## 4 Discussion
This reduction in the bioavailability of trace elements was related to the properties of the bentonite, specifically its strong sorption capacity and the large number of adsorption sites within the interlayer space, on the outer surface and on the edges. Introducing the sorbent into the soil also increased the pH (from 8.30 to 8.53), promoting the precipitation of TEs in the form of hydroxides. [PERSON] and [PERSON] [83] confirmed the usefulness of bentonite in immobilising nickel and copper in mining-contaminated soils. They demonstrated that adding 10% bentonite reduced the exchangeable fraction of nickel by 56% compared to the control object (without the additive) and increased the fraction bound to organic matter and Fe and Mn oxides by 69% and 73%, respectively. In the case of copper, the exchangeable fraction was reduced by more than 2.7-fold compared to the control sample, while the fraction bound to organic matter increased by 68%, and the fraction bound to Fe-Mn oxides increased by 91%. According to the researchers, applying bentonite also reduced soil acidity (pH 7.9 versus 7.2). Under alkaline soil conditions, the adsorption of trace elements onto the surface of Fe-Mn oxides increases [49], as does the stability of the soil organic matter-metal complex [86]. At the same time, the concentration of iron, aluminium and magnesium ions decreases, facilitating the adsorption of the ions of trace elements on soil colloid surfaces [83]. The main mechanism of action of liming is an increase in soil pH, leading to the precipitation of trace elements in the form of less soluble and bioavailable hydroxides, carbonates and phosphates [87]. In our study, adding calcium oxide reduced the accumulation of Co, Fe, Ni and Cr in the soil. [PERSON] et al. [88] conducted a comprehensive analysis of the effects of various calcium compounds on cadmium immobilisation in soils affected by gold mining activities. Of the alkaline materials tested (CaCO\({}_{3}\), Ca(OH)\({}_{2}\), CaSO\({}_{4}\)-2H\({}_{2}\)O and oyster shell meal), hydrated lime (Ca(OH)\({}_{2}\)) was found to be the most effective at increasing soil pH and the net negative charge, and at reducing the concentration of extractable Cd. Adding 8 Mg ha\({}^{-1}\) reduced the bioavailability of cadmium by 54% relative to the control. An increase in the net negative charge, which is associated with the dissociation of H\({}^{+}\) from the weakly acidic functional groups of organic matter and some clay minerals, increases the adsorption of TE cations onto soil colloids. Additionally, liming reduces Cd mobility by precipitating it as carbonates, phosphates, hydroxides and oxides at a high soil pH [89]. [PERSON] et al. [90] evaluated the effect of hydrated lime on the immobilisation and phytoavailability of trace elements in soils contaminated by zinc smelter operations. They demonstrated that adding 1% (\(w/w\)) of Ca(OH)\({}_{2}\) reduced the mobility, toxicity and bioavailability of Pb (by 21.6-31.7%), Cd (by 22.2-57.6%), Cu (by 29.8-50.0%) and Zn (by40.8-61.3%) compared to the control soil without the additive. [PERSON] et al. [91] showed that lining with CaCO\({}_{3}\) (0.4% \(w/w\)) significantly reduced the leachability of Cd, Pb and Zn from mining-contaminated soil. They confirmed that the immobilising effect of calcium compounds on soluble trace element content is mainly due to an increase in soil pH (from 4.23 to 7.14). This increase in pH promotes the precipitation of TEs in less accessible forms and increases their adsorption onto colloids of different charges, such as organic matter or iron oxides and aluminium oxides. Furthermore, the authors state that lining significantly increases microbial and enzymatic soil activity. The slightly different results of our study compared to those of the cited authors in terms of the effect of the lining process on the immobilisation of specific TEs may be due to differences in the form of the calcium compound, the granulometric composition of the soil and the nature of the contamination studied.