Prediction of free metal ion activity in contaminated soils using WHAM VII, baker soil test and solubility model

Highlights

• Free ion activity of metal is the most potent index of availability and ecotoxicity.

• This index decides the suitability of agricultural land for cultivation.

• Assessing free ion activity of metal in soil solution is tedious and cumbersome.

• Novel protocol was developed for routine assessment of free metal ion activity.

• This protocol will go a long way in helping risk assessment of metal polluted soil.

Abstract

Bioavailability and ecotoxicity of metals in contaminated soils depend largely on their solubility. The present investigation was carried out to predict the free ion activity of Zn²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Cd²⁺ in contaminated soils as a function of pH, organic carbon content and extractable metal concentration. Twenty-five composite soil samples were collected from various locations which had a history of receiving sewage sludge (Keshopur and IARI, Delhi), municipal solid waste (Kolkata, West Bengal), polluted river water (Madanpur, Delhi) and industrial effluents (Debari, Rajasthan and Sonepat, Haryana). Four composite soil samples were also collected from adjacent fields which had not received contaminated amendments. Free ion activities (-log₁₀ values), viz. pZn²⁺, pCu²⁺, pNi²⁺, pPb²⁺ and pCd²⁺ as measured by the Baker soil test, were 10.1 ± 1.12, 13.4 ± 1.23, 12.9 ± 0.85, 11.6 ± 0.74 and 12.6 ± 2.26, respectively. Free metal ion activities were also determined using the geochemical speciation model WHAM-VII following extraction of soil solution with porous Rhizon samplers from the rhizosphere of growing plants. pH dependent Freundlich model based on soil properties could explain the variation in pZn²⁺, pCu²⁺, pNi²⁺, pPb²⁺ and pCd²⁺ to the extent of 84, 52, 73, 60 and 70%, respectively, in the case of data from Rhizon samplers coupled with speciation modelling. Whereas, C-Q model could explain 84, 57, 82, 72 and 74% variability in pZn²⁺, pCu²⁺, pNi²⁺, pPb²⁺ and pCd²⁺, respectively, based on soil properties and free metal ion activity as determined with integrated use of Rhizon-WHAM-VII. Modelling approach was superior compared to that based on the Baker soil test solution.

Introduction

The accumulation of heavy metals in agricultural land from various sources (e.g. sewage sludge, polluted river water, industrial effluents or municipal solid wastes) is of increasing concern due to its detrimental effects on soil ecosystems and the potential risks to human and animal health (Golui et al., 2019). Metals can be transferred from contaminated soils to the edible portion of crops, depending mainly upon the extent of their solubility in soils (Ray and Datta, 2017). As a consequence, human health may be adversely affected due to dietary intake of trace toxic metals. For example, central nervous system toxicity, nephro-toxicity and cognitive behavioural effects, haemolysis, Wilson’s disease, itai-itai, renal damage, hepatic necrosis, metabolic acidosis, sinusitis, anosmia, aminoaciduria, proteinuria, glucosuria, renal tubular dysfunction, encephalopathy and failure in reproduction have all been ascribed to metal toxicity (Rattan et al., 2009; Golui et al., 2019).

Precise assessment of bioavailability of metals is a prerequisite for successful risk assessment of contaminated soils (Groenenberg et al., 2010). However, total metal content in soil is normally used as a simple index of hazard in the legislation of most countries in the world (Datta and Young, 2005). Such indices do not consider the soil properties that alter the solubility of metal in contaminated soils. In fact, water-soluble and exchangeable pools usually constitute less than 10% of total metal content in soils (Mortvedt et al., 1991). Consequently, poor correlations are generally observed between the total metal content and the bioavailable fraction in metal contaminated soils (Zhang et al., 2001; Francois et al., 2004). Research to relate the toxic effects of heavy metals to operationally defined extractable fractions continues (Quevauviller, 1998; Sauve et al., 1998; Young et al., 2000; McLaughlin et al., 2000; Hooda et al., 1997; Ray et al., 2013).

Most research indicates the need for a shift from measurement of ‘quantity’ to ‘intensity’ of metals in soils for an accurate prediction of bioavailability with the ultimate aim of risk assessment (Datta and Young, 2005; Meena et al., 2016; Golui et al., 2017). This is particularly important because the concentration of metal in soil solution has a direct bearing on phytoavailability and ecotoxicity to microorganisms (Minnich et al., 1987; Sauve et al., 1998; Vulkan et al., 2000; Groenenberg et al., 2010; Golui et al., 2018). Soil pore water can be extracted through porous tubes, such as Rhizon samplers, which involves in-situ extraction of solution from the rhizosphere of growing plants (Tipping et al., 2003; Golui et al., 2018; Mishra et al., 2019). Centrifugation and squeezing have also been used to displace the pore water (Rowell, 1994; Datta and Young, 2005). Free metal ion activity in soil solution can then be determined using geochemical speciation models, such as WHAM VII and others (Mao et al., 2017; Marzouk et al., 2013; Tipping et al., 2011). Alternatively, the ‘intensity parameter’ of ions in soil can also be determined by simpler inferential soil testing procedures, as developed by Baker and Amacher (1981), which are easier to implement in non-specialist laboratories. The Baker soil test assumed that at equilibrium, ions are distributed between soil and solution phases with no net movement of any ion from one phase to another. Therefore, few ions in soil solution remain complexed with DTPA (component of Baker soil test solution) and other ions remain in free hydrated form (Datta et al., 2013). In Baker soil test, activity co-efficient of individual ion can be computed through Debye-Huckel expression based on ionic strength of the solution. Using the activity co-efficient, free ion concentration of respective ion can be estimated considering mass balance expression for each metal and equilibrium constant (log K values) for formation of metal-DTPA complex through iteration until equilibrium distribution of free ions is achieved (Datta et al., 2013). The Baker soil test has seldom been used to measure the free ion activity in soils but direct extraction of soil pore water and chemical analysis for a large number of model parameters may be considered too demanding to adopt on a regular basis to evaluate the bioavailability of metal in polluted soil. More empirically based alternatives have been tried, such as the use of pH-dependent Freundlich equations to predict free metal ion activity in soil solution, based on easily measurable soil properties, such as pH, soil organic carbon and extractable metal content (Zan et al., 2013; Golui et al., 2014, 2017; Meena et al., 2016). In similar line, Groenenberg et al. (2010) formulated transfer function for solid solution portioning of metal in soil using regression approach. But all these approaches have been investigated in soils of temperate region (Zan et al., 2013). Such models/mathematical formulation need wider evaluation, particularly under tropical conditions where organic matter contents are lower and mineralogy is different from soils in temperate regions (Hough et al., 2003; Datta and Young, 2005; Gandois et al., 2010; Groenenberg et al., 2010; Loncaric et al., 2010). Also, an adequate database has not yet been generated comparing speciation using WHAM with simpler approaches such as the Baker soil test or C-Q model to indicate the general applicability of these approaches. In this paper, we develop a tool to predict free ion activity of Zn, Cu, Ni, Pb and Cd in metal-contaminated soils, based on easily measurable soil properties including extractable metal content. We also attempted to work out the efficacy of Baker soil test to assess free metal ion activity in soil.