Arsenic and Cadmium Co-Contamination in Agronomic Soils: Risks and Options for Management

Abstract

<p>Arsenic (As) and Cadmium (Cd) are Group 1 carcinogens that are toxic to biota. These elements occur widely in the environment as contaminants from many anthropogenic sources. Arsenic and Cd show different chemical properties and fate processes in the soil environment, and thereby the needs for risk management differ in contaminated soils when the two elements co-occur. Effective management for co-contaminated soils is, however, limited by the little information available on competing or synergistic sorption effects in soils, interactions for plant uptake, with very few studies are available on remediation or management in soils. Arsenic and Cd sourced from agrochemicals contaminate agronomic soils worldwide. For example, the dry zone of Sri Lanka, the major agricultural region in the country, has one of the world’s highest occurrences of chronic kidney disease with unknown etiology (CKDU) and exposure to As and Cd is considered one of the possible causes. Agrochemicals used extensively in rice farming have been attributed as likely sources of the As and Cd in these agronomic soils. The sources and distribution of As and Cd in the dry zone of Sri Lanka, however, is still under debate and any links between As, Cd and CKDU not yet clear.</p> <p>The aim of this thesis was to assess the key drivers and mechanisms influencing As and Cd mobility in co-contaminated agronomic soils, particularly considering the metal(loid) soil concentrations and soil characteristics, and also modification induced by agronomic amendments. This was achieved first by a systematic investigation of baseline values and potential sources of As and Cd in dry zone farm soils of Sri Lanka, followed by a series of laboratory based experiments using spiked and aged As and Cd co-contaminated soils. These included a column leaching experiment under different conditions of water flow, and phosphorus and lime amendment with soil obtained from the dry zone Sri Lanka, batch sorption and desorption experiments on acidic and mildly alkaline soils and a column leaching experiment using rice husk biochar as a soil amendment and potential leaching mitigation measure.</p> <p>The first part of this work describes a comprehensive, systematic field sampling study conducted to investigate the baseline concentrations and the potential sources of As and Cd in agricultural surface soils in the dry zone Sri Lanka. Soil, fertiliser and rice seeds samples were collected from the CKDU affected (Medawachchiya (M), Padaviya (P) and Giradurukotte (G)) and non-affected (Hambanthota (H)) agronomic regions in 2017 and 2018. In this study, all inorganic fertilisers collected, showed low concentrations of As (<30 mg kg⁻¹ ) and Cd (<1.25 mg kg⁻¹ ) and were within European Union (EU) guideline values. In the soil, the maximum As concentration in CKDU affected areas was 3.55 mg kg⁻¹ and for Cd was 0.2 mg kg -1 but the CKDU non-affected area showed wider As and Cd ranges with higher concentrations of As of 3.84 mg kg⁻¹ and Cd of 3.00 mg kg⁻¹ , and the Cd concentration was greater than Canadian guidelines for agricultural use of soil. The As and Cd concentrations in rice seeds were less than detection limits (<0.1 mg kg⁻¹ ) and smaller than Codex Alimentarius Commission maximum levels. This work provides clarity around baseline values for As and Cd in certain farm soils for dry zone Sri Lanka and provides no substantive evidence for surface soils as a As and Cd exposure route currently for the local agricultural populations. To fully understand any possible links to CKDU in Sri Lanka, and leachability, bioavailability and risk for these elements in the soils, additional sampling of As and Cd in sub-surface soils and concentrations in water resources is needed.</p> <p>The dry zone of Sri Lanka receives high annual rainfall (1000-1750 mm) compared to other tropical locations. Phosphorus fertiliser application and liming are also common agronomic practices in rice paddy farming to enhance soil fertility. Therefore, a column leaching experiment was conducted over 48 h to assess the influence of high rainfall, phosphorus (P) and lime amendment on As and Cd mobility in soil collected from dry zone Sri Lanka (Padaviya location), an acidic Alfisol soil with sandy loam texture. Arsenic (20-100 mg kg⁻¹ ) and Cd (3-20 mg kg⁻¹ ) were spiked to the soil and aged for one year prior to leaching. The acidic Sri Lankan soil showed a low affinity for As and leaching was strongly affected by treatments, whereas Cd was strongly retained and typically less affected by treatment conditions. With high rainfall, As leached increased at 48 h in all treatments, but was significantly reduced when Cd was present at the high concentration tested (20 mg kg⁻¹ ). In contrast, Cd total mass leached was not significantly different at low Cd concentration (3 mg kg⁻¹ ), but significantly decreased by 48 h in the higher Cd co-concentration treatment with As present (Cd 20 mg kg⁻¹, As 100 mg kg⁻¹ ) compared to the control. Arsenic-Cd bridging complexes were considered responsible for the enhanced retention of As and Cd in soil at the higher As/Cd concentrations. Phosphorus addition increased As leaching at the higher As concentration tested which was further increased with Cd present suggesting P addition disrupted any co-sorption effect for As. The Cd leached, however, with P amendment decreased at the higher Cd concentration tested and was further reduced with As present, significantly at higher As/Cd concentrations suggesting As-Cd bridging complexes were important for Cd sorption in this soil. The As and Cd leached over 48 h decreased with lime application, and effects of one contaminant on leaching of the other were not evident, possibly due to strong lime induced sorption mechanisms that masked any co-contaminant sorption. Effects of the co-contaminants was most significant at the highest soil co-concentrations examined (As (100 mg kg⁻¹ ) and Cd (20 mg kg⁻¹ )) in treatments. This column leaching experiment demonstrated that in co-contaminated soil, As or Cd could affect the mobility of the other but the effect varied with the treatment and became significant at the high concentration element values tested in this soil: As at 100 mg kg⁻¹ and Cd at 20 mg kg⁻¹.