CHEMICAL STABILIZATION OF Cd CONTAMINATED SOIL USING FRESH AND AGED WHEAT STRAW BIOCHAR

摘要

Soil pollution can adversely affect the ecosystem services provided by the soil. Polluted soils reduce land productivity by reducing crop yields and polluting groundwater. Also, both crops and water in polluted lands may unsafe for the consumption by animals or humans. Release of chemicals or toxic substance can happen through industrial and agricultural activities. Metal mining and smelting to separate minerals is one such of activity which can introduce large quantities of heavy metals into the environment which persist in the soil for long periods even after those activities are ended. The Campine area on the border of Belgium and the Netherlands contains Cd contaminated sites due to historic metal smelting activities. A soil collected in that region containing 11±0.5 ppm Cd exceeding soil remediation standards was taken in consideration for this soil remediation study. Biochar is increasingly getting attention as a remediation tool to immobilizing heavy metals in contaminated soils. However, long-term provisioning of such service is mainly depends on the biochar carbon stability. Biochar carbon stability is mainly depending on the biochar production conditions, nature of the feedstock material and the biotic and abiotic environmental conditions that biochar is being used. Also, the heavy metal immobilization process heavily depends on the soil and biochar pH and the nature of the functional groups present on biochar surfaces such as carbonates and phosphates. Within this context, three types of wheat straw biochar were produced using a screw reactor at 400 °C, 500 °C, and 600 °C. To age the biochar samples, biochar samples were subjected to accelerated aging using a method suggested by Cross and Sohi, 2013 , This method can be used as proxy for environmental aging of biochar approximately 100 years under temperate conditions. Then these six biochar samples (BC_(400F), BC_(500F), BC_(600F), BC_(400A), BC_(500A), BC_(600A)) were characterized for elemental analysis, ash content, volatile matter content and fixed carbon content, pH, EC, phosphate and carbonate content and FT-IR analysis. The soil used in this study was characterized for the soil texture, elemental contents, organic matter content, pH and EC. Six months of laboratory incubation study was conducted with contaminated soil amended with each type of biochar at 2 % rate (w/w). Rhizon extractions were collected at the end of each month to quantify the Cd concentration, pH and total organic carbon content in the soil pore water. At the end of the six months of incubation time, Cd concentration in the pore water ranged from 100.36 ppb in BC600A to 249.85 ppb BC400A. The Cd concentration in each treatment was BC_(600A)< BC_(400F)< BC_(600F)< BC_(500f)< Soil only (control) < BC_(500A)< BC_(400A). According to the FT-IR analysis of the six biochar samples, more carboxylic-C and carbonate- C functional groups were present in aged biochar samples compared to the freshly produced wheat straw biochar samples. Also, biochar produced at lower temperatures were characterized by lower pH and a lower amount of stable C compounds compared to the biochar produced freshly and in higher production temperatures. These results suggest that the stability of biochar carbon and pH of both biochar and soil have a significant impact on the stabilization of heavy metals in the soil environment. Therefore, the selection of biochar with desired qualities thus choosing of suitable biochar production conditions is essential in decision-making processes to keeping the biochar services in the long run.