Evaluation of reactive cap sorbents for in-situ remediation of contaminated sediments

Abstract

Contaminated sediments can be treated using in-situ treatment methods that aim to either degrade or sequester contaminants, reducing their bioavailability. The main purpose of this research was to develop and evaluate a reactive capping mat that can be used for in-situ remediation of contaminated sediments. This study investigated the interferences caused by humic acid on the adsorption of co-planar and non-co-planar polychlorinated biphenyls (PCBs) including 2-chlorobiphenyl, 2, 2\u27, 5, 5\u27-tetrachlorobiphenyl, 3, 3\u27, 4, 4\u27-tetrachlorobiphenyl, 2, 2\u27, 4, 4\u27, 5, 5\u27-hexachlorobiphenyl and 3, 3\u27, 4, 4\u27, 5, 5\u27-hexachlorobiphenyl and polycyclic aromatic hydrocarbons (PAHs) including naphthalene, phenanthrene and pyrene on two types of sorbents being evaluated for use in a mat: activated carbon and organoclays. Several kinetic and isotherm studies have been conducted using several formulations of activated carbons and organoclays as sorbents to treat individual PCB congeners and PAHs. The results showed that preloading of sorbents with humic acid, and simultaneous adsorption of humic acid and contaminant, significantly reduced the adsorption capacity for all selected PCB congeners and PAHs. Experiments conducted without preloading of sorbent surfaces demonstrated that desorption upon subsequent spiking with humic acid, to simulate the long-term exposure to porewater that contains high humic acid concentrations, was not pronounced and varied with co-planarity of PCBs and number of rings of PAHs. Also, humic acids were found to interfere to a much greater extent with adsorption to activated carbon than with organoclay formulations evaluated in this work. Experiments were also conducted to determine the effects of Suwannee River fulvic acid (FA), humic acid (HA) and natural organic matter (NOM) obtained from International Humic Substance Society (IHSS) and pore water isolated from sediment of the Hudson River and the Passaic River to understand the influence of different fractions of dissolve organic carbon that will be present in real site conditions. The results demonstrated enhancement in adsorption of PCB and PAH in presence of fulvic acid on both type of sorbents including activated carbon and organoclay but the effect of humic acid and NOM varied with contaminant. The humic acid had more reducing effect on PCB adsorption as compared to NOM and NOM had more reducing effects on PAH adsorption. A structural analysis using Scanning Electron Microscopy for activated carbon and X-Ray Diffractometry, Atomic Force Microscopy and Scanning Electron Microscopy for organoclay were conducted to observe differences caused by humic acid on the surfaces of the sorbents. BET surface area analysis has also been conducted to determine the surface area of activated carbon and organoclays. Thermo gravimetric analysis of organoclays was done to determine the % organic content which increases the hydrophobicity and thereby adsorption capacity of organoclays. This research indicate that organic acids, which are quite concentrated in sediment porewater, have a significant impact on the efficacy of reactive cap components and are an essential factor in the design and ultimate performance of this type of in-situ sediment management approach