|Ph.D Student||Azerrad Sara|
|Subject||Integrated Treatment of Reverse Osmosis Brines for|
Removal of Micropollutants
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Carlos Dosoretz|
|Full Thesis text|
Dense membrane filtration, i.e., reverse osmosis (RO) is a generic means for purification of treated wastewater effluents enabling sustainable water recycle while confining contaminants in brines. Advanced oxidation processes (AOPs) emerge as a useful technique for micropollutants removal from brines. However, brines components perform a considerable quenching to hydroxyl radicals, thus reducing the rate and efficiency of transformation.
The overall aim of this research is to study the fate and key aspects governing the oxidation of micropollutants in brines, with special emphasis on the oxidation patterns of iodinated contrast media. A systematic characterization and quantification of the quenching influence of main RO brines components and accumulation of transformation products (TPs) was performed with diatrizoate (DTZ) and iopromide as model compounds and use of synthetic solutions mimicking real RO brines. The results were validated using fresh RO brines. AOP was performed with nonthermal plasma (NTP) and UVA-TiO2. The influence of reactive species involved in these two oxidation techniques (hydroxyl radicals, ozone, direct photolysis) on transformation of DTZ was comparatively assessed. An integrative process to improve micropollutants oxidation in brines was studied as well, involving chemical coagulation (CC) or electrocoagulation (EC) as pretreatment. In this stage, additional model compounds were added: caffeine, carbamazepine, dexamethasone and ketoprofen.
The major contribution to quenching in RO brines resulted from organic matter (~70%) followed by bicarbonate alkalinity (~30%). Interestingly, humic-like materials performed higher quenching than the soluble microbial products fraction in spite of their relative lower concentration. In total fourteen DTZ transformation products were identified of which five new TPs were elucidated. Accumulation of TPs was markedly different in RO brines in comparison to UPW. This evidences the influence of brines components in the formation of reactive species that will further attack DTZ/TPs or the scavenging performed by the components themselves will limit further transformation of TPs. DTZ transformation resulted in formation of iodide in both oxidation techniques with the subsequent oxidation of iodide to iodate by ozone at NTP conditions. Five plausible degradation pathways are proposed for DTZ transformation. The main reactions taking place in DTZ transformation included hydroxylation, deiodination, oxidation of hydroxyl groups, cleavage of amine moiety, decarboxylation, deacetylation and ipso-attack at carboxylic acid or iodine moieties.
Of the pretreatment tested, iron dosed-CC and anodic Al-EC exhibited similar efficiencies for removal of dissolved organic carbon (~40-50%), alkalinity (42-95%) and phosphate (>99%). The major limitation of anodic iron-EC is the incomplete oxidation of iron at low pH leaving residual iron in the supernatant. Moreover, the main disadvantage of chemical coagulation is the increase of chloride content. Coupled in tandem, Fe-CC or Al-EC of brines and UVA-TiO2 oxidation of the supernatants increased transformation of the micropollutants tested due to reduced concentration of quenching components, enabling complete transformation of all models compounds out of DTZ.
Concluding, the results of this research helped to understand and quantitatively evaluate the role of the main RO brines matrix components during advanced oxidation and physico-chemical pretreatment as well as to assess their influence on the formation and accumulation of transformation products.