Sunlight-driven persulfate oxidation as a sustainable solution for the degradation of trypan blue in aqueous medium

Water pollution resulting from the release of dye contaminated wastewater poses a serious threat to aquatic ecosystems and human health. Sunlight-based water treatment is an emerging method of sustainability practice. This study proposes the use of persulfate-based advanced oxidation processes for the degradation of a dye pollutant, using trypan blue (TB), an azo dye, as a model pollutant. The degradation was carried out under natural sunlight by adding persulfate to the dye solution. The effect of parameters such as light intensity (sunny weather, cloudy weather, and dark conditions), varying persulfate concentration, initial pH conditions, and co-existing inorganic matrices on the degradation efficiency was investigated. At a PS concentration of 10 mM, the degradation of TB was increased from about 28% under ambient room conditions to 60% under cloudy conditions and 88% under direct sunlight at 60 min, explaining the strong influence of irradiation conditions and indicating that higher PS doses enhanced the generation of reactive radicals and consequently improved the process performance. Increasing the PS concentration to 25 mM enhanced the degradation to about 99%, while only about 22% degradation was observed under direct sunlight without PS. An efficient degradation of about 99% was achieved under sunny weather when the PS concentration was 25 mM. pH is an important parameter that controls the degradation efficiency, and both alkaline and acidic pH favoured practical degradation of TB. The transformation of reactive species caused the difference in degradation. Among the selected inorganic ions, sulfate ions, didn't show much impact on degradation, while chloride and nitrate ions showed an inhibitory effect on degradation. However, carbonate ions enhanced the degradation of TB under the same conditions. In short, sunlight-driven PS activation is an effective and sustainable method for TB with potential applicability to a broad spectrum of recalcitrant organic pollutants in wastewater treatment systems, suggesting its strong dependence on solar irradiance and its potential as sustainable, catalyst-free treatment technology for dye-contaminated wastewater.