Effect of multiple factors on the adsorption of Cd in an alluvial soil from Xiba, China

Highlights

• The effects of organic matter, Fe/Mn oxides on Cd adsorption.

• The effects of pH, ionic strength, background electrolyte on Cd adsorption.

• Adsorption thermodynamics and kinetics of Cd.

Abstract

The effects of organic matter, free Fe oxides and Mn oxides in an alluvial soil on adsorption of Cd were studied through selective chemical extraction and adsorption experiments. Compared to untreated soil, after H₂O₂ treatment for removal of organic matter and NH₂OH·HCl treatment for removal Mn oxides, the distribution coefficient (K_d) decreased by a maximum of 25.2% and 64.1%, respectively. After dithionite-citrate-bicarbonate treatment for removal of free Fe oxides, K_d increased by 1670.2%. After increasing the solution pH from 2 to 3, K_d increased by 2842.1%, whereas after increasing the solution pH from 3 to 7, the adsorption tended stabilize. As the ionic strength increased from 0.001 M to 0.1 M NaNO₃, K_d gradually decreased, whereas at the same ionic strength, K_d decreased as the initial concentration of Cd increased. The effects of different background electrolytes on Cd adsorption was as follows: Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ for cations and Cl⁻ ≈ SO₄²⁻ > NO₃⁻ for anions. The adsorption capacity of Cd increased as the increased of temperature, and it's a spontaneous endothermic process. The pseudo second-order rate model described the process of Cd adsorption well.

Introduction

Cd is the seventh most toxic heavy metal as per ranking among environmental pollutants by the Agency for Toxic Substance and Disease Registry (ATDSR, 2017). The world health organization (WHO, 2000) reported that the average background concentration of Cd was 0.1–0.4 mg kg⁻¹ in soil. in recent years, with the development of industry and intensive agriculture, a large amount of Cd has entered into the soil environment, causing the content of Cd in the soil in some areas to exceed the recommended level. Cd is more bioavailable and mobile than other metals (Li et al., 2015). It can easily enter the human food chain, causing a range of acute and chronic disease symptoms, as well as cause serious damage to the kidneys, lungs, and liver (Bedoui et al., 2008).

Soil is a complex heterogeneous system. As soil components, iron oxides, manganese oxides, and organic matter are important adsorption carriers, which play an important role in the transfer, transformation, and other environmental geochemical behaviors of Cd in soil (Dong et al., 2000; Li et al., 2015). A variety of methods are used to study the effects of soil components on Cd adsorption. For example, firstly, we can study the adsorption capacity of synthetic or semi-synthetic pure substances to Cd and then perform an extrapolation (Covelo et al., 2007; Tan et al., 2018). Secondly, we can analyze the correlation between mineral composition and content/distribution of Cd in soil (Lee, 2006; Holm et al., 2003). Thirdly, we can compare the adsorption capacity of Cd after separation of specific component by selective chemical extraction (Li et al., 2015; Zhao et al., 2014). However, overall analysis by selective chemical extraction technology is an effective method for accurately evaluating the adsorption effect of specific components on heavy metals in soil and their adsorption mechanism (Sarkar et al., 2014).

It is well known that the conditions of solution chemistry (e.g., pH, ionic strength, background electrolyte type etc) can also significantly affect the Cd adsorption in soil (Chen et al., 2011; Loganathan et al., 2012). The pH mainly affects Cd adsorption by changing the speciation of Cd, such as Cd(OH)₃⁻, Cd(OH)₂⁰, CdOH⁺, and Cd²⁺ (Hu et al., 2014), and the surface charge of the adsorbent (Hu et al., 2011). The ionic strength and background electrolyte mainly affect the adsorption of Cd by affecting the structure of the double electrode layer of the adsorbent (Wang et al., 2013; Malamis and Katsou, 2013), competing with Cd for adsorption sites or forming soluble complexes with Cd (Wang et al., 2013; Dai et al., 2013). However, the effects of the three types of solution chemistry conditions on Cd adsorption show different characteristics due to the differences in the physicochemical features of the studied soil.

The adsorption equilibrium is influenced not only by soil components and solution chemistry conditions but also by the kinetic and the thermodynamics processes. The adsorption of Cd, as with other heavy metals, is recognized as a two-step process (Axe et al., 2006). That is, adsorption is divided into the initial rapid adsorption stage, followed by the slow adsorption stage. The temperature affects the adsorption process mainly through the following mechanism. As the temperature increases, the viscosity of the solution decreases (Nouri et al., 2007) and the kinetic energy of the ions in the solution increases, increasing the diffusion rate of the metal ions (Malamis and Katsou, 2013; Wang et al., 2019). The increase in the temperature promotes the hydrolysis of metal ions, reducing the electrostatic repulsion between the sorbate and the surface of the adsorbent (Fan et al., 2009). Moreover, an elevated temperature results in an increase in the number of the active adsorption sites on the solid (Goyal et al., 2003; Puranik and Paknikar, 1999). However, it has also been reported that an increase in the temperature enhances the tendency of Cd to escape from the solid surface, thus reducing the ability of the adsorbent to remove Cd (Öztürk et al., 2014; Sharma et al., 2007).The thermodynamic parameters (i.e.,△G⁰,△H⁰ and△S⁰) will also change with temperature (Chen and Wang, 2006).

Riverine alluvial plains are usually densely populated, with relatively well developed agriculture, due to the flatness of the terrain and good quality of the soil; however, this also leads to heavy metal pollution of the latter. To date, the reports in the literature regarding the effect of multiple factors on Cd adsorption in alluvial acidic soils have been inadequate. In this study, selective chemical extraction technology and adsorption experiments were applied to investigate: (1) the quantitative analysis of the effects of organic matter, free Fe oxides, and Mn oxides in an alluvial soil on Cd adsorption; (2) the effects of solution chemistry conditions (pH, ionic strength, background electrolyte) on Cd adsorption; and (3) the thermodynamics and kinetic characteristics of Cd adsorption. The research results should provide a theoretical basis and data support for soil Cd pollution prevention, treatment, and remediation.