Abstract
This study investigated the feasibility of using sodium hexametaphosphate (SHMP)- amended calcium (Ca) bentonite in backfills for slurry trench cutoff walls for the containment of lead (Pb) contamination in groundwater. Backfills composed of 80 wt% sand and 20 wt% either Ca-bentonite or SHMP-amended Ca-bentonite were tested for hydraulic conductivity and sorption properties by conducting laboratory flexible-wall hydraulic conductivity tests and batch isothermal sorption experiments, respectively. The results showed that the SHMP amendment causes a one order of magnitude decrease in hydraulic conductivity of the backfill using tap water (1.9 to 3.0 × 10−10 m/s). Testing using 1000 mg/L Pb solution resulted insignificant variation in hydraulic conductivity of the amended backfill. Moreover, SHMP-amendment induced favorable conditions for increased sorption capacity of the backfill, with 1.5 times higher retardation factor relative to the unamended backfill. The Pb transport modeling through an hypothetical 1-m-thick slurry wall composed of amended backfill revealed 12 to 24 times of longer breakthrough time for Pb migration as compared to results obtained for the same thickness slurry wall with unamended backfill, which is attributed to decrease in seepage velocity combined with increase in retardation factor of the backfill with SHMP amendment. Overall, SHMP is shown to be a promising Ca-bentontie modifier for use in backfill for slurry trench cutoff wall for effective containment of Pb-contaminated groundwater.
Highlights
Soil-bentonite (SB) slurry trench cutoff walls usually consist of backfill composed of sodium (Na) bentonite mixed with either in-situ excavated soil or appropriate imported soil, and are commonly used in the U.S as vertical barriers to prevent lateral migration of contaminated groundwater [1].The SB slurry walls may be used as an interim measure for the short-term containment of impacted groundwater until an efficient and effective treatment technology is developed and deployed [2].Low permeability and the contaminant sorption capacity are the two dominated properties of the backfill that control the containment performance of the walls [3,4]
As the hydraulic conductivity was higher with tap water, no further testing was conducted with Pb solution as permeant liquid
The replicate sodium hexametaphosphate (SHMP)-amended backfill specimens (SHMP-20CaB) exhibited hydraulic conductivity of 1.9 × 10−10 m/s and 3.0 × 10−10 m/s when tap water was used as permeant, and 1.7 × 10−10 m/s and 1.6 × 10−10 m/s when Pb solution was used as permeant. These results showed that Pb solution decreases hydraulic conductivity 0.89 to 0.53 times as compared to that with tap water as permeant
Summary
Soil-bentonite (SB) slurry trench cutoff walls usually consist of backfill composed of sodium (Na) bentonite mixed with either in-situ excavated soil or appropriate imported soil, and are commonly used in the U.S as vertical barriers to prevent lateral migration of contaminated groundwater [1].The SB slurry walls may be used as an interim measure for the short-term containment of impacted groundwater until an efficient and effective treatment technology is developed and deployed [2].Low permeability and the contaminant sorption capacity are the two dominated properties of the backfill that control the containment performance of the walls [3,4]. Soil-bentonite (SB) slurry trench cutoff walls usually consist of backfill composed of sodium (Na) bentonite mixed with either in-situ excavated soil or appropriate imported soil, and are commonly used in the U.S as vertical barriers to prevent lateral migration of contaminated groundwater [1]. The SB slurry walls may be used as an interim measure for the short-term containment of impacted groundwater until an efficient and effective treatment technology is developed and deployed [2]. Low permeability and the contaminant sorption capacity are the two dominated properties of the backfill that control the containment performance of the walls [3,4]. To enhance service life and/or service performance of the SB slurry walls, novel bentonites including multiswellable bentonite MSB, contaminant-resistant bentonite SW101 [5], and bentonite-polymer composite BPC [6] were developed. Public Health 2020, 17, 370; doi:10.3390/ijerph17010370 www.mdpi.com/journal/ijerph
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
