Investigation of cadmium immobilization in a contaminated calcareous soil as influenced by biochars and natural zeolite application

The effect of natural zeolite and biochars addition on the immobilization of cadmium in a calcareous soil was investigated using a combined factorial experiment under laboratory conditions. The following factors were evaluated: zeolite applied at 0, 3 and 6% (w/w) in combination with five different biochars at 3% (w/w), namely wheat straw biochar, corn straw biochar, licorice root pulp biochar, rice husk biochar and sheep manure biochar. Two different methods including a sequential extraction procedure and desorption kinetic experiment (using 0.01M EDTA) were used to assess the effectiveness of applied treatments for cadmium stabilization in soil. It was observed that with increasing the levels of zeolite application from 0 to 6%, the concentration of water-soluble plus exchangeable, carbonate-bound, Fe-Mn-oxide-bound and organic-bound fractions was significantly reduced, while the residual content of cadmium was increased. Changes in chemical fractions of cadmium and their transformation into more stable forms were also observed with application of all biochars. Use of all amendments led to a significant decrease in cadmium desorption during 48 h compared to the control soil, with sheep manure biochar + 6% zeolite combined treatment being the most effective. This was mainly attributed to the high-lime, P and O + S functional group content of the sheep manure biochar and the high pH and CEC of the natural zeolite. Ultimately, it was concluded that addition of sheep manure biochar + 6% zeolite combined treatment was best for enhancing the immobilization of cadmium in the contaminated calcareous soil. [GRAPHICS] .