Investigation of Effect of Biochar and Natural Zeolite Application on Copper Stabilization in a Cu-Polluted Calcareous Soil

Background and objectives: In situ immobilization procedures concern the reduction of heavy metals bioavailability in contaminated soils using the materials which are compatible with the environment. In recent years, biochars effectively have been used to minimize the heavy metals uptake by agricultural crops in polluted sites using the immobilization process. Also, zeolite is a porous alkali alumino-silicate mineral that its application as an inorganic modifier is developing particularly in stabilizing the soil heavy metals. Accordingly, the aim of this study was to investigate the copper (Cu) stabilization as influenced by biochars (derived from different organic materials) and natural zeolite application and their interactions in a Cu-polluted calcareous soil.
Materials and methods: 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 500 mg kg-1 Cu supplied as CuSO4, 5 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 assess the efficiency of applied amendment materials in soil for stabilization of Cu, the sequential extraction procedure, reduced partitioning index (IR) and the parameters of two-first order kinetic model using EDTA extractant (Q1, Q2 and Q3) were used.
Results: Relative percentage of Cu chemical fractions in different treatments in soil were as: carbonatic (Car) > residual (Res) > organic (OM) > adsorbed (Ads) > soluble+exchangeable (WsEx). With increasing the application of zeolite levels from Z0 to Z2, the concentration of WsEx, Ads, OM and Car fractions were significantly reduced by 5.46, 7.80, 10.40 and 15.30 percent respectively while, the Res form of Cu was increased. Application of all biochars except RHB, significantly caused the decrease of WsEx fraction compared to control. Use of all biochars led to reduction of the concentration of OM and Car fractions of Cu significantly while, Res fraction was significantly increased compared to control. Also, IR value was significantly increased as affected by zeolite levels and biochars application. The SMBZ2 combined treatment had the highest IR value. Soil pH showed a significant and positive correlation with Res fraction and IR value of Cu, which it indicates the important role of pH enhancement in stabilization of Cu in tested soil as affected by applied treatments. Copper desorption by EDTA extractant in all treatments was initially rapid, then continued at a slower rate, which probably indicating the incorporation of several sites with different bond energy in Cu desorption from soil. The Q1/Q3 and Q2/Q3 ratios were significantly decreased by application of all biochars and zeolite levels. The lowest of these ratios was observed in WSBZ2 combined treatment. There was a significant and positive correlation between the Q1 and Q2 parameters with Car fraction and the Q3 parameter with Res fraction of Cu in soil.
Conclusions: Application of all biochars and zeolite levels caused the transformation of Cu chemical fractions from more bioavailable and mobile forms to more stable forms. According to the results, it seems that SMBZ2 and WSBZ2 combined treatments were more effective to stabilize Cu in soil than other treatments.