Effect of Tetrakis (Hydroxymethyl) Phosphonium Sulphate Biocide on Metal Loss in Mild Steel Coupons Buried in a Water-logged Soil

Abstract

Mild steel strip coupons were buried in water-logged clay soil sitesin the Niger Delta for 190 days, with one site untreated and the other site treated with a tetrakis (hydroxymethyl) phosphoniumsulphate (THPS)-based biocide. Post-exposure analysis of the coupons showed that there was an increasing trend in metal loss in the coupons as the exposure days increased, for both the untreated and treated soil sites. The trend of metal loss showed an average cumulative increase of 46.7% in the untreated soil site and 34.3% in the treated soil site. Average percentage weight loss (APWL) after the 40, 100 and 190-day observational periods, were 2.3%, 5.5% and 8.5% respectively, in the untreated soil; and 1.2%, 2.0% and 2.8% respectively, in the treated soil. Over the period, there was a cumulative 5.4% metal loss in the coupons from the untreated soil and 2.0% in the treated soil. With biocide treatment of the soil, there was a 59.5% decrease in cumulative APWL, comparing the untreated soil and the treated soil sites during each of the observational periods. Total bacterial counts determined by quantitative polymerase chain reaction (qPCR) showed a 5-log, 2-log and 1-log reduction in total bacterial counts after 40, 100 and 190 days, respectively, representing between 94-100% reduction in the bacterial numbersin the soil treated with 250 ppm of the biocide.