Radical change of Zn speciation in pig slurry amended soil: Key role of nano-sized sulfide particles.

Spreading livestock manure as fertilizer on farmlands is a widespread practice. It represents the major source of heavy metal(loid)s (HM) input in agricultural soils. Since zinc (Zn) is present at high concentrations in manure, it poses special environmental concerns related to phytotoxicity, groundwater contamination, and introduction in the food chain. Therefore, investigations on the fate and behavior of manure-borne Zn, when it enters the soil environment, are necessary to predict the environmental effects. Nevertheless, long-term field studies assessing Zn speciation in the organic waste matrix, as well as within the soil after manure application, are lacking. This study was designed to fill this gap. Using SEM-EDS and XAS analysis, we reported the following new results: (i) ZnS made up 100% of the Zn speciation in the pig slurry (the highest proportion of ZnS ever observed in organic waste); and (ii) ZnS aggregates were about 1-µm diameter (the smallest particle size ever reported in pig slurry). Moreover, the pig slurry containing ZnS was spread on the soil over an 11-year period, totaling 22 applications, and the resulting Zn speciation within the amended soil was analyzed. Surprisingly, ZnS, i.e. the only species responsible for a nearly 2-fold increase in the Zn concentration within the amended soil, was not detected in this soil. Based on SEM-EDS and XAS observations, we put forward the hypothesis that Zn in the pig slurry consisted of nano-sized ZnS crystallites that further aggregated. The low stability of ZnS nanoparticles within oxic and complex environments such as the studied soil was the key explanation for the radical change in pig slurry-borne Zn speciation after long-term amendments.

Copper and zinc accumulation and fractionation in a clayey Hapludox soil subject to long-term pig slurry application.

Pig slurry (PS) recycling as fertilizer is commonly practiced as an option for minimizing livestock waste. Successive PS applications on the soil can lead to crop toxicity and environmental risk. Despite extensive investigation of macronutrient behavior, the fate of trace metals remains uncertain and only a few long-term field studies have been reported to date. This study was designed to assess the impact of 11-year continuous PS spreading on Cu and Zn accumulation and fractionation in a Brazilian clayey Hapludox soil. Three different PS application rates--50, 100 and 200m3 ha(-1)year(-1)--were monitored at six soil depths in comparison to a non-amended control soil. The modified Geological Survey of Canada sequential extraction protocol was applied. A statistically significant increase in Cu and Zn total concentration (assessed by the sum of fractions) was noted only within the 0-5 cm soil layer for the 50 and 100 m3 ha(-1) year(-1) (PS50 and PS100) treatments, and up to 10-15 cm for the 200 m3 ha(-1)year(-1) (PS200) treatment. The mass balance, determined for the 22 PS amendments over the period, confirmed the overall exogenous Cu and Zn accumulation within the surface layers. More than 70% of the natural heavy metal content was originally in the residual fraction. However, this was the only fraction not influenced by the PS amendments. After PS input, the exogenous Cu was mainly detected in the fraction bound to organic matter (66.4%) within the PS200 0-5 cm soil layer. Exogenous Zn was mainly distributed between the adsorbed fraction (36.7%) and the organic matter fraction (32.0%) within the PS200 0-5 cm layer.

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.

Modeling field-scale vertical movement of zinc and copper in a pig slurry-amended soil in Brazil.

Organic amendments often represent a source of trace metals (TMs) in soils, which may partly leach into the groundwater. The objectives of this study were (1) to validate Hydrus-2D for modeling the transport of Zn and Cu in an Alfisol amended with pig slurry (PS) by comparing numerical simulations and experimental field data, and (2) to model the next 50 years of TM movements under scenarios of suspended or continued PS amendments. First, between 2000 and 2008, we collected detailed Zn and Cu data from a soil profile in Santa Maria, Brazil. Two hypotheses about Zn and Cu reactivity with the solid phase were tested, considering physical, hydraulic, and chemical characteristics of six soil layers. Using a two-site sorption model with a sorption kinetic rate adjusted based on laboratory EDTA extractions, Hydrus simulations of the vertical TM transport were found to satisfactorily describe the soil Zn and Cu concentration profiles. Second, the long-term fate of Zn and Cu in the soil was assessed using the validated parameterized model. Numerical simulations showed that Zn and Cu did not present risks for groundwater pollution. However, future Cu accumulation in the surface soil layer would exceed the Brazilian threshold for agricultural soils.

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.