Biochar–Carboxymethylcellulose Composite as an Injectable Colloidal Suspension for In Situ Groundwater Remediation

Injectable Permeable Reactive Barriers (IPRBs) represent a promising in situ technology for groundwater remediation, with sustainable adsorbents like biochar offering an alternative to activated carbon. This study optimized an IPRB process using a colloidal suspension of pinewood biochar stabilized with sodium carboxymethylcellulose (BC@CMC). The research first characterized the suspension stability under varying hydrochemical conditions, finding optimal colloidal stability at neutral to basic pH (6–9.4), while high ionic strength (>50 mM NaCl) and extreme pH values prompted aggregation. To prevent clogging, a key operational challenge, pre-filtration through a 64-µm sieve was implemented preventing column clogging and facilitating successful deep-bed distribution. The BC concentration was optimized to 3 g L−1, maximizing injectable adsorbent mass. Batch adsorption tests demonstrated the biochar’s high affinity for toluene (TOL) and tetrachloroethylene (PCE), with performance comparable to commercial activated carbon, particularly for PCE. The complete IPRB process was successfully validated through continuous-flow adsorption tests, where columns containing distributed BC@CMC showed high contaminant retention, with experimental retardation factors (Rx) of 144 ± 4 for TOL and 360 ± 6 for PCE. The study confirms that the optimized BC@CMC suspension enables highly efficient IPRB implementation, establishing this approach as a viable and sustainable strategy for field-scale groundwater remediation.