Biochar: an environmentally sustainable approach for removing potentially toxic elements (PTEs) from contaminated water

Nowadays, to mitigate the negative effects caused by pollution, particular attention is being paid to the eco-sustainability of human and industrial activities and the development of alternatives to the use of charcoal as a source of energy production. Biochar fits perfectly into these objectives, being used as a soil amendment or as an adsorbent of organic and inorganic environmental contaminants [1,2]. Moreover, it is obtained through a sustainable process, which yields biofuels, synthetic gases, and clean energy as by-products. An important source of contamination in the Friuli Venezia Giulia Region is the Raibl dismissed mining district, located in Cave del Predil (UD), which was active for the extraction of galena (PbS) and sphalerite (ZnS) until 1991. A large amount of mineralised waste-rocks and tailings from the flotation plant operating at the Raibl mining district were disposed from 1976 to 1991 in tailings impoundments set up on the western bank of the Rio del Lago stream. Leaching processes involving tailings and the subsequent release and dispersion of potentially toxic elements (PTEs), especially Thallium (Tl), are promoted during periods of intense rainfall and high flow river conditions [3]. The main aim of this study was to experimentally determine the ability of biochar to adsorb PTEs (such as Pb, Tl and Zn) from contaminated water. The biochar used in this study is obtained from the processing of vineyard pruning waste, produced by slow pyrolysis, and characterised by a high degree of carbonisation, aromaticity, hydrophobicity, and porosity. Several adsorption tests were performed with two types of biochar, fine-grained and heterogeneous coarse-grained, in different solvents (MilliQ water and fluvial water from the Tagliamento River). A quantity of 200 mg of biochar was exposed to 20 mL of solutions with concentrations of 1, 5, 10, 25 and 50 mg/L of the three elements individually. The tests were also repeated with solutions containing the three elements in a mixture to assess if competition occurred between them in absorption. Using ICP-AES, the residual metals in solution were quantified, and then the adsorbed metals were calculated, expressed both as a percentage in solution and per gram of biochar used. The tests showed that fine biochar has a higher adsorption capacity than coarse biochar due to its greater homogeneity, and that the maximum amount adsorbed by this type of biochar was approximately 2000 μg/g of metal. Furthermore, neither pH nor the presence of cations in fluvial waters (Na, Ca, Mg and K) affect adsorption. By comparing the results of the single-element and multi-element tests, it was concluded that the three metals compete in adsorption, with a recurring order of affinity Pb2+ > Tl+ > Zn2+ most likely due to the formation of a double layer on the surface of the biochar in contact with the solution, in agreement with Stern’s theory. References: [1] M. Ahmad, A. U. Rajapaksha, J. E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage, S. S. Lee, Y. S. Ok, Chemosphere 2014, 99, 19 – 33. [2] W. Xiang, X. Zhang, J. Chen, W. Zou, F. He, X. Hu, D. C. W. Tsang, Y. S. Ok, B. Gao, Chemosphere 2020, 252, 126539. [3] N. Barago, E. Pavoni, F. Floreani, M. Crosera, G. Adami, D. Lenaz, S. Covelli, Journal of Geochemical Exploration2023, 245, 107129.