</p> <p>The different drivers of As and Cd mobility were clearly evident in the column leaching experiment and suggested effects on sorption when both elements co-occurred. Therefore, a series of batch sorption/desorption experiments were conducted with As and Cd contaminated (singly and co-contaminated) agronomic soils, at various As and Cd co-concentrations and soil pH values to assess competitive or synergistic effects on As and Cd sorption/desorption when the elements co-occurred. A slightly alkaline Chromosol soil and an acidic Chromosol soil with similar texture (sandy loam and loamy sand) were used in batch experiments. Initially, the influence of co-occurring Cd(II) (3-100 mg kg⁻¹ ) on sorption of As(V) at soil concentrations 0-3000 mg kg⁻¹ and with varying pH (~4 - ~10) was examined. The findings indicated that As(V) sorption (acidic soil K_F ^sorp = 76.35 L kg⁻¹ and slightly alkaline soil K_F ^sorp = 53.70 L kg⁻¹ ) and retention (acidic soil K_F ^des = 110 L kg⁻¹ ; slightly alkaline soil K_F ^des = 70 L kg⁻¹ ) were greater in the acidic soil, and % As sorbed decreased as As concentration increased. Arsenic(V) sorption and retention in both soils increased with the presence of Cd, and this was considered the result of a combination of coordination bridging complexes of Cd with arsenate anions and some co-precipitation. Arsenic sorption increased with increasing pH to a maximum at pH 7 in the slightly alkaline soil whereas in the acidic soil, As sorption decreased with pH increase to a minimum at approximately pH 8. This study suggested that Cd would increase As retention in soils under different conditions.</p> <p>The same agronomic soils (slightly alkaline and acidic) were subsequently used for another batch sorption study to examine Cd(II) sorption and desorption, as a function of initial Cd concentration (0-800 mg kg⁻¹ ), with co-occurring As(V) (20-500 mg kg⁻¹ ) and varying pH (~3 - ~11). This study showed that the Cd sorption was greater in the slightly alkaline soil (K_F ^sorp = 177.83 L kg⁻¹ ) compared to the acidic soil (K_F ^sorp = 13.18 L kg⁻¹ ), and Cd desorption was also less in the same soil (slightly alkaline soil K_F ^des = 213.93 L kg⁻¹ ; acidic soil K_F ^des = 14.13 L kg1 ). The presence of As(V) increased sorption and retention of Cd in both soils, but the effect was more pronounced in the acidic soil. The cumulative effect of surface electrostatic attraction for the arsenate anion and specific adsorption resulting in significant enhancement of negative surface charge was proposed as the explanation for this finding. Cadmium desorbed was also reduced with the presence of As, considered a consequence of the stronger sorption observed. With pH variation, Cd sorption gradually increased with increasing pH in both soils up to pH ~7, however, there was no significant effect of As(V) observed on Cd sorption in the two soils with changing pH, possibly a result of the ionic solutions used for pH manipulation. This batch experiment also suggested that As(V) can increase Cd sorption and retention in soils under specific conditions. The outcomes of the sorption studies demonstrated that in As and Cd cocontaminated soil systems the presence of one element can increase sorption of the other, but the extent of this directly depends on the soil characteristics.</p> <p>Taking into account the differences observed for As and Cd sorption when the elements cooccurred and in different soils, targeted management strategies are required for cocontaminated soils. Therefore, a risk management column leaching study was conducted over 48 h with the same soils (mildly alkaline and acidic) to assess the influence of rice husk biochar (RHB) amendment on the mobility of As and Cd in As(V) (0, 20 and 100 mg kg⁻¹ ) and Cd (0 and 20 mg kg⁻¹ ) co-contaminated agricultural soils. One-year aged As and Cd spiked soils were used. In the control, without RHB amendment, the influence of As or Cd on leaching of the other was evident at 48 h only for As in the acidic soil and was considered a result of As-Cd bridging complex formation. The RHB treatment significantly increased As leaching in the slightly alkaline soil only up to 12 h probably due to the increase in pH and reduced As retention. There was no significant effect with RHB amendment in the acidic soil in which As was more strongly sorbed. In contrast, RHB treated acidic soil significantly reduced Cd in leachate to 48 h, and the effect was also evident in the slightly alkaline soil but only at early sample collection times. Arsenic or Cd leaching with RHB addition was not influenced by the other element in both soils, in contrast to the control non RHB treated soils, and was considered due to increased element retention by the RHB and reduced availability for complexation. The results of this column leaching trail suggested that addition of RHB was effective for immobilisation of Cd, but not As, at least over 48 h. However, with RHB addition, cumulative As leached did not increase over 48 h, therefore RHB may be considered a potential risk management measure for low level contaminated soils. </p> <p>This thesis as a whole has gone some way towards enhancing our understanding of risk and management of As and Cd co-contaminated agronomic soils. The field sampling in Sri Lanka suggested that As and Cd were not present in dry zone surface soils at concentrations considered a risk for human health but additional sampling of subsurface soils and water resources would supplement understanding of risk. Key findings from the series of laboratory experiments investigating mobility and sorption in As and Cd co-contaminated soils clearly demonstrated that different factors impact As and Cd mobility in soils, and that synergistic or antagonistic effects on retention can occur when both elements co-occur. The soil As/Cd concentrations, pH and different soil amendments all importantly modify the interactions. The work in this thesis would be extended by confirmation of the As and Cd sorption mechanisms and speciation using solid-state analysis such as synchrotron-based spectroscopy, and by incorporating real field co-contaminated soils into experiments. This will further inform risk assessment and management for these elements in co-contaminated agronomic soils. </p>