Potential Release of Zinc and Cadmium From Mine-Affected Soils Under Flooding, a Mesocosm Study

Abstract

Metal-contaminated mining soils pose serious environmental and health risks if not properly managed, especially in mountainous areas, which are more susceptible to perturbation. Currently, climate change is leading to more frequent and intense rain events, which cause flooding episodes, thereby altering soil redox equilibria and contaminants stability. We evaluated the potential release of Zn and Cd (two of the most common inorganic contaminants) and the factors regulating their solubility and speciation in two heavily contaminated soils representative of a Zn-mining area. The soils were flooded under aerobic (for 24 h) and anaerobic (for 62 days) conditions using mesocosm experiments, sequential extractions, and geochemical modelling. Leaching trials under aerobic conditions showed a high release of Zn and Cd (10 times the legislative limits), with metals possibly migrating via water infiltration or runoff. Under anaerobic conditions Zn and Cd were initially released. Then, solution concentrations decreased gradually (Zn) or sharply (Cd) until the end of the experiment. Sequential extractions and multisurface modelling indicated that both metals precipitated mainly as carbonates. This was confirmed by a geochemical multisurface modelling, which also predicted the formation of sulphides after 60 days in one soil. The model calculated metals to be preferentially complexed by organic matter and well predicted the observed soil solution concentrations. The results showed that during flooding episodes contaminants could be promptly transferred to other environmental compartments. The use of multisurface modelling coupled with laboratory experiments provided useful indications on the potential release and speciation in case of anoxic conditions.

Why comparison between different chemical extraction procedures is necessary to better assess the metals availability in sediments

“…In their opinion, further studies comparing the metals availability estimated by various chemical procedures for a specific site are necessary. According Padoan et al (2020), the use of chemical extractions provides useful indications to estimate the metals release to the environment in the case of flooding episodes.…”

Integrated Assessment of Affinity to Chemical Fractions and Environmental Pollution with Heavy Metals: A New Approach Based on Sequential Extraction Results

“…Differences in the set of chemical fractions taken into account when calculating RAC, MF, and BF are associated with the fact that the dissolution of HM fractions can occur due to changes in the physicochemical properties of the environment, that is, RAC takes into account only bioavailable HM fractions, while MF and BF additionally consider the content of potentially mobile HM forms, the migration rate of which depends on water regime, temperature, and redox potential [9], pH value and content of carbonates [8], electrical conductivity, particle size fractions, and magnetic susceptibility [95], content of organic matter [45], intensity of the biological cycle [96]. The main limitation of ICF, RAC, MF, BF, MC, SC, and PC is non-selectivity, that is, the impossibility of defining the chemical affinity of HMs to a specific geochemical phase.…”

“…Each next reactant has greater chemical activity than the previous one, allowing the stepwise dissolution of more stable compounds [7], which are considered less mobile, bioaccessible, and environmentally hazardous of HM fractions [8]. Thus, a sequential extraction procedure provides essential information about the strength of HMs binding to particulates and the degree of their "anthropogenicity" or "geogenicity" [6], which is crucial in understanding the relationship between geochemical processes and HMs mobilization as well as the availability of HM forms for organisms and, accordingly, potential environmental risks [5,9].…”

“…Unlike the A. Tessier et al scheme, a shorter set of chemical fractions is isolated by the BCR: F1acid-soluble (exchangeable and bound to carbonates), F2-reducible (bound to Fe/Mn oxides), F3-oxidizable (bound to organic matter and sulfides), F4-residual (metals within lithogenic minerals) fraction. BCR is also applied in some modifications to assess the ratio of HM forms in mangrove soils [47], soils in mining [9,48], industrial [49], and urban [5] areas; soils amended with waste composts or bio-sorbent materials [50,51]; airborne suspended particles [52]; river suspended sediments [53], soakaway sediments [54]; road dust in megacities [55], on various types of roads [56], and from industrial areas [57]; sewage sludge [58]; fly ash from wood biomass [59], and municipal waste incineration [10]; mining wastes [12], and ore-processing wastes [60]. For the sequential extraction of HMs from bottom sediments, the schemes according to M. Kersten and U. Förstner [61], H. Zeien and G.W.…”