Adsorption of chlorinated solvents and heavy metals onto low-cost materials (biochars) in groundwater remediation

Soil contamination by mixture of compounds represents a challenging environmental problem to face, especially for the risk connected to the human health, due to the possible diffusion of the pollution in the groundwater. One of the most used remediation technology is adsorbtion on carbonaceus material and Activated Carbon (AC) is the most used one, even if it is usually connected to high costs. Biochar (BC) is proposed as an alternative low-cost material for the removal of wide range of contaminants in the environment (Alhashimi et al., 2017). BC is a carbon-rich product obtained by thermal decomposition of organic raw material, under different conditions of process (e.g. feedstock, gasification and pyrolysis). Its efficiency to reduce the bioavailability of heavy metals and to immobilize organic pollutants has been reported. Moreover, it is possible to combine retention of contaminants with biological degradation (bioremediation strategies), increasing the value of this by-product (Zhu et al., 2017). In this study two different BCs, Pine Wood Biochar (PWB) and Rice Husk Biochar (RHB), have been used for the adsorbtion of trichloroethylene (TCE), and lead (as Pb(II)), representing target contaminants for chlorinated solvents and heavy metals. PWB is obtained by pine wood gasification (800°C and limited supply of oxygen), while RHB is produced from rice husk pyrolysis (350°C, absence of oxygen). Both materials were characterised in terms of physical-chemical properties: Scanning Electron Microscope (SEM) analysis, Cation exchanged capacity (CEC), Fourier tranform infrared spectroscpy (FT-IR)). Kinetic and equilibrium batch tests (with mono-component contaminated solution) have been carried out to assess the BCs' adsorption capacities, changing solid/liquid ratio (from 1 to 5 mg mL-1). Furthermore, PWB-column and RHB-column were performed to better simulate the groundwater conditions. The fixed bed columns were filled with inert sand mixed with adsorbent material (4%w/w) and feeded with a bicomponent contaminated solution (5 ppm of TCE and 20 ppm of Pb(II)). The breaktrough curves for both contaminants has been assessed in order to get informations of the operational capacity of the materials. Results suggest PWB has higher affinity for TCE and confirm its good efficiency in immobilizing hydrophobic organic contaminants (HOCs)(Silvani et al.,2017), possibly due to the higher temperature achieved during gasification which leads to a graphite-like structure. On the other hand, RHB shows major Pb(II) removal. The low temperature of the RHB pyrolysis possibly allows to maintain a higher content of functional groups involved in cations exchanging and binding. The RHB-column study confirm the lower adsorption capacity for TCE considering that the saturation of the material has happened first, while the breaktrough point for Pb(II) has been observed later, compared to PWB. These results are very promsing, encouraging to consider BC as cost-effective alternative to AC in groundwater contamination scenarios.