Continuous Flow Process for Cr(VI) Removal from Drinking Water through Reduction onto FeOOH by ISRs E. Kaprara, F. Pinakidou, E. Paloura, A. Zouboulis and M. Mitrakas Aristotle University of Thessaloniki, Greece 13th IWA Specialized Conference on Small Water and Wastewater Systems (SWWS) 5th IWA Specialized Conference on Resources ‐ Oriented Sanitation
Introduction Chromium • Naturally occurring element found in rock, soil and groundwater. • Commonly present in the environment in two forms: Cr(III) and Cr(VI). Special interest • Cr(III): essential element for human and animal nutrition. • Cr(VI): toxic, causing various types of cancer and DNA damage. Cr(VI) origin in water • Natural: Oxidation of Cr(III) to Cr(VI) onto soils derived from ultramafic sediments and ophiolitic rocks. • Anthropogenic: industrial wastes from manufacturing processes, such as metal’s plating. 13th IWA Specialized Conference on Small Water and Wastewater Systems (SWWS) 2 5th IWA Specialized Conference on Resources ‐ Oriented Sanitation
Chromium regulation Cr(VI) priority pollutant Cr total < 0.1 mg/L Cr total < 0.05 mg/L ( ‐ ) Lack of regulation limit for Cr(VI). ( ‐ ) Total Cr limit underestimates the risk. (!) Strong intention for establishing regulation limit for Cr(VI) in drinking water. (!!!) U.S. State of California: MCL=10 μg Cr(VI)/L (1/7/2014). 13th IWA Specialized Conference on Small Water and Wastewater Systems (SWWS) 3 5th IWA Specialized Conference on Resources ‐ Oriented Sanitation
Cr(VI) in drinking water of Greece 4 Kaprara et al (2015), J. Hazard. Mater. 281, 2–11.
Chemical reactions for geogenic Cr(VI) formation Contact with ultramafic rocks activates Cr(VI) formation by a catalytic mechanism. Cr(VI) concentration in water samples accounts for 96% of total chromium in average. MnO 2 (s) + Mn 2+ → MnO 2 (s) • Mn 2+ 2Cr 3+ + 3MnO 2 (s) + 2H 2 O → 2HCrO 4 ‐ + 3Mn 2+ + 2H + MnO 2 (s) • Mn 2+ + O 2 → 2MnO 2 (s) + + 3MnO 2 (s) + 2OH ‐ → HCrO 4 ‐ + 3Mn ΟΟΗ (s) Cr(OH) 2 2MnO 2 (s) + 2Mn ΟΟΗ (s) → 2MnO 2 • Mn ΟΟΗ (s) 2MnO 2 • Mn ΟΟΗ (s) + 1/2O 2 → 4MnO 2 + H 2 O
Treatment technologies for Cr(VI) removal Chemical reduction and precipitation. Adsorption. Ion exchange. Membrane separation NF, RO. Electro‐dialysis. Electro‐coagulation. Phyto‐remediation. Flotation. Solvent extraction. 13th IWA Specialized Conference on Small Water and Wastewater Systems (SWWS) 6 5th IWA Specialized Conference on Resources ‐ Oriented Sanitation
Evaluation criteria of technologies for drinking water treatment Residual concentration of Cr(VI) at low ppb level. Low reaction time. Feasibility for full scale operation. Sustenance of physical and chemical characteristics of water. Low capital and running cost. 13th IWA Specialized Conference on Small Water and Wastewater Systems (SWWS) 7 5th IWA Specialized Conference on Resources ‐ Oriented Sanitation
Qualified treatment technology for Cr(VI) removal from drinking water hemical reduction of Cr(VI) and removal of C(III) by: ‐ precipitation ‐ adsorption
emical reduction and precipitation ‐ adsorption ost widely practiced methods ighly efficient and fast and educes Cr(VI) to the non‐toxic insoluble Cr(III) form mon reductants ro‐valent iron (ZVI) surface passivation, enriches treated water with dissolved Fe(II) rrous iron salts (FeSO 4 , FeCl 2 ) BAT ‐ production of sludge organic sulphur reductants, ISRs (NaHSO 3 , Na 2 S, CaS 5 , Na 2 S 2 O 3 , Na 2 S 2 O 4 ) eresting results high dose demand
mical reduction and precipitation norganic Sulfur Reductants perimental batch evaluation: e maximum dose 50 mg S/L (increase SO 4 2 ‐ by 150 mg/L) ISRs proved very effective at pH ≤ 4 (!) eptable efficiency at pH 7 only by sodium dithionite (Na 2 S 2 O 4 ) uence of reaction pH on Cr(VI) removal Influence of ISRs dose on Cr(VI) removal (Cr(VI)=100μg/L, pH 7, reaction time 24 h, 20±1 o C) (Cr(VI)=100μg/L, C ISR =10 mg S/L, reaction time 24 h, 20±1 o C)
Aim of the Study imization of Cr(VI) removal from drinking water by ISRs Through surface “catalysis” Under continuous flow configuration.
