The removal of dissolved organic matter (DOM) from water using photocatalysis coupled with coagulation

Abstract

Dissolved organic matter (DOM) comprises of both synthetic and natural organic compounds such as pharmaceuticals, pesticides, natural organic matter (NOM), found in the environment. Most of the DOM pollute drinking water sources, which inevitably end up in water distributed to communities for consumption. There are several methods that are currently employed in water treatment plants to eliminate DOM from drinking water, but the removal efficiencies are not of required standard. The existence of NOM in drinking water is undesirable because it decreases the aesthetic merit of water. Moreover, NOM can result in the generation of disinfection by-products (DBPs) when it reacts with chlorine-based disinfectants. Pesticides are also a major concern as they contribute to drinking water pollution. Water pollution resulting from organic materials such as pesticides have been linked to several adversative effects on the environment and human health. This work is divided into two parts, with both aimed to evaluate a photocatalysis-coagulation integrated process for the removal of DOM in water. The first part of the study focussed on the photocatalytic-coagulation of a herbicide, mecoprop using titanium dioxide (TiO2) as a photocatalyst and ferric sulphate (Fe2(SO4)3) as a coagulant, under Ultraviolet-C (UVC) irradiation. The aim was to facilitate simultaneous removal of mecoprop, background organic matter and turbidity, as well as the removal and recovery of TiO2 nanoparticles (NPs) from surface water. Jar tests were performed to optimize the coagulation conditions ([Fe2(SO4)3] and pH). Subsequently, oxidative degradation experiments were conducted with UVC radiation in a bench scale collimated beam system. Control tests were performed, where removal of mecoprop was evaluated under photolysis, catalysis and coagulation, respectively. Furthermore, the combination of UV-coagulation, UV-TiO2, TiO2-coagulation were employed for the removal of mecoprop from surface water samples. Up to 88% removal of mecoprop was achieved by direct photolysis at a maximum UV fluence of 8000 cm2.mJ-1. Comparatively, photocatalysis with TiO2, displayed complete degradation of mecoprop at UV fluence of 4500 cm2.mJ-1and TiO2 concentration of 100 mg/L. However, when photocatalysis (UV-TiO2) and coagulation (Fe3+) were combined, a maximum degradation rate constant of 0.0034 cm2.mJ-1 was obtained. This was followed by the UV-Fe3+ process, with a rate constant of 0.0031 cm2.mJ-1. The improved mecoprop removal in the photocatalysis-coagulation was due to the synergy between a Fenton-like process (UV/Fe3+) and photolysis (UV), which overall lead to an improved production of hydroxyl radicals. However, the addition of TiO2 into the system improved the degradation rate by 0.0003 cm2.mJ-1, which is negligible. Therefore, the degradation of mecoprop could be performed without the photocatalysis, but with the UV/Fe3+ system alone. The second part of the study entailed the photocatalytic-coagulation removal of humic acid as a model NOM pollutant at a concentration of 10 mg/L, which is the concentration that is usually recorded in natural water. Titanium dioxide was modified by co-doping with varying concentrations of nitrogen and sulphur (1 g, 2 g, 4 g of thiourea, denoted as 1NS-TiO2, 2NS-TiO2, 4NS-TiO2) to achieve a visible light active catalyst. Coagulation experiments were performed using ferric chloride (FeCl3) to evaluate the recovery of NS-TiO2 nanoparticles and background organic matter. Subsequently, coagulation and photocatalysis processes were performed individually as controls and to optimize parameters such as coagulant dose, pH and photocatalyst dose. The photocatalysis-coagulation process was conducted under the optimized conditions ([FeCl3]= 30 mg/L, pH= 6, [2NS-TiO2]= 150 mg/L) under visible light irradiation (250 W). Optical differences were observed between the doped and undoped TiO2. Consequently, the pristine TiO2 (3.19 eV) band gap decreased when doped with nitrogen and sulphur and continued to decrease further with an increase in dopant (1NS-TiO2 = 3.18 eV, 2NS-TiO2 = 2.55 eV and 4NS-TiO2 = 2.41 eV). The results demonstrate that the combined photocatalysis-coagulation treatment process has a higher humic acid removal rate than the photocatalysis, coagulation individual processes (photocatalysis-coagulation k1 = 0.0143 min-1, photocatalysis k1 = 0.0066 min-1, coagulation k1 = 0.0074 min-1). In this case, both processes have been conclusively demonstrated to work synergistically to degrade and remove humic acid.