Urban Water Security Research Alliance Electrochemical Treatment of Reverse Osmosis Concentrate: Strategies to Minimise the Formation of Halogenated By-products Arseto Yekti Bagastyo Electrochemical Treatment of Problematic Water Recycle Waste Streams Science Forum, 19-20 June 2012
- Sent to power stations, industry and agriculture. - Supplement for drinking water supplies (emergency drought response) RO spiral wound modules Reverse osmosis Concentrate (ROC) - 15-25% of the feed water. - Rejected contaminants are concentrated up to 7 times! Electrochemical oxidation of ROC High conductivity of ROC lowers the energy consumption. No use of chemicals!. Using appropriate electrode material and anode potential, a series of oxidant species is generated at the anode (e.g. OH ▪ , H 2 O 2 , O 3 ). 2
Boron doped diamond (BDD) 2- 2- H 2 O / OH ▪ 2- / CO 3 H 2 O 2 / H 2 O 2- / SO 4 O 3 / H 2 O O 2 / H 2 O Cl 2 / Cl - C 2 O 6 S 2 O 8 (Rychen et al., 2010) OH ▪ (Comninellis, 1994) are generated by water electrolysis at the electrode surface (M): M (OH ▪ ) + H + + e - M+H 2 O Since (BDD)OH ▪ are quasi-free, i.e. not adsorbed by the anode and similar to aqueous OH ▪ , oxidative degradation of organic matter will be enhanced (Bejan et al., 2012): M (OH ▪ )+R O + H + + e - M + mCO 2 + nH 2 Any ions present in the solution (e.g. Cl - , SO 4 2- , CO 3 2- ) will also be oxidized at the electrode surface or by the generated OH ▪ : +2e - Cl - Cl 2 ROC has typically ≥ 1g/L of Cl - , and intense 2SO 4 2- S 2 O 8 2- +2e - electrochemical hypochlorination may lead to the formation of toxic, chlorinated by- 2- C 2 O 6 2- +2e - 2CO 3 products!!!
BDD electrochemical oxidation at acidic and neutral pH and HOCl/OCl - will be affected by the pH, with pH ≥ 6 favouring the Competition between OH ▪ participation of OH ▪ . Relative distribution of active chlorine Relative mass distribution of chloride radicals species (Deborde and Von Gunten, 2008) and hydroxyl radicals (De Laat et al, 2004) ▪ - pH < 3: HOCl/Cl 2 pH < 4: Cl 2 pH > 6: OH ▪ / HOCl ▪ - pH 6-10: HOCl/OCl - 4
Results. Removal of COD and DOC • t : 96 h pH 2 • vol : 5 L ROC pH 6-7 • I : 0.5 A pH 2: 48% DOC • E AN : 3.4-3.7 V pH 6-7: 54% DOC removal COD 0 : 136 mg/L DOC DOC 0 : 42 mg/L (3.5 mM) COD pH 2: 64% DOC removal pH 6-7: 68% DOC removal Faster COD removal at pH 2 intense electro-chlorination by the dominant HOCl species. Faster DOC removal at pH 6-7 enhanced participation of OH • and possibly other oxidants 2- and HC 2 - ) in oxidative degradation of organics. (e.g. S 2 O 8 O 8 Incomplete DOC removal at both pH remaining DOC 32-36 % (persistent organic fraction, not oxidisable by the COD test kit). 5
Results. Chloride oxidation and measured free available chlorine (FAC) Chloride Faster oxidation of pH 2 FAC Cl - to Cl 2 at pH 2. pH 6-7 Lower FAC in acidic pH is observed due to Cl 2 stripping to the gas phase. . 7.2 mM Cl - 2.3 mM Cl - Both Cl - and HOCl/OCl - can act as scavengers of OH • and generate less reactive chloro-radicals (e.g. OCl • , Cl • and Cl 2 • ). 6
Results. Formed trihalomethanes (THMs) and haloacetic acid (HAAs) pH 6-7 Polychlorinated species were dominant. - Trichloromethane (TCM): 70-80% of THMs - Trichloroacetic acid (TCAA): 40-50% of HAAs THMs TCM pH 2 Higher concentrations of THMs/HAAs measured at pH 6-7. - Release by hydrolysis of other DBPs which were not measured in this study, e.g. haloacetonitriles, haloacetaldehydes and haloacetamides (Chen, 2011). pH 6-7 pH 2 Decrease in THMs and HAAs was observed TCAA HAAs with the increasing the electrolysis time. - chloro-THMs+HAAs (as molar conc. Cl - ): 16-28% of AOCl (at 5.2 Ah L -1 ) was decreased to 4-8% of AOCl (at 10.9 Ah L -1 ). 7
