Development and application of Fenton and UV-Fenton processes at natural pH using chelating agents for the treatment of oil sands process-affected water

• Author / Creator

  Zhang,Ying

• The increasing amount of oil sands process-affected water (OSPW), its high toxicity towards aquatic organisms due to naphthenic acids (NAs) and other organics, and the zero discharge practice of the oil sands industry urge researchers to seek effective approaches for its treatment. Advanced oxidation processes (AOPs) have been successfully used to degrade recalcitrant NAs and reduce the overall toxicity of OSPW towards selected test organisms. Fenton and photo-Fenton processes as very common AOPs have not been applied on the OSPW treatment yet. Due to the precipitation of Fe at pH > 3, nitrilotriacetic acid (NTA) and [S,S]-ethylenediamine-N,N’-disuccinic acid ([S,S]-EDDS) have been employed to form complex with iron and enable Fenton reactions at high pH. The overall goal of this research was, therefore, to investigate the reaction mechanisms and kinetics of the (UV-)chelate-modified Fenton processes on the treatment of OSPW. In the first stage, this research focused on the optimization of the NTA- and EDDS-Fenton processes in the degradation of cyclohexanoic acid (CHA), a model NA compound. The final CHA removal under the optimum conditions was 87% for NTA-Fenton and 64% for EDDS-Fenton. Hydroxyl radical (•OH) was the main radical responsible for the CHA removal in both processes, while superoxide radical (•O2-) played a minor role. Oxy-CHA, hydroxyl-CHA, and dihydroxyl-CHA were detected as the CHA oxidation products. The second-order rate constants of •OH with CHA, NTA, and EDDS at pH 8 were investigated and obtained as 4.09±0.39×109, 4.77±0.24×108, and 2.48±0.43×109 M-1s-1, respectively. The consecutive addition of hydrogen peroxide (H2O2) and Fe-EDDS in the EDDS-Fenton process led to a higher removal of CHA compared with adding the reagents at a time. However, the H2O2 dosing mode in the NTA-Fenton process did not have a significant impact on the CHA removal. The mechanisms of Fe(II) oxidation and Fe(III) reduction in the NTA/EDDS-Fenton processes were proposed and discussed based on the redox potentials of Fe(III/II)NTA, Fe(III/II)EDDS, O2/O2•-, and H2O2/O2•-. In the second stage of the research, UV irradiation was applied in different processes at pH 3 and 8. Compared to UV-H2O2, Fenton, UV-Fenton, NTA-Fenton, and UV-Fe-NTA processes, UV-NTA-Fenton at pH 3 exhibited the highest efficiency for the H2O2 decomposition, CHA removal, and NTA degradation. H2O2-Fe(III)NTA adduct was related to the high amount of Fe in the UV-NTA-Fenton system at pH 8 compared to that in the UV-Fe-NTA system. The co-complexing effect of borate buffer helped to keep iron soluble at pH 8; however, it imposed a negative influence on the CHA degradation in the UV-NTA-Fenton process. In the third stage of the research, the removals of phenol and hydrogen sulfide (H2S) as toxic contaminants in OSPW were investigated. Tetrapolyphosphate (TPP) is a chelating agent of iron and the catalytic ability of Fe(III)TPP in the oxidation of H2S was investigated. The formation of Fe(III)TPP-HS- intermediate in the reaction of Fe(III)TPP and HS- was proposed. The second-order rate constant of the direct reaction between Fe(III)TPP and HS- at pH 9 was obtained as 4.36±0.17 M-1s-1. The oxidation of Fe(II)TPP by O2 followed a four-step reaction mechanism, and the reaction kinetic parameters were calculated based on the half-wave potential (E1/2) of Fe(III/II)TPP. Low level of phenol degradation was found in the Fe(III)TPP-catalytic desulfurization process. At the end of the research, the application of the UV-NTA/EDDS-Fenton processes in the treatment of OSPW at natural pH was investigated. The photodecomposition of Fe(III)NTA and Fe(III)EDDS under UV irradiation in MilliQ water and OSPW was described and compared. UV-NTA-Fenton exhibited higher efficiency than UV-EDDS-Fenton in the removal of acid extractable organic fraction and aromatics. NAs removal in the UV-NTA-Fenton process was much higher than that in the UV-H2O2 and NTA-Fenton processes. Both UV-NTA-Fenton and UV-EDDS-Fenton presented promoting effect on the acute toxicity of OSPW towards Vibrio fischeri. No significant change of the NTA toxicity occurred during the photolysis of Fe(III)NTA; however, the acute toxicity of EDDS towards Vibrio fischeri increased as the photolysis of Fe(III)EDDS proceeded. Considering the contaminants removal and the toxicity effect, NTA is a much better agent than EDDS for the application of UV-Fenton process as a very promising approach for OSPW remediation, and the process deserves more in-depth research on the toxicity depletion and organics removal.

• Subjects / Keywords

  • Chelating agents
  • Oil sands process-affected water
  • (UV-)Fenton
  • Natural pH

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3W669D9Q

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.