A novel functionalized zeolite coating for mitigation of microbiologically influenced concrete sewer corrosion

An evaluation of the resistance and antibacterial characteristics of mortar/concrete specimens coated with silver-loaded zeolite to microbiologically influenced corrosion was carried out using Acidithiobacillus thiooxidans as corrosive agent. The experiments were undertaken by exposing uncoated (UC), epoxy coated (EP), blank zeolite coated, zeolite without silver-loading (ZC) and silver-loaded zeolite coated specimens (AC, AK, Z1, Z2, or Z3) to basal nutrient medium (BNM) inoculated with the corrosive bacterium. The antibacterial characteristics of zeolite and nano-copper oxide coated concrete pipes were also compared. Specimens were characterised by X-ray powder diffraction (XRPD), and Field-emission scanning electron microscopy (FE-SEM), optical microscopy and surface profiler. The antibacterial characteristics were evaluated by measuring the variation in sulphate production, sulphur oxidation, cellular ATP, oxygen uptake rate (OUR), biomass dry cell weight (DCW), pH and microscopic bacterial cell count (MBCC). The protective role from surface deterioration zeolite coating offers due to dissolution was evaluated by measuring the bioleaching rate of calcium and silicon. The rate of the silver release was studied to estimate the service life of the antibacterial zeolite coating. Silver partition from the zeolite coating into the BNM as well as its incorporation to bacterial biomass was investigated to elucidate the mechanism of bactericidal action of the silver-loaded zeolite coating. In mortar specimens coated with zeolite containing 3.5% silver and 5% copper (AC) and 5% silver and 14% zinc (AK), significant reduction in Acidithiobacillus thiooxidans activity was observed compared to that of uncoated and epoxy coated specimens indicating the antibacterial properties of silver-loaded zeolite. As demonstrated from biomass total suspended solid (TSS), biological sulphate production and live bacterial cell count; the activity of the aforementioned bacterium was higher in AC compared to AK In all the experiments where epoxy to zeolite to cure ratio by weight of 2:2:1 (Z2) and 1:3:1(Z3) were used, a significant biomass growth inhibition was observed. No A. thiooxidans growth inhibition was observed for uncoated, epoxy coated and blank zeolite coated and Z1 specimens where epoxy to zeolite to cure ratio by weight of 3:1:1 were employed. From XRPD and FE-SEM analysis, it was deduced that gypsum and ettringite crystals, which are the main indicators of bacterially induced corrosion in concrete sewers, were absent in ZC, Z2 and;Keyword: Zeolite coating, silver-loaded zeolite, epoxy, antibacterial, Acidithiobacillus thiooxidans, biofilm, concrete corrosion, ATP, OUR, XRPD, FE-SEM, gypsum, nano-copper oxide, biological sulfuric acid, biological sulfur oxidation, bioleaching, powder coating, mechanism of inhibition.;Z3 specimens, while there was an indication of the aforementioned corrosion crystals in UC, EP and Z1 specimens. The leachate of biotoxic metals from the antibacterial zeolite exerted no inhibitory impact on the respiratory activity of wastewater treatment plant sludge. Biofilm formation on powder coated specimens was found to be negligible. The activity of A. thiooxidans was negligible upon exposure to 2.6% silver-loaded chabasite (Z-Ag-2.6 %), and there was indication of some bacterial activity in the 18% silver-loaded chabasite (Z-Ag-18%). In both Z-Ag-2.6% and Z-Ag-18%, a progressive decrease in soluble silver (Ag-S) was observed due to uptake of silver by the bacteria as confirmed by an increase in particulate silver. The leaching rate of silver was only 0.001 d-1 and 0.0115 d-1 for Z-Ag-2.6% and Z-Ag-18%, respectively with corresponding half-life values of 700 d and 60 d. The activity of A. thiooxidans was negligible in Z2 compared to severely corroded (SC-N), moderate corroded (MC-N) and uncorroded (UC-N) concrete pipes coated with nano-copper oxide. As demonstrated by increased biomass concentration, OUR, ATP production and decrease in pH, the order of bacterial growth inhibition was SC-N > MC-N > UC-N. While there were no indicator of A. thiooxidans activity in the biofilm of SC-N and MC-N, there were active bacterial cells in the biofilm ofUC-N.