WHAT S UP WITH Cr(VI)?

WHAT S UP WITH Cr(VI)? Ilke McAliley, P.E., HDR Inc. Peter C. D Adamo, Ph.D., P.E., HDR Inc. Phil Brandhuber, Ph.D., P.E., HDR Inc. ABSTRACT Chromium is a naturally occurring element in the environment. In aquatic systems, chromium occurs in two oxidation states: trivalent chromium Cr(III) and hexavalent chromium - Cr(VI). Cr(III) is considered to be a micronutrient and dietary guidelines have been established by the National Institute of Health (NIH). Cr(VI), on the other hand, is considered carcinogen. Chromium can be introduced into water sources either by natural weathering of chromite containing minerals or by contamination from a variety of industrial sources such as leather tanning, metal plating, wood treatment and corrosion control. Relying on Cr(III) occurrence data to estimate Cr(VI) occurrence has been a very common method due to limited availability of Cr(VI) occurrence data. Currently, the existing Maximum Contaminant Level (MCL) set by the U.S. EPA is for total chromium. Total chromium is the sum of trivalent and hexavalent chromium. The existing MCL for total chromium is based on non-cancer based health effects. Currently, there is no federal regulation (MCL) for hexavalent chromium. In 2008, the National Toxicology Program (NTP) published studies which concluded that oral ingestion of hexavalent chromium (Cr(VI)) caused an increase in the incidence of cancer in rats and mice. After the completion of Integrated Risk Information System (IRIS) Toxicological Review of Cr(VI) in 2010, the U.S. EPA has proposed to classify Cr(VI) as probable human carcinogen and lower the Cr(VI) reference dose (RfD) for non-cancer effects by a factor of three. These changes to IRIS will require EPA to review the existing chromium MCL and possibly establish a Cr(VI) specific MCL. Based on the proposed changes to IRIS, a Cr(VI) specific MCL in the low part per billion to sub-part per billion range is possible. The Unregulated Contaminant Monitoring Rule 3 (UCMR 3) requires monitoring of 30 contaminants including Cr(VI) during 2013-2015 period. In February 2012, a new schedule was developed for completing IRIS Cr(VI) assessment. U.S. EPA anticipates that the draft assessment (oral and inhalation) will be released for public comment and external peer review in 2013. This presentation will review Cr(VI) occurrence data and public health concerns, provide a regulatory update for Cr(VI) including recent events, provide an overview of chromium chemistry, and review potential Cr(VI) treatment technologies. The presentation will benefit utilities and regulators as they prepare for possible MCL standards for Cr (VI). KEYWORDS Chromium, Hexavalent Chromium, INTRODUCTION In recent years the hexavalent form of the element chromium, has received increased focus as a drinking water contaminant due to its potential human carcinogenicity when ingested in trace amounts. In aquatic systems, chromium occurs in two oxidation states: trivalent chromium Cr(III) and hexavalent chromium - Cr(VI). Cr(III) is considered a micronutrient and dietary guidelines have been established by National Institute of Health (NIH). Alternatively, Cr(VI). is considered a carcinogen associated with inhalation exposure routes, and a Federal toxicology review of Cr(VI) by ingestion is in progress. Research concerning Cr(VI) toxicity, occurrence and treatability in drinking water is ongoing. Meanwhile, there has been increased activity toward regulating Cr(VI) as a drinking water contaminant. The State of California has already established a non-enforceable Public Health Goal (PHG) that could be followed by

