Fixation of cadmium, copper, nickel and zinc in soil : kinetics, mechanisms and its effect on metal bioavailability

Abstract

The total concentrations of trace metals such as Cu, Zn Ni and Cd increase with time in the majority of soils due to diffuse contamination. These metals become toxic at elevated concentrations. The bioavailability of these metals can decrease over time by fixation reactions in soil and can partly offset the risk due to increasing total metal concentrations. The term fixation is used here with reference to the slow immobilization reactions that take place in the weeks and years after the short-term (hours) immobilisation reaction. The objectives of this work are to characterize the extent of fixation of metals (Ni, Zn, Cd and Cu) in various soils, to assess which soil constituents are involved in metal fixation and to identify the role of fixation on metal availability to plants. It is postulated that metal fixation takes place in micro-porous amorphous iron (Fe) oxyhydroxides and in carbonates. Twenty eight soils with varying properties (pH 3.4-7.7) were sampled, contaminated with an environmental relevant dose of Ni, Zn, Cd and Cu and incubated in open air with free drainage for 850 days. Soils included two toposequences (Spain, Vietnam) with varying weathering stage and concentrations of Fe oxyhydroxides, and 13 European soils ranging from acid podzols to calcic cambisols. Fixation was measured as the increase of the fraction of added metals that was not isotopic exchangeable and was, on average among all soils, 43% (Cu), 41% (Zn), 41% (Ni) and 28% (Cd) after 850 days. Fixation of Cd, Zn and Ni was mainly explained and positively correlated with pH. Metal fixation within samples from each toposequence was positively related to total Fe oxyhydroxide concentration for Zn, Ni and Cd. Fixation of Zn and Cd in soils with pH > 7.0 increased with increasing concentrations of carbonates at initial ageing times. Fixed fractions of Zn, Ni and Cd were significantly released when experimentally removing 50% of carbonates by acidification. Fixation of Cu was most poorly related to soil properties. These data suggest that fixation is related to a pH dependent diffusion into oxyhydroxides (Cd, Zn and Ni) and to a diffusion/coprecipitation in carbonates (Cd, Zn). Fixation of Ni at pH neutral conditions may also be related to stabilisation of precipitates which readily form in soil. The role of Fe-oxyhydroxide structure and crystallinity on metal fixation was studied using 4 synthetic Fe oxyhydroxides (Hydrous Ferric Oxide (HFO), Ferrihydrite (FH), Goethite (GT), Hematite (HT)). Metal adsorption kinetics was characterised in suspensions during 70 days at varying initial metal concentration and various pH values (3.8-7.0) maintained within 0.2 units. The slow reactions, defined as those occurring beyond day 1, were most pronounced in GT and least in HT. The extent of slow reaction was surprisingly unrelated to crystallinity or microporosity as determined by N2 adsorption. Amorphous hydroxides such as HFO probably unfold in suspension thereby increasing their specific surface but reducing their micro-porosity and capacity for metal diffusion. Metals fixed on GT after 70 days, determined by isotopic exchange were 90% (Ni), 90% (Zn) and 54%Cd. Freundlich adsorption isotherms fitted to the data show that ageing between 1 and 70 days decreases metal concentrations in solution by factors 2-30 (Ni), 1-20 (Zn) and 1-4 (Cd) depending on the type of oxyhydroxide. These factors become significantly larger at lower concentration for Ni and Zn in some oxyhydroxides. This concentration dependency suggests that slow reactions are not only related to diffusion but that substitution for Fe in the structure might be involved as confirmed by spectroscopic evidence elsewhere. The extent of slow reactions on metal immobilization on GT decreased with decreasing pH and was less than a factor 2 within 70 days at pH 3.7. The role of soil organic matter on metal fixation was studied in humic acids and peat samples. Metal fixation was measured as the increase of the fraction of added metals that was not isotopic exchangeable after 70 days incubating the organic matter fractions with metals in suspensions at pH=7. Metal fixation varied from insignificantly different from zero to maximally 31% and was, on average for 5 organic matter samples, 19% (Cu), 17% (Zn), 7% (Ni) and 8% (Cd). These results show that the majority of metal added to soil binds reversibly with organic matter although a statically significant fraction can be fixed. Soil organic matter and oxyhydroxides are the dominant reactive surfaces for metal binding in