Transport and retention of xanthan gum-stabilized microscale zero-valent iron particles in saturated porous media

Microscale zero valent iron (mZVI) is a promising material for in-situ contaminated groundwater remediation. However, its usefulness has been usually inhibited by mZVI particles' low mobility in saturated porous media for sedimentation and deposition. In our study, laboratory experiments, including sedimentation studies, rheological measurements and transport tests, were conducted to investigate the feasibility of xanthan gum (XG) being used as a coating agent for mZVI particle stabilization. In addition, the effects of XG concentration, flow rate, grain diameter and water chemistry on XG-coated mZVI (XG-mZVI) particle mobility were explored by analyzing its breakthrough curves and retention profiles. It was demonstrated that XG worked efficiently to enhance the suspension stability. and mobility of mZVI particles through the porous media as a shear thinning fluid, especially at a higher concentration level (3 g/L). The results of the column study showed that the mobility of XG-mZVI particles increased with an increasing flow rate and larger grain diameter. At the highest flow rate (2.30 x 10(-3) m/s) within the coarsest porous media (0.8-1.2 mm), 86.52% of the XG-mZVI flowed through the column. At the lowest flow rate (0.97 x 10(-4) m/s) within the finest porous media (0.3 -0.6 mm), the retention was dramatically strengthened, with only 48.22% of the particles flowing through the column. The XG-mZVI particles appeared to be easily trapped at the beginning of the column especially at a low flow rate. In terms of two representative water chemistry parameters (ion strength and pH value), no significant influence on XG-mZVI particle mobility was observed. The experimental results suggested that straining was the primary mechanism of XG-mZVI retention under saturated condition. Given the above results, the specific site-related conditions should be taken into consideration for the design of a successful delivery system to achieve a compromise between maximizing the radius of influence of the injection and minimizing the injection pressure. (C) 2015 Elsevier Ltd. All rights reserved.