Abstract
This study examined the removal of natural organic substances (humic acids-HA, and fulvic acids-FA) from model solutions using photocatalysis and ultrafiltration. The effect of two nano titanium dioxide types (P25 and P90) with different active surface areas and two UV lamps (low-pressure and medium-pressure) on the effectiveness of FA and HA removal during photocatalysis was tested. An integrated photocatalytic + ultrafiltration system was also analyzed to determine the effectiveness of FA and HA removal and the changes in the relative permeate flux (Ultrafiltration Membrane Fouling-UF). We observed that photocatalysis using the P90 nano titanium dioxide was more efficient than the P25 due to the larger surface area (2×). The decomposition of organic substances proceeded efficiently up to 30 min of solution exposure, and the use of a medium-pressure lamp accelerated compound decomposition relative to the low-pressure lamp. The applied photocatalysis + ultrafiltration system was characterized by a high degree of FA removal while improved hydraulic efficiency was observed during ultrafiltration.
Highlights
Natural organic matter (NOM) found in aquatic ecosystems is a mixture of water-soluble humic substances/solvents; these include fulvic acids (MW ≤ 2000 Da, soluble at all pH levels), humic acids (MW > 2000 Da, insoluble at pH < 2), hymatomelic acids and insoluble compounds called humins [1,2]
Titanium dioxide particles have surface OH groups which serve as adsorption sites for organic compounds
A disadvantage of photocatalysts with high specific surface areas is a large number of crystalline defects that favor electron recombination as well as holes, both of which contribute to a reduction in their photocatalytic activity [13]
Summary
Natural organic matter (NOM) found in aquatic ecosystems is a mixture of water-soluble humic substances/solvents; these include fulvic acids (MW ≤ 2000 Da, soluble at all pH levels), humic (humus) acids (MW > 2000 Da, insoluble at pH < 2), hymatomelic acids (soluble in alcohol) and insoluble compounds called humins [1,2]. The dissolved NOM fraction accounts for approximately 80–90% of the total NOM amount in natural waters In their structural composition, natural organic substances have methyl groups responsible for by-product formation during water chlorination and are suspected to possess carcinogenic and mutagenic properties [2,3,4]. The specific structures of humic substances suggest they are characterized by high chemical activity and can react with various organic and inorganic admixtures in natural waters This means they may carry toxic micro-pollutants and can significantly complicate water purification technology. The most commonly used technological systems consist of classic unit processes such as coagulation, flocculation, and filtration in sand filters None of these processes, even when they are combined with sorption on activated carbon, result in highly efficient water purification due to the significant seasonal variability of natural organic properties and a tendency for increased concentrations. A key operation in the design and operation of water treatment processes is the need to develop advanced NOM removal methods that improve or even replace conventional water treatment methods [5]
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