Research article
Evaluating Cr behaviour in two different polluted soils: Mechanisms and implications for soil functionality

https://doi.org/10.1016/j.jenvman.2020.111073Get rights and content

Highlights

  • Pollution by aqueous Cr(VI) and industrial sewage sludge is simulated in two soils.

  • The methodology used allows to discriminate the variables controlling the fate of Cr.

  • Cr reactions in soil depends on pH and specific organic and mineral functional groups.

  • Cr bioavailability determines microbiological activity and plant growth.

  • Cr(VI) spills can overcome the natural reduction capacity of alkaline and poor soils.

Abstract

This work investigates the mechanisms determining Cr speciation and availability in two different soils polluted with two chromium sources (an industrial sludge, highly polluted with Cr, and Cr(VI) solution) and the influence of these parameters on the recovery of the soil functions related with biological quality and plant growth. The experiment was carried out in greenhouse conditions using 36 pots of 17 kg for the growth of Silene vulgaris for 21 months. Logistic Regression Model using Lasso estimator shows that soil organic matter (SOM) and pH control Cr availability in studied soils. In soils treated with the sludge, X ray Absorption spectroscopy showed that Cr was present as Cr(III), biological quality indicators increased and plants were able to grow. However, in soils polluted with Cr(VI), Cr availability was significantly different in the two soils. In the alkaline and poor in organic matter soil, 12% of Cr(VI) remained in the soil leading to the decrease of soil quality indicators and the total inhibition of plant growth. In the neutral soil, Cr(VI) was totally reduced to Cr(III) by soil organic matter (SOM), quality indicators were not affected and plants grown properly. Infrared Spectroscopy showed that different functional groups reacted with Cr in the two soils. This study highlights the importance to understand the mechanisms underlaying Cr redox and adsorption reactions in Cr polluted soils as they determine the potential recovery of the functions related with biological quality indicators and plant growth. The methodology proposed allows this study in complex soil samples at realistic concentrations and may be useful for risk assessment and for the planning of managing strategies in Cr polluted soils.

Introduction

Chromium is a transition metal with anticorrosive properties, it is widely used in many industrial applications including metallurgy, stainless-steel production, electroplating, leather tanning, wood preservation, ceramics glazes, paints and pulp and paper production (Jobby et al., 2018; Shahid et al., 2017). Solid and liquid wastes from these industries are considered major sources of Cr pollution in soils (Shahid et al., 2017). Besides this, atmospheric deposition and the use of fertilizers (phosphorus fertilizers and biosolids) also contributes to increase Cr concentration in soils (Borges et al., 2015; Nicholson et al., 2003; Vogel et al., 2015). Given this context, concern about Cr pollution in soils is gaining significant attention since it can potentially cause human health problems by entering the food chain (Ahmed et al., 2016), through soil dust inhalation (Broadway et al., 2010) or the transfer to ground and surface waters (Hausladen et al., 2018; Paustenbach et al., 2003).

The most common oxidation states of Cr in the environment are Cr(III) and Cr(VI). Cr(VI) is an oxyanion, it is found forming soluble forms in soils so it can be easily leached into the groundwater or be absorbed by plants. It possesses a high oxidation power causing severe oxidative injuries and DNA damage on living organisms. In humans, it is classified as carcinogen of Group A (USEPA, 1998). Cr(III) is between 10 and 100 times less toxic than Cr(VI) (De Flora et al., 1990). It is mainly found as cation, forming insoluble forms that are adsorbed to the negatively charged particles of soils, or precipitated as oxides and hydroxides.

Some biological (e.g. oxalic, citric and humic acids) and synthetic (e.g. EDTA) chelating agents can, however, form Cr(III) soluble complexes (Bartlett, 2014; Carbonaro et al., 2008; Saad et al., 2017). About 35% of the Cr that arrives to soils from anthropogenic activities is Cr(VI) (Dhal et al., 2013). Soil organic matter (SOM) plays as an electron donor to reduce Cr(VI) to Cr(III), this process is also controlled by other soil characteristics such us texture, pH or cation exchange capacity (Bartlett, 2014; Shahid et al., 2017; Taylor et al., 2000). However, the quantity of Cr(VI) coming from anthropogenic activities could overcome soil redox buffering capacity.

Besides the above-mentioned risks to human health, the presence of Cr in soils can impair soil ecological services such as nutrient cycling, carbon storage or water cycling. The establishment of a vegetation cover in polluted soils can be a strategy to improve soil structure and to reduce soil erosion and pollutant mobilization helping in the recovery of these services (Burges et al., 2018; Vangronsveld et al., 2009). In this sense, previous studies showed the intrinsic tolerance of Silene vulgaris (Moench) Garcke to high levels of Cr (Pradas del Real et al., 2014, 2016) and its ability to shape the structure of rhizosphere microorganism community in Cr polluted soils (García-Gonzalo et al., 2017b, 2017a).

