Cadmium (Cd) is a non-essential element that is ubiquitously found in soil, water, and the atmosphere. Due to its high bioavailability and carcinogenic properties in humans, Cd has been blacklisted by the World Health Organization [1
]. Cd is widely applied in industry for purposes such as electroplating, mining, and smelting, because of its metallic properties. This leads to the release of large amounts of Cd into soil through wastewater, irrigation, and sewage disposal [2
]. The overuse of Cd-containing pesticides and fertilizers may also increase the concentration of Cd in the soil [3
]. Due to its widespread occurrence, and its bioaccumulating and biomagnifying effect throughout tropic chains, Cd monitoring has become an area of increased research focus [4
]. Excavation, phytoremediation, leaching, electro remediation, solidification, and stabilization are some of the many engineering and chemical techniques that have been proposed to reduce Cd bioavailability in contaminated soils [6
]. However, all of these methods have apparent disadvantages. Due to its comparatively high destruction of native organisms in the soil environment, the excavation approach can easily disrupt ecosystem stability and therefore, is not an eco-friendly method [7
]. Phytoremediation and phytoextraction are more eco-friendly-based approaches that may also be long-lasting and cost-effective. Using these approaches, Cd is accumulated in plant tissues and removed from the soil through the harvest of these plants [8
]. However, due to the high selectivity of the biotic for the target elements, the capacity of plants to contain Cd, and the mobility of the element in soil, phytoremediation also does not supply an adequate solution to the reduction of Cd bioavailability in the soil [5
]. Consequently, due to the severe implications of Cd contamination in the soil, the availability of a robust tool for the immobilization of Cd labile fractions is critical.
Previous investigations have shown performance inconsistencies of organic amendments in Cd contaminated soil. Some studies have revealed that the addition of organic materials might increase the ability of soil to bind the target element, making these contaminants available for removal through the remediation processes of sorption, precipitation, or complexation [9
]. Yao et al.
] have also indicated that typical organic pig manure could significantly reduce Cd toxicity interrestrial plants, such as wheat and maize. Additionally, the addition of organic materials could supply sufficient nutrients to plants and improve water retention in soil. Moreover, it was also reported that organic material can be beneficial by reducing phototoxicity and improving growth and survival of the biota [6
]. Related studies have indicated that biochar, which may be the reactive group in the organic material, contains essential nutrients for plant germination and growth and, furthermore, could cover large areas to increase soil porosity and immobilize Cd fractions [14
]. However, many investigations have also shown the increasing addition of the organic materials may enhance fraction mobility in soils and restrain biota growth [6
]. Consequently, there are two prerequisites for organic materials that determine its applicability for soil amendment. One is that the organic amendment must be capable of reducing accumulated Cd in plant tissues during the plant’s lifetime. The other prerequisite is that the addition of the organic material must benefit the plant biomass. Accordingly, further investigations on the effectiveness of organic amendments for the remediation of Cd contaminated soil are necessary to give the overall understanding of organic amendment application. Colza cake is the collected residue of rapeseed after the refining process and is a common fertilizer for cultivation. However, there are no systematic investigations explaining its effect on metal mobility and bioavailability in Cd contaminated soils. Accordingly, colza cake was applied as the amendment material in this study.
Diffusive gradients in thin films (DGT), as an in situ
measurement, has become an increasingly popular method for constructing eco-security and early-warning systems [18
]. This technique, based on Fick’s first law, passively collects a target element while avoiding the influences of the base solution concentration and the surrounding environment [18
]. The DGT device locally collects target element particles, while responding to fractions of resupplied labile species in a solution and the labile pools in the solid phase [19
]. Consequently, the element concentration measured by the device is the mean concentration of the target element during deployment time. Due to the dynamic nature of the measurements, the DGT-measured concentration could effectively mimic the biota uptake process of metals such as Cu [20
], Zn [20
], Cd [21
], and Hg [23
]. Tian et al.
] also demonstrated that the DGT obtained a concentration of a target element that was not influenced by the physiochemical properties of various metals and metalloids in soil, sediment, and water in the environment. Consequently, the use of DGT, as an in situ
method, could dynamically reflect the bioavailability of Cd in soil. The traditional methods such as soil solution concentration [7
], the free ion activity model, single or sequential extraction methods [26
], and the isotope dilution exchange method are widely applied in the related investigations. These approaches have a lower time-cost ratio and simple operation procedures. These ex situ
methods are of great significance when evaluating Cd bioavailability in a static manner [28
]. Due to the different advantages of the in situ
and ex situ
methods, both the DGT technique and traditional chemical methods were applied in this study to reflect the immobilization of labile Cd fractions after the addition of organic material.
Typical cash crops, wheat and maize, were used to accumulate Cd. The DGT technique and five traditional static testing methods, including soil solution concentration and four commonly used chemical extraction methods (chelating extractant EDTA (ethylene diamine tetraacetic acid), acid extractant HOAc (aqua regia), and salt solutions CaCl2 and NaOAc (sodium acetate)), were selected to evaluate how these approaches reflect Cd bioavailability. The aim of this study was to systematically investigate Cd bioavailability in Cd-contaminated soil after the addition colza cake at varying concentrations. Pearson correlation coefficients, which determine differences between the bioavailable concentrations of Cd obtained by different indicators and the concentration of Cd accumulated in plant tissues, were used to compare bioaccumulation. The ratio of the DGT-measured concentration and the labile fractions in the soil solution under different addition concentrations of colza cake were also shown to reflect the resupply of labile Cd fractions. This further demonstrated the interaction between colza cake and labile Cd.