IN-SITU REMEDIATION OF ARSENIC CONTAMINATED GROUNDWATER USING PERMEABLE REACTIVE BARRIER

Abstract

Arsenic pollution is posing one of the serious public health concerns due to its vast scale presence in groundwater, especially in South Asia along the foothills of the Himalayas. The presence of arsenic in soil and water is primarily due to geogenic sources induced by anthropogenic activities. Arsenic availability and its concentration level in groundwater are significantly controlled by pH and redox conditions of the soil-water system and it commonly exists in forms of inorganic compounds of As(III) and As(V). The toxicity of As(III) is relatively higher than As(V) due to its high binding affinity towards thiol or sulfhydryl groups in tissue proteins of human body organs. The population exposed to a higher concentration of arsenic can develop several severe diseases including arsenicosis, hyperkeratosis, and cancers. The use of permeable reactive barriers (PRB) seems one of the promising technologies for in-situ remediation of arsenic-containing groundwater. Pollutant removal by PRB mainly relies on sorption processes of the reaction zone made of a suitable reactive media. As adsorption based techniques for arsenic removal are well proven and widely used in ex-situ treatments, up-gradation of these adsorbents, along with their use in PRB, is required for in-situ treatment of arsenic polluted groundwater. Amongst the various adsorbents used for removing arsenic, iron-based nanoparticles are considered most promising due to their very high adsorption efficiency. However, these adsorbents are mostly used as fine to ultrafine powder forms making their field application very limited due to the formation of aggregates, which reduces the hydraulic conductivity and reactivity of adsorbents. Nonexpanding base materials are thus needed to develop composites of these nanoparticles for effective dispersion of nanoparticles and to preserve the hydraulic conductivity without compromising with their adsorption efficiency. In this research work, zero valent iron nanoparticles loaded pumice (P-nZVI 10, P-nZVI 20) and zeolite (Z-nZVI 10 and Z-nZVI 20) composites and iron oxide maghemite phase loaded pumice, namely P-maghemite composite, are developed and assessed for As(III) and As(V) removal. The synthesized composites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermo-gravimetric analysis (TG/DTA). Thereafter, a series of batch experiments are conducted to study the removal of As(III) and As(V) using the developed composites under varying experimental conditions. Fixed bed column experiments are then conducted to assess the feasibility of using P-nZVI 20 and P-maghemite as the reactive materials in a continuous flow remediation system. Finally, the experiments are conducted for assessment of the most effective developed P-maghemite composite in PRB for in-situ arsenic removal in two modes viz columnar PRB and well-integrated PRB. The characterization of composites using SEM and XRD indicates the effective loading and dispersion of nanoparticles on the surface of the base materials. The removal of As(III) and As(V) is observed to increase linearly with contact time for all the composites until an equilibrium is achieved around 10-12 hours in the batch experiments. At the equilibrium, more than 80 % of As(III) and As(V) removal is observed by PnZVI 20 and P-maghemite. Both the composites are found effective for removing As(III) and As(V) at varied pH; however, an optimum pH range of 5-9 for PnZVI 20 and 2-7 for P-maghemite is observed. The adsorption kinetics and isotherms study for all the adsorbents reveals that the interaction of As(III) and As(V) with the developed adsorbent is spontaneous and physical in nature. A regeneration efficiency of almost 95 % is observed for both of the composites. Further, more than 99 % of As(III) removal is observed in the fixed-bed column experiments conducted using P-nZVI 20 and P-maghemite. A slight reduction in hydraulic conductivity is observed for both the composites at the end of the column experiments. PRB experiments are carried out under controlled conditions using P-maghemite as the adsorbent for the removal of arsenic from laboratory prepared synthetic groundwater. For the columnar PRB, 16 cm bed depth provided adequate residence time for the removal of arsenic as more than 99 % of arsenic removal is achieved throughout the experiment. The results from the well-integrated PRB shows arsenic concentration little above the permissible limit after 15 days of operation. However, the 4 cm well integrated PRB is able to reduce the arsenic concentration from 652 μg/L to less than 20 μg/L, showing a great potential for arsenic remediation at slower flow rates. Other than arsenic, concentration of all the ions in the effluent is observed below permissible limit except Fe. The reduced removal efficiency of well screen integrated PRB seems due to the high flow rate of groundwater through the reactive well screen, which did not allowed sufficient time of contact between pore water and the surrounding reactive material. An improved version of the PRB with low well discharge can help in more effective removal arsenic from heavily polluted groundwater using the developed composites. The high removal efficiency of As(III) and As(V) in batches as well as in continuous flow-through remediation systems with a reasonably high regeneration efficiency indicates the usability of the developed iron-based nanoparticles composite in remediation of As polluted water.