a maximum contaminant level (MCL) in the near future. EPA is also considering development of a Federal MCL to regulate Cr(VI) after finalization of the toxicity review. Greater understanding of Cr(VI) drinking water occurrence and treatability is required to understand the implications of regulation. REGULATORY REVIEW In 2008, the National Toxicology Program (NTP) published studies which concluded that oral ingestion of hexavalent chromium (Cr(VI)) caused an increase in the incidence of cancer in rats and mice. After the completion of the draft Integrated Risk Information System (IRIS) Toxicological Review of Cr(VI) in 2010, the U.S. EPA has proposed to modify the status of Cr(VI), as follows, pending peer-review and comment: (i) classify Cr(VI) as a probable human carcinogen and (ii) lower the Cr(VI) reference dose (RfD) for non-cancer effects by a factor of three (Table 1), Table 1:Proposed Cr(VI) status changes based on draft 2010 IRIS Review Results Current IRIS Value Proposed IRIS Value Oral RfD 0.003 mg/kg-day 0.0009 mg/kg-day Cancer Slope None 0.5 (mg/kg-day) -1 In February 2012, a new schedule was developed for finalizing the IRIS Cr(VI) assessment. U.S. EPA anticipates that a new draft assessment incorporating updated information on Cr(VI) carcinogenicity and modes of action will be released for public comment and external peer review in 2013. The results of this review will impact the potential regulatory developments for hexavalent and total chromium. The proposed changes to IRIS require EPA to review the existing total chromium MCL. The current EPA MCL for total chromium is 100 ug/l. Total chromium is the sum of trivalent and hexavalent chromium. This total chromium MCL is based on non-cancer health effects (i.e. allergic dermatitis), and does not account for the Cr(VI)-specific carcinogenicity discussed above. Evaluation of the existing total chromium MCL could lead to development of a Cr(VI) specific MCL. Based on the proposed changes to IRIS, a Cr(VI) specific MCL in the low part per billion to sub-part per billion range would not be unlikely. In April 2012, EPA decided to include monitoring of Cr(VI) in the third Unregulated Contaminant Monitoring Rule (UCMR3). As result of research and UCMR3, EPA will decide either to review total chromium regulations or set a new MCL for Cr(VI). The State of California is considering independent regulation of Cr(VI) in drinking water. In 2011, the California Office of Environmental Health Hazard Assessment (OEHHA) established a non-enforceable Public Health Goal (PHG) for Cr(VI) for 0.02 µg/l. The PHG is a non-enforceable standard set at a onein-one million excess cancer lifetime risk level due to exposure to Cr(VI) in drinking water. California, at the time of this study, has not promulgated an enforceable Cr(VI) standard. However, the availability of a final PHG enables California Department of Public Health (CDPH) to proceed with setting a primary drinking water standard. CDPH estimates that an enforceable MCL will be established between July of 2014 and 2015 if no major delay in the process occurs (CDPH Website). Table 2 compares regulatory levels for chromium that have been set by various agencies throughout the world. At present Cr(VI) is not regulated in Europe or Canada, nor has the World Health Organization WHO published a Cr(VI)-specific regulatory recommendation. California regulates total chromium at a level similar to these other organizations (see Table 2), but it remains to be seen if the California PHG represents a level at which Cr(VI) might be regulated in the future Table 2:Comparison of Chromium regulations by various world health agencies Agency Contaminant Drinking Water Limit USEPA Total Chromium 100 µg/l MCL State of California Total Chromium 50 µg/l MCL

