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Permeable reactive barrier for groundwater remediation

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A Permeable Reactive Barrier (PRB) is an in situ permeable treatment zone designed to intercept and remediate a contaminant groundwater plume. PRB was first implemented in the early 1990s and PRBs have become an important remediation technique among the various technologies available to remediate groundwater contamination. Soil mix technology (SMT) is a relatively recent development PRB installation technique in which reactive materials are mixed in-situ with the native soil to intersect the flow of contaminated groundwater. SMT has shown to be cost effective and versatile system with many environmental advantages over other remediation techniques. Project SMiRT (Soil Mix Remediation Technology) is a real field remediation trial involved the execution of the largest research field trials in the UK for the application of SMT in land remediation. The main aim of the project was field validation, testing and assessment to increase stakeholder confidence in and uptake of SMT for land remediation in the UK. PRB technology represents one of the remediation trials within SMiRT project where a range of aluminosilicates, organoclays, inorgano-organo bentonites and natural zeolite were used in different combinations as reactive materials. The treatment process associated with these materials is sorption and their high sorptive capacity and ability to deal with a range of contaminants make them ideal for use in PRB systems. The objectives are to investigate the (i) materials relative adsorption efficiency, (ii) reaction kinetics (iii) mass balance of the contaminants (iv) monitor the groundwater remediation through PRB system in field scale (v) and to evaluate the SMT PRB performance in field scale. Inorgano- organobentonites (IOB) are produced by the exchange of organic quaternary ammonium structure for inorganic ions (Na+, Ca2+) on the internal and external surfaces of bentonite. Pillaring agents with Aluminium are used to intercalate QAC within bentonite layers. IOB have significantly higher sorptive capacity for many organic pollutants relative to conventional bentonite. Three different IOBs were used in the current study. The material were characterised using X-ray diffraction (d-spacing changes) and thermo-gravimetric analysis (hydrophilic nature and carbon content). According to batch tests with model contaminant solutions (T, E, X) the maximum adsorption order was IOB2 -OCg-Zlt >= OCg > IOB -OC>> IOB1 >IOB2> IOB3 > IOB2-Zlt. The adsorption was modelled by Langmuir and Freundlich isotherms and showed high adsorption capacity. The materials showed higher affinity to the real contaminated site groundwater (average 80mg/g) with greater adsorption capacity than model contaminants (30mg/g) and the adsorption was mainly due partitioning effect. Desorption tests were carried out and showed strong contaminants bonding. Materials also showed greater adsorption capacity towards hydrocarbons and (dodecane) with more than 550 mg/g. Column test using same materials indicated similar results with similar sorptive preference order. The column tests performed complemented the batch tests in examining flow conditions, groundwater and model contaminants to simulate the field conditions. Materials showed slight to no effect on aquifer permeability and the mixes have same order of magnitude. Site cores from within the PRB walls showed high vulnerability with indications of good mixing quality and consistency down to 5 m depth. Groundwater pumping through the PRB indicated occasional improvement in groundwater quality parameters where more pumping still needed. Solid state sensors were developed to facilitate real time groundwater monitoring. It was tested in lab scale with the use of model contaminated solution. Sensor step curve with MTBE and xylene showed high response up to 100ppm. Results indicated that sensors response time depends on material fabrications and temperature. The results of this research confirmed the efficiency of the used materials in PRB application with the varied efficiency due to materials properties. It also emphasises the difference in materials performance at lab and field scale. More time and work are needed to draw general conclusions on field trial quality assurance.

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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