Materials and methods nts ants examined: NaHSO 3 , Na 2 S 2 O 3 , Na 2 S 2 O 4 , Na 2 S 2 O 5 and Na 2 S. catalyst: synthesized FeOOH mainly consisting of Fe 16 O 16 (OH) 10 (SO 4 ) 3 ∙ 10H 2 O. Artificial water prepared with composition close to that of natural ones according to the NSF standard. Cations (mg/L) Anions (mg/L) Na + ‐ 88.8 HCO 3 183 Ca 2+ 40.0 SO 4 2‐ 50 Mg 2+ Cl ‐ 12.7 71 NO 3‐ ‐N 2 F ‐ 1 3‐ ‐P PO 4 0.04
Materials and methods rimental procedure d Small Scale Column Tests (RSSCTs) sment of treatment efficiency r continuous flow conditions. rption columns: OOH granules (0.25 – 0.5 mm). CT: 3 min, pH: 7.0, T: 20 o C. /h of 100 μg/L Cr(VI) in artificial water. 5 L/h of ISR solution. ation y to decrease Cr(VI) concentration below 10 μg/L S: Investigation of Cr(VI) sorption mechanism
esults and discussion Assessment of FeOOH effectiveness to adsorb Cr(VI) under continuous flow conditions ( ‐ ) low adsorption capacity (+) ability to decrease residual Cr(VI) to sub‐ppb levels. (+) Promotes electron transfer Cr(VI) + 3e+ → Cr(III) ugh curve of Cr(VI) adsorption at FeOOH column (initial Cr(VI):
esults and discussion apid Small Scale Column Tests ISR inflow ISR outflow Cr(VI) outflow O 2outflow Addition of ISRs solution at doses mg S/L mg S/L μ g/L mg/L of 20 and 40 mg S/L. 20 8 47 3.5 O 3 Na 2 S 2 O 4 , Na 2 S: highest efficiency. 40 26 26 2.5 Na 2 S: unpleasant odour to 20 14 50 3.5 O 3 treated water. 40 33 28 2.5 NaHSO 3 , Na 2 S 2 O 3 , Na 2 S 2 O 5 : 20 13 ND 4.5 O 4 ( ‐ ) failed to decease Cr(VI) to single 40 31 ND 2.5 ppb levels, 20 4 42 1.5 O 5 Na 2 S 2 O 4 : qualified and further 40 21 29 <1 examined at column experiments. 20 <1 ND <1 S
esults and discussion apid Small Scale Column Tests For better estimation of Na 2 S 2 O 4 dose: The FeOOH column was initially saturated at 100 μg/L Cr(VI)and • The surplus was evaluated by the curve’s slope 10 mg/L S‐Na 2 S 2 O 4 : Cr(VI) 5 ± 2 μg/L 15 and 20 mg S‐Na 2 S 2 O 4 : Cr(VI) < 1 μg/L (+) “buffer” adsorption capacity of FeOOH column. kthrough curves of Cr(VI) uptake by FeOOH column different Na S O concentrations (i iti l C (VI) 100
esults and discussion ptake mechanism gation of Cr(VI) reduction reaction and sorption mechanism of Cr. measurements H / absence of ISRs : Cr(VI) is physisorbed onto the FeOOH surface. H / addition of Na 2 S 2 O 4 : Cr(III) forms inner sphere complexes, Cr(VI) involved in sphere complexes. Fe Fe Cr(III) Fe
esults and discussion ptake mechanism dsorption mechanism of Cr oxyanions onto surfaces Chemisorption Physisorption inner sphere complexes outer sphere complexes Bidentate Monodentate Mononuclear ( 2 E) Mononuclear (3.00‐3.05Å) ( 2 C) ( 1 V) (3.60Å) 6Å) Cr
onclusions Significant contribution of FeOOH on Cr(VI) removal by ISRs. Highest efficiency: Na 2 S 2 O 4 , Na 2 S. Na 2 S: residual unpleasant odour additional treatment step (‐) Continuous flow configuration: • A 10 mg S/L dose of Na 2 S 2 O 4 ensures the reduction of 100 μ g/L Cr(VI) concentration below the upcoming limit of 10 μ g/L. • Higher Na 2 S 2 O 4 dose (15 mg/L) can diminish Cr(VI) below D.L. • “Buffer” uptake capacity for more than 3 d (~1.500 BV). EXAFS study: chromium uptake onto FeOOH proceeds via both physisorption and chemisorption. • Cr(VI): outer sphere complexes • Cr(III): inner sphere complexes.
tinuous Flow Process of Cr(VI) Removal from Drinking Water through Reduction onto FeOOH by ISRs E. Kaprara, F. Pinakidou, E. Paloura, A. Zouboulis and M. Mitrakas Thank you for your attention!
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