In order to assess the risk associated with Cr soil pollution and to develop adequate soil managing strategies, it is important to understand the processes governing Cr behavior in soils. In general, the risk assessment of a contaminated site uses data of total metal concentration (Kumpiene et al., 2017). However, these data provide limited information on the impact of the pollutant on soil biota. Metal bioavailability, which refers to the accessible fraction that could be absorbed by living organisms, it is being considered a better indicator to evaluate the risk (Kumpiene et al., 2017). This is especially relevant in the case of Cr, as its availability strongly changes with speciation.

Recent studies have investigated the role of soil components on redox, complexation and adsorption reactions of Cr with special attention to the role of humic acids (HA) (Huang et al., 2012; Ohta et al., 2012; Wu et al., 2017; Zhang et al., 2018) and Fe (oxy) hydroxides (Hori et al., 2015; Saad et al., 2017; Whitaker et al., 2018). By using Fourier Transform Infrared Spectroscopy (FTIR), Wen et al. (2018) showed that different soil organic matter (SOM) functional groups are implicated in Cr(III) complexation in rhizospheric and non rhizospheric soils. Veselská et al. (2019) investigated the redox and adsorption reactions of Cr(VI) in synthetic mixtures of silicates, clays, oxides and humic acids. The authors tested different Cr(VI) concentrations, pH, background electrolyte concentrations and sorbate/sorbent ratios and they combined results from mechanistic surface complexation modelling with experimental spectroscopic and isotopic analysis. The study concluded that soils rich in organic matter and clay minerals would be more efficient in Cr(VI) reduction and Cr(III) retention but that complex combination of factors, particularly the presence of Fe(oxy)hydroxides can lead to very different scenarios. Although they provide significant mechanistic information, the above-mentioned studies used model batch systems based on isolated synthetic or extracted soil components. There is then a lack of mechanistic studies in more realistic experimental systems.

The objectives of the present study were: i) to investigate the mechanisms explaining Cr speciation and bioavailability in artificially polluted soils after a long pot experiment and, ii) to evaluate their influence on the recovery of the soil functions related with biological quality and plant growth. To this end, two soils with contrasting characteristics were polluted with either a Cr(VI) solution, to simulate pollution from wastewaters and mine and industrial effluents; or with a sewage sludge from a wastewater treatment plant, as an example of solid industrial waste containing Cr. S. vulgaris was grown in these soils for 21 months. An exhaustive sampling and multivariate analysis were developed to ensure the robustness of the results.

Section snippets

Soil characterization and pollution process

Two soils from Madrid region (Spain) were investigated in this study: Soil A (El Escorial: 40,579140; 4,129067) and soil B (Alcala de Henares: 40,521987; 3,288817). For their characterization, soil samples were air dried, disaggregated and sieved (<2 mm) and then analyzed according to the official Spanish methodology for soil analysis (MAPA (Ministerio de Agricultura Pesca y Alimentación), 1994). Metal concentrations were determined after acid mineralization as described in supporting

Soil and biological quality variables

Considering the large number of variables in our experimental system that may affect the biological activity of soils and plant growth, we seek to identify the main soil factors describing the variability of the experimental data set. Due to the high correlation among studied variables, PCA did not allowed to identify the main variables (data not shown). We then applied Logistic Regression Model using LASSO estimator. From the twenty soil variables investigated in this study, the following nine

Discussion

In the present study Cr bioavailability has been determined in two soils with contrasting characteristics 21 months after being polluted with either an industrial sewage sludge or Cr(VI) solution. Cr concentration has been determined in the soil pore water, which has been previously shown to be an adequate method for the quantification of the Cr bioavailable fraction in soils (Choppala et al., 2013). Although total Cr concentrations were similar in both soils and Cr treatments, significant

Conclusions

This study shows the role of specific functional groups of SOM and other soil components in Cr redox and adsorption reactions in two soils polluted with Cr(VI) solution and an industrial sludge. The presence of these functional groups together with pH determine Cr speciation and bioavailability which strongly influence the recovery of the soil functions related with soil biological activity and plant growth 21 months after pollution. In calcareous soils, like soil B in this study, with pH > 7

CRediT authorship contribution statement

A.E. Pradas del Real: Methodology, Investigation, Formal analysis, Writing - original draft, Visualization. A. Pérez-Sanz: Conceptualization, Methodology, Supervision, Writing - review & editing. P. García-Gonzalo: Data curation, Validation, Writing - review & editing. H. Castillo-Michel: Software, Methodology, Data curation. M.J. Gismera: Investigation, Validation. M.C. Lobo: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Projects SINCROPLANT (FP-18 IMIDRA) and REHABILITA CTM2016-78222-C2-1-R (AEI/FEDER, UE). Ana Pradas del Real thanks Atracción de Talento Program from Madrid Region. Authors want to thank Leticia Monico for providing Cr(OH)3 reference spectra and Marine Cotte for her advice in Cr XANES acquisition.

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    Present address: Department of Agricultural Chemistry, Universidad Autónoma de Madrid, 28049, Madrid, Spain.

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