Results. Formation of halogenated organics (AOX) pH 2 AOCl pH 6-7 AOBr AOCl • As expected, adsorbable organic chlorine (AOCl) was the AOBr dominant AOX species. • AOCl and AOBr formation was higher at pH 2 . 3% of initial [Cl - ], i.e. 39.5 mM after 11 Ah L -1 was incorporated into the remaining organics. However, the ratio DOC:AOCl in the final sample was 1.25:0.9 (pH 2) and 1.1:0.8 (pH 6-7) , i.e. the remaining organics were highly chlorinated. Decrease in AOBr at longer oxidation time brominated organics are further oxidised. 8
) for Cl ‐ Novel strategy. Electrodialysis of ROC (ROC ED separation prior to electro ‐ oxidation Electrodialysed ROC (ROC ED ): • 91% Cl - separated (lowered from 37.5 to 4 mM) • 17% COD permeated • 12% DOC permeated • Conductivity decreased from 5.2 to 1.4 mS cm -1 • pH: 6.8 Electrochemical oxidation of ROC ED : In order to maintain the conductivity and investigate the effects of electro-generated OH ▪ /ROS and/or S 2 ▪ - /OH ▪ 2- /SO 4 O 8 species on the oxidative degradation of organics for the same initial ROC ED (at pH 6-7) addition of NaNO 3 and Na 2 SO 4 to ROC ED Re-addition of NaCl was done for control experiments.
Results. Removal of COD and DOC, and formed THMs and HAAs COD removal was lowered from 100% ([Cl - ]=37 mM) to 60-74% ([Cl - ]=4 mM). DOC removal was increased from 38% ([Cl - ]=37 mM) to 51% ([Cl - ]=4 mM) , particularly in ▪ - . 2- electrolyte, due to the contribution of S 2 2- /SO 4 the presence of SO 4 O 8 (5.6 Ah L -1 ) (5.6 Ah L -1 ) The formed THMs and HAAs was significantly decreased when [Cl - ] was lowered from 37 to 4 mM. Increased formed THMs and HAAs (in sulfate ): oxidation of Cl - to reactive chloro-species in ▪ - radicals. 2- ions, and/or in the vicinity of the electrode surface by SO 4 the bulk by S 2 O 8 10
Conclusions Conclusions • The most efficient COD removal is observed in the presence of high concentrations of Cl ‐ ions. It is mainly achieved by electro ‐ chlorination , which is favoured at pH 2 . • Faster DOC removal was observed at pH 6 ‐ 7 , likely due to the enhanced participation of OH • in the indirect oxidation mechanism. • At both acidic and neutral pH the formation of THMs, HAAs and AOX was observed , with AOCl being the dominant species. While THMs and HAAs are degraded by prolonging the electrolysis time, AOCl is continuously formed. The toxicity of the remaining organic fraction remains to be determined. • Separation of chloride ions prior to electrochemical oxidation seems to be the only option for an application of this process for the treatment of highly saline waste streams such as ROC.
Acknowledgements Acknowledgements • Advisors: • Australian Research Council (grants LP0989159) A/Prof Damien Batstone (UQ) Dr Jelena Radjenovic (UQ) Prof Korneel Rabaey (UQ/UGent) • Project team: Prof Jurg Keller (UQ-Project Leader) Dr Wolfgang Gernjak (UQ) QLD Health and Forensic Analytical Service • Collaborators: • UQ International and APAI Scholarships Curtin Water Quality Research Centre Dr Ina Kristiana and A/Prof Cynthia Joll • Co-authors: Damien Batstone, Wolfgang Gernjak, Korneel Rabaey and Jelena Radjenovic
Urban Water Security Research Alliance THANK YOU www.urbanwateralliance.org.au
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