Cr(VI) 0.02 µg/l PHG World Health Organization Total Chromium 50 µg/l MCL EU Drinking Standards Total Chromium 50 µg/l MCL Canada Total Chromium 50 µg/l MCL Sub-part-per-billion levels of Cr(VI), such as the California PHG, when proposed for regulatory purposes have, until recently, presented obstacles for accurate and consistent measurement. USEPA Method 218.7 Determination of Hexavalent Chromium in Drinking Water by Ion Chromatography with Post- Column Derivatization and UV-Visible Spectroscopic Detection, presents a reliable method that is capable of measurement of concentrations in this range, and other technologies are in development. The limited available technology and potential high costs associated with Cr(VI) sampling and analysis have important implications for implementation of an MCL in this concentration range. Development of an MCL is further confounded by the fact that expectations for occurrence of Cr(VI) in drinking water systems are not well-informed. Only recently have improvements in Cr(VI) analysis techniques facilitated collection of a substantial body of Cr(VI) occurrence data, though the occurrence picture remains incomplete. SOURCES AND OCCURRENCE Chromium can occur naturally and also be introduced into water sources by natural weathering of chromite containing minerals. In general, total chromium concentrations in the part-per-billion range are expected in most natural waters, but this occurrence usually consists of Cr(III). Outside of a handful of recently identified natural Cr(VI) occurrences, Cr(VI) has been associated with anthropomorphic contamination. Cr(VI) contamination may have its source in a variety of industrial processes such as leather tanning, metal plating, wood treatment or corrosion control (McNeill et al, 2012). Until recently, Cr(VI) occurrence information has been limited by analytic capability. Reliance on total chromium occurrence data to estimate Cr(VI) occurrence has been common.. This is especially problematic in light of toxicological information that emphasizes concentrations of Cr(VI) in the sub-partper-billion range. With historical limitation in Cr(CI) analytic techniques, and no historical federal regulatory framework for mandating collection of Cr(VI) sampling information, a reliable picture of Cr(VI) occurrence within the continental United States has yet to emerge. Figure 1 shows a summary of total chromium sampling data collected by EPA for its 6-year review of regulated contaminants (Seidel et al(2012)). It must be noted that this map is based on results obtained only from the systems which report total Cr results. All total chromium results are much higher than the PHG established by the State of California (0.02 ug/l). This means that if regulation of Cr(VI) is to be implemented based on the PHG or similar concentrations, many utilities around the country will be considered out of compliance and will be responsible for installing new treatment technologies to remove chromium. While natural weathering and industrial contamination are recognized as sources of chromium in source waters, treatment processes and chemicals can also be source of chromium in tap water. Chromium can be present at trace levels in treatment chemicals such as alum and lime which can introduce the contaminant into the treated water leaving the plant (McNeill et al (2012), Song et al (2013)).

CHROMIUM SPECIATION Figure 1. Total Chromium Occurrence (Seidel et al, 2012) Chromium is present in the aqueous phase in various oxidation states (from +6 to -2) but the two most ubiquitous forms are in hexavalent or trivalent state. Aqueous speciation and solubility of chromium are determined primarily by ph level and re-dox conditions, as shown in the diagram in Figure 2. Figure 2. pe-ph Diagram for Chromium Speciation (McNeill et al, 2012) Cr(III) can be found in water samples in a variety of forms including soluble species, low solubility Cr(OH) 3 solids, sorbed or fixed onto ferric oxides, or complexes with humic and fulvic acids. In general, trivalent chromium occurs as: (i) a cation at low ph, (ii) an insoluble hydroxide at moderate ph and

(iii) an anionic hydroxide at high ph. While Cr(III) is predominantly an insoluble hydroxide across a range of typical drinking water ph conditions, Cr(VI) species remain soluble as CrO 4-2 species under most ph conditions. Significantly, Cr(VI) is favored thermodynamically under more oxidative conditions, similar to those that might occur in distribution systems where a strong oxidant disinfectant residual (i.e. sodium hypochlorite) is present. Based on thermodynamic stability and evaluation of typical ranges for the redox reaction rate associated with the presence of free chlorine, it has been posited that if total chrome is not adequately removed in the treatment plant, it can potentially be oxidized to Cr(VI) in the distribution system (McNeill et al, 2012), Lindsay et al (2012)). Potentially Cr(III) could also be oxidized by distribution system materials such as ferrous iron leached from cast iron pipes. These considerations associated with the speciation of Cr(VI) have meaningful implications for regulation and monitoring approaches, as well as establishing treatment requirements that will be stringent enough to prevent Cr(VI) formation in the distribution system. At the time of this writing, sampling data exploring the formation of Cr(VI) in the distribution system is not available. CHROMIUM (VI) TREATMENT ALTERNATIVES The effectiveness of treatment for removal of chromium, with emphasