Effect of potassium fertilizers on immobilization remediation of Cd-polluted soils using sepiolite

Cadmium (Cd) is one of the most toxic pollutants in soil environments because of its persistence, toxicity and potential for bioaccumulation. Natural sepiolite has recently been found as a cost-effective material for immobilization remediation of metal-contaminated soils due to its low cost, high cation exchange capacity, and high specific surface area associated with the small particle sizes. In agricultural production, the application of various fertilizers is vital, but the effects of fertilizer addition to polluted soils on immobilization remediation have been little investigated. In previous studies on immobilization experiments, only remediation effects were emphasized. The effects of nutrient elements on remediation process promotion or inhibition has been largely ignored. For large application of immobilization remediation in different areas with various fertilizer forms, the impact of fertilizers on the process must be determined. In this research, natural sepiolite (10 g·kg^-1) was used as immobilization agent and meanwhile potassium chloride (KCl) and potassium sulphate (K₂SO₄) used as representative potassic fertilizers in rape pot experiments to determine the effects of potassic fertilizers on the process of immobilization remediation of Cd-polluted soil. The potassium fertilizer content was calculated as K₂O, with 0.1 g·kg^-1, 0.2 g·kg^-1 and 0.3 g·kg^-1, respectively. The results showed that rape biomass significantly increased (by 6.06%-10.05%) after the application of K₂SO₄, compared with sole sepiolite treatment. Cd contents in shoot increased respectively by 16.38%-60.73% and 15.62%-25.19% after the application of KCl and K₂SO₄. KCl and K₂SO₄ had little effects on soil pH, but increased exchangeable Cd concentration significantly (respectively by 25.51%-34.65% and 18.5%-24.96%). Sepiolite conduced Zeta potential of soil samples to shift in negative direction, while the addition of KCl and K₂SO₄ made the Zeta potential of soil samples increase. The maximum adsorption of Cd by sepiolite in aqueous solution was 5.30 mg·g^-1, but KCl and K₂SO₄ reduced sorption of Cd on sepiolite, with maximum sorption of respectively 2.87 mg·g^-1 and 4.92 mg·g^-1. Bioavailable fractions of K, Mn, Cu and Zn were enhanced significantly by the additions of KCl and K₂SO₄. Considering the various factors during passivation of sepiolite to Cd-contaminated soils therefore, the effect of application of K₂SO₄ on passivation was less than that of application of KCl. On the whole, K₂SO₄, rather than KCl, was recommended potassic fertilizer for remediation of Cd-contaminated soils using sepiolite.