soil. Soil organic matter can, therefore, keep metal in a reversible pool and limit their fixation in Fe-oxyhydroxides. A model system consisting of Fe oxyhydroxides and amberlite (as a model for organic matter with fully reversible binding) was made and showed that the relative changes of soluble Ni during long-term equilibration with goethite were significantly smaller when a resin was added to the sorption system. The resin acts as a reversible pool that limits the slow reactions of Ni with oxyhydroxides. A data analysis by modeling suggested no interactions among both adsorbents. The solubility of metals in the 28 soils as a function of ageing was predicted with an assemblage model (WHAM VI) considering soil organic matter, Fe-oxyhydroxides and clay minerals as reactive surfaces. The predictions were only successful by adjusting the specific surface area of Fe-oxyhydroxides to a value of 600 m2/g. The effects of time on solubility was included by using the measured values of isotopic exchangeable metals as reactive metals and these predictions were better than using total metal concentrations. The fixation observed in the Fe-oxyhydroxide studies was used to predict metal fixation in soil with the 2-component additive model described above. The assemblage model predicted the fraction of total metal present on the iron oxyhydroxide surfaces at the start of soil spiking and this fraction increased with increasing pH. The observed versus predicted fixed fractions agreed reasonably well for Ni and Zn which show that slow adsorption reactions in iron oxyhydroxides are capable of explaining metal ageing in soils. Finally, it was checked if ageing also reduced metal availability to plants. Italian ryegrass was grown simultaneously on the 28 different soils spiked with a mixture of Ni, Zn, Cu and Cd salts and aged for > 2 years and on corresponding freshly spiked soils with the same total metal concentration. Ageing significantly reduced shoot Ni concentrations in 6 and Cd and Zn concentrations in 3 soils out of 19 on which growth was successful. Changes in shoot metal concentrations were significantly explained by corresponding changes of soluble soil metal concentrations for Ni or by isotopic exchangeable concentrations for Ni and Cd. Unfortunately, there was a significant effect of soil ageing on plant yield in several soils which confounded the effects on shoot metal concentrations. Multivariate analysis including yield showed that ageing effects on Cd, Ni and Zn uptake were significant when using total soil metal concentrations whereas the isotopic exchangeable fractions explained these ageing effects, thereby predicting that shoot Cd and Ni concentrations decreased proportionally with decreasing isotopic exchangeable fractions during ageing. It was concluded that changes in metal supply due to ageing were relatively small compared to other factors that affect metal bioavailability. Our data show that Zn, Ni, Cu and Cd can be gradually fixed and can have reduced availability by ageing reactions, however the extent of these reactions, expressed as reduction in the labile fraction, is less than a factor 5 with some exceptions such as Ni in high pH soil and Zn in soils with high concentrations of carbonates. Our data suggest that fixation takes place in iron oxyhydroxides and in carbonates and that binding to soil organic matter is mainly reversible. The microporosity of Fe-oxyhydroxides in soil is likely to play a role in slow reactions; however there is no tool to quantify that factor. Metal fixation reduces metal mobility but effects of ageing on bioavailability are obscured by other factors that also affect bioavailability and which may change by ageing.Samenvatting iii Summary vii Abbrevations, notations and symbols xi Chapter 1 Fixation of trace metals in soils: an overview of existing concepts and objectives for this work 1 Chapter 2 Role of soil constituents on fixation of soluble Zn, Cu, Ni and Cd added to soils 13 Chapter 3 Long-term reactions of Ni, Zn and Cd with iron oxyhydroxides depend on crystallinity and structure and on metal concentrations 33 Chapter 4 Labile and inert fractions of Cu, Zn, Ni and Cd in soil organic matter detected by an isotopic exchange/resin sorption technique 53 Chapter 5 Fixation of Ni in iron oxyhydroxide and amberlite mixtures: evidence for additive reactions 63 Chapter 6 Prediction of Cd, Zn, Ni and Cu solubility during >2 years ageing reactions with an assemblage model 71 Chapter 7 Ageing of Cd, Zn, Cu and Ni in soils does not consistently reduce metal uptake by Lolium perenne L 91 Chapter 8 Major conclusions and suggestions for further research 107status: publishe