to an increase of Kd (dry soil to water partition coefficient). Predicted concentrations in plants were close to measured levels in plant residues (straw), but higher than measured concentrations in grains. Initially, Pb was mainly predicted to deposit from air into plants (82% in 1998); the next years, uptake from soil became dominating (30% from air in 2006), because of decreasing levels in air. For Cd, predicted uptake from air into plants was negligible (1–5%).

Using a two site-reactive model for simulating one century changes of Zn and Pb concentration profiles in soils affected by metallurgical fallout.

Predicting the transfer of contaminants in soils is often hampered by lacking validation of mathematical models. Here, we applied Hydrus-2D software to three agricultural soils for simulating the 1900-2005 changes of zinc and lead concentration profiles derived from industrial atmospheric deposition, to validate the tested models with plausible assumptions on past metal inputs to reach the 2005 situation. The models were set with data from previous studies on the geochemical background, estimated temporal metal deposition, and the 2005 metal distributions. Different hypotheses of chemical reactions of metals with the soil solution were examined: 100% equilibrium or partial equilibrium, parameterized following kinetic chemical extractions. Finally, a two-site model with kinetic constant values adjusted at 1% of EDTA extraction parameters satisfactory predicted changes in metal concentration profiles for two arable soils. For a grassland soil however, this model showed limited applicability by ignoring the role of earthworm activity in metal incorporation.

Effect of acid mine drainage on the mineralogy of a dystric cambisol

Abstract Two natural soil profiles non-acidified and acidified with acid mine drainage (AMD) were selected for study. Interactions between acid mine drainage and the soil were evaluated by the changes in the mineral composition applying XRD, transmission electron microscopy (TEM), analytical electron microscopy (AEM), and selective area electron diffraction (SAED). The main differences between the acidified and the non-acidified soil were in the presence of jarosite and in the authigenesis of two types of amorphous and/or poorly crystalline matter (Fe-rich; Al, Si-rich). The most frequent Fe-rich matter was identified as ferrihydrite. It appears to be in two morphological varieties: “massive” (composed of spheroidal particles) and “mohair” (composed of layered particles).

Prospective modeling with Hydrus-2D of 50 years Zn and Pb movements in low and moderately metal-contaminated agricultural soils.

Results of detailed modeling of in situ redistribution of heavy metals in pedological horizons of low and moderately metal contaminated soils, considering distinctly different long-term land use, are scarcely reported in literature. We used Hydrus-2D software parameterized with abundant available local soil data to simulate future Zn and Pb movements in soils contaminated by metallurgical fallout in the 20th century. In recent work on comparing different modeling hypotheses, we validated a two-site reactive model set with adjusted chemical kinetic constant values by fitting the 2005 Zn and Pb concentration profiles in soils, with estimated 1901-1963 airborne Zn and Pb loads (Mallmann et al., 2012a). In the present work, we used the same approach to simulate 2005-2055 changes in Zn and Pb depth-distribution and soil-solution concentrations, comparing two hypotheses of chemical equilibrium: i) the validated two-site model (one site at equilibrium and the other involved in kinetic reactions with pore water) set with adjusted kinetic EDTA extraction constants, and ii) a non-linear one-surface site adsorption equilibrium model. Simulated transfers were found generally lower and more realistic when using the two-site model. Simulations showed that consistent Zn redistribution and loss occurred in the moderately contaminated soil until 2055, i.e., more than one century after the main metal deposition, but negligible in low contaminated soils. Transfer of Pb was small in the three soils and under both hypotheses. In 2055, simulated Zn outflow concentrations remained under threshold values for drinking water.

Trace metal availability in soil horizons amended with various urban waste composts during 17 years – Monitoring and modelling

Recycling organic residues in agrosystems presents several benefits but faces the question of contaminants, among them a few trace metals which eventually accumulate in soils following regular applications of organic waste products (OWP) and represent an ecological risk. The increase of total trace metal contents in amended topsoils can be predicted by a mass balance approach, but the evolution of their available fractions is a more intricate issue. We aimed at modelling this evolution by using the dataset of a long-term field experiment of OWP applications (manure and three urban waste composts). Two operationally-defined fractions of 6 trace metals have been quantified in the OWP and amended topsoils between 2002 and 2015: the soluble and potentially available metals, extracted in 0.01 M CaCl2 and 0.05 M EDTA solutions, respectively. The potentially available metals have progressively increased in amended topsoils, at rates depending on elements and types of OWP. For Zn, these increases corresponded in average to inputs of potentially available Zn from OWP. But the soil stocks of potentially available Cu increased faster than from the inputs of EDTA-extractable Cu, showing linear regression slopes between 1.4 and 2.5, depending on OWP type. The influence of OWP has been provisionally interpreted in the light of their efficiency to increase soil organic matter and their inputs of reactive oxides. Soluble copper has increased with repeated amendments. But soluble cadmium, nickel and zinc have generally decreased, as they are influenced by changing soil variables such as pH and organic matter. Statistic models were used to unravel the relationships between soluble and EDTA-extractable metals and other soil variables. For Cu, the most satisfactory models just relate soluble and potentially available Cu. Developing such models could contribute to predict the long-term effects of a precise scenario of agricultural OWP recycling upon available trace metals in soils.