Selected antimicrobial applications of oxidised coal products

Abstract

South African bituminous coal was converted by controlled wet oxidation to a highly oxidized water-insoluble product, oxicoal, and a water-soluble product, oxifulvic acid. Oxihumic acid was produced by the extraction of the oxicoal with base and precipitation of the extract with acid. Due to the reported antimicrobial activity of humid substances, antimicrobial applications for the coal-derived products were investigated since a market exists for the production of low cost and effective antimicrobial compounds in South Africa. iii Oxifulvic acid was the most bactericidal of all the coal-derived products evaluated since 150.0 mg/I was required for bactericidal activity against seventeen bacterial tests isolates. The antibacterial activity of oxifulvic acid was pH dependant, with the optimum pH for bactericidal activity between pH 3.0 and pH 4.0. A linear relationship was exhibited between the calcium concentration and bactericidal concentration of oxifulvic acid. An eight-fold increase in oxifulvic acid was required for bactericidal activity in the presence of organic matter. iv Oxifulvic acid was bactericidal at 100 mg/1, in in vitro studies, against a range of twelve bacteria isolated from South African water-cooling systems, but was unable to control the growth (oxifulvic added at 200.0 and 400.0 mg/1) of sessile and planktonic bacteria during in situ studies. The concentrations of oxifulvic acid required for fungicidal ( > 4000.0 mg/1) and algicidal (400.0 mg/1) activity indicated that oxifulvic acid does not have potential for use as a fungicidal or algicidal compound. Oxifulvic acid (800.0 mg/1) proved to be more effective than a commercially available coal-tar disinfectant (48 000.0 mg/1). A concentration of 96.0 mg/1 oxifulvic acid was viricidal against coliphage V1 within 10 min. Oxifulvic acid did not exhibit any sporicidal activity. Oxifulvic acid (25.0 mg/1) in combination with sodium dodecyl sulphate (250.0 mg/1) exhibited synergistic activity. This combination has potential for use as a surface disinfectant on hard non-porous surfaces. Oxifulvic acid showed potential for use in the control of acid mine drainage formation by iron-oxidizing bacteria, since it inhibited iron-oxidation by these bacteria at 100. 0 mg/1 in in vitro studies. The antibacterial action of oxifulvic acid against Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli strain K12 was ascribed to the failure of the pH homeostasis mechanism of the bacteria, to maintain a constant intracellular pH. Pseudomonas aeruginosa, S. aureus and E. coli strain K12 were able to tolerate 125.0, 250.0 and 250.0 mg/1 higher concentrations of oxifulvic acid after 10 subcultures in the presence of oxifulvic acid. This increased tolerance to oxifulvic acid was ascribed to the habituation of the bacteria to a sub-lethal pH. Oxicoal removed 100% of the coliphages of E. coli strain K12 present in sterile distilled water and raw sewage in in vitro studies. The results indicated that oxicoal could be used as a substitute for coal in a dual-media sand/coal filter for the removal of enteric viruses from treated and untreated water.