Effect of Biochar and Natural Zeolite Application on Desorption Kinetic and Chemical Fractions of Zinc in a Zn-Contaminated Calcareous Soil

The use of organic and inorganic amendments is one of the effective methods for reducing the deleterious effects of heavy metals in contaminated soils and their immobilization. Addition of biochar as an organic soil amendment may change some soil chemical properties and provides suitable condition for immobilization of heavy metals in soil. Also, zeolite is a porous alkali alumino-silicate mineral that its application as an inorganic modifier is developing particularly in stabilizing of heavy metals in soil. Therefore, the aim of this study was to evaluate the effectiveness of biochar (derived from different organic materials) and natural zeolite application and their interaction on immobilization of zinc in a Zn-contaminated calcareous soil.
The appropriate amount of soil from the surface horizon (0-30 cm) of a calcareous soil, was collected, air dried and passed through 2 mm sieve. Then, the amount of 400 mg kg-1 Zn supplied as ZnSO4, 7 H2O was added to each soil sample (200 g). A factorial experiment in a completely randomized design was done with three replications. Factors included zeolite at three levels (0 (Z0), 3 % (Z1) and 6 % (Z2) (w/w)) and biochar at six levels (without biochar application (C), wheat straw biochar (WSB), corn straw biochar (CSB), licorice root pulp biochar (LRB), rice husk biochar (RHB) and sheep manure biochar (SMB) each at 3 % (w/w)). Contaminated soil samples were treated according to the experimental design and were kept for 90 days at room temperature (22±2 ˚C) and about field capacity moisture with distilled water. To evaluate the effectiveness of applied amendment materials in soil for immobilization of zinc, the sequential extraction procedure (sing et al, 1988), mobility factor (salbu and kreckling, 1998) and the parameters of two-first order kinetic model (Q1, Q2 and Q3) (Santos et al, 2010) were used.
with increasing zeolite levels from Z0 to Z2, the concentration of soluble-exchangeable (WEx), carbonatic (Car), organic (Om) and FeMn-Oxides fractions were decreased significantly while residual (Res) form of Zn was significantly increased. Application of all biochars significantly caused the decrease of WEx, Car and MnOxides (MnOx) forms in soil while crystalline Fe oxides (CFeOx) fraction was significantly increased compared to control (C). Res fraction was also increased significantly as influenced by different biochars application compared to control, so that, the impact of WSB, CSB, RHB and SMB treatments were similar and more than LRB treatment. The mobility factor (MF) value in different biochar treatments were as: C > LRB > RHB > CSB > SMB > WSB. Combined treatment of WSB and Z2 (WSBZ2), had the lowest MF of Zn in soil. The Q1 parameter of the kinetic model had significant and positive correlations with WEx and Car forms of Zn while Q3 and Q2 parameters had a significant and positive correlation with MnOx, FeOx and Res fractions. Furthermore, with application of all five biochars and increasing zeolite percentage, Q1/Q3 and Q2/Q3 ratios were significantly decreased compared to control. The lowest values of Q1/Q3 and Q2/Q3 ratios were observed in CSBZ2 and WSBZ2 combined treatments respectively.
application of all biochars and zeolite levels, led to the redistribution of Zn in soil, so that, Zn was transformed from more available forms to more stable forms. According to the results, it seems that the combined application of WSB and zeolite (Z2) was the best treatment to stabilize Zn in the studied soil.