Can Biochar be used as an alternative low-cost/cost effective sorbent for groundwater remediation?

Contaminated sites by mixture of contaminants are diffused worldwide: DNAPLs, VOCs or heavy metals are commonly revealed in the aquifer. Groundwater’s remediation technologies need to be more effective at lowering concentration imposed by local legislation, more sustainable and cost-effective. The “Pump&Treat” remains the most chosen technology combined with ex situ treatment, like adsorption onto sorbent materials, such as Activated Carbon. New strategies are interested in low-cost carbonaceous materials, like solid waste obtained by thermal treatment of agricultural waste. In this context lots of research are studying capacity of “biochar” (BC) in immobilizing different class of compounds from contaminated solutions. Some works report the BC’s efficiency in immobilizing nutritional compounds for plants and microorganisms and its eco-friendly being. These aspects are interesting for an innovative in situ technology, where the BC could be injected directly into the aquifer for selective contaminants immobilization and for supporting natural attenuation carried out by microorganisms. Furthermore, in the majority of papers, laboratory-made BCs were used. In this study (i) pine wood BC, obtained in a gasification unit in Germany, a (ii) rice husk BC, from a pyrolysis small-scale farmers unit in Indonesia, and a (iii) BC (from Eupatorium sp. shrubs) modified during the pyrolysis process adding oxide-hydroxides of Iron, were used. SEM and X-ray analysis, meso-, micro-porosity analysis and determination of the surface area (BET) confirm the morphological difference between these BCs, due to the different feedstock and process conditions (presence/absence of O2; temperature; residence time). Adsorption tests with contaminated solutions by Trichloroethylene and Lead (Pb2+) are carried out to investigate the behaviour and the efficiency in contaminants removal by batch and column reactors. Kinetic tests in batch reactors are performed to verify the equilibrium time required to write the isotherm curve. Experimental data are fitted with Langmuir and Freundlich model in order to obtain the optimized parameters that allows to compare the materials. To simulate real groundwater conditions, fixed-bed reactors are realized in plexiglass columns packed with a mixture of sand and reactive material and the breakthrough curves are drawn. The early results are very encouraging: at the equilibrium concentration of 5 mg L-1 of TCE the amount of adsorption, with Langmuir modeling, is 20 mg g-1 (i); 4 mg g-1 (ii) and 5 mg g-1 (iii) (tests onto a commercial Activated Carbon report 40 mg g-1); on the other hand at 20 mg L-1 of Pb(II) we obtained 5,5 mg g-1 and 23 mg g-1 for (i) and (ii) respectively (texts onto material (iii) are ongoing). These results are confirmed by the column tests and batch experiments with Toluene contaminated solutions are underway. Moreover, a fluidized-bed reactor (turbulence in the reactive zone) is going to be set up, in order to avoid problems linked to the column experiments (such as dead zone or preferential routs of the solutions) and other adsorption tests, combined with microorganisms’ activity, will be designed.