Effect of willow biochar and fly ash-derived zeolite in immobilizing heavy metals and promoting enzymatic activity in a contaminated sandy soil

Abstract

Great difficulties associated with the reclamation of contaminated soils make it necessary to improve its methods and search for new, innovative technological solutions. Both biochars and synthetic zeolites have properties to control the mobility, and bioavailability of heavy metals in the soil through surface adsorption, ion exchange, or complexation. The aim of this study was to determine the effectiveness of soil supplementation with biochar and zeolite in immobilizing heavy metals (Cd, Pb, Zn) and promoting enzymatic activity in contaminated loamy sandy soil. The study was conducted on biochar produced from plant (willow) biomass at 350 °C and 550 °C, zeolite hydrothermally synthesized from fly ash and obtained from conventional coal combustion, as well as mixtures of these materials. Experimental pots were filled with loamy-sandy soil. The soil was contaminated with the following toxic metals: Cd, Pb, and Zn. Two factors were used in the experiment: (i) grass species (Dactylis glomerata L., Festuca arundinacea Schreb.) and (ii) soil treatment. Soil supplementation with functionalized materials reduced the share of mobile and carbonate-bound forms of cadmium, lead, and zinc. The study indicated that functionalized materials, such as biochar and zeolite, are moderately effective when used separately, but better results can be obtained by applying these materials together as mixtures. The results of the two-year study carried out on soil freshly contaminated (spiked) with heavy metals proved that the time after which significant changes in the bioavailable metal contents are observed is an important element in the assessment of soil supplementation with functionalized materials. The beneficial effect of the applied functionalized materials and their mixtures in promoting soil enzymatic activity is closely related to the effect of these materials on the chemical properties, such as pH or EC. Therefore, in addition to the chemical properties of contaminated soils, the design of heavy metal immobilization processes should also consider the microbial response as a key element in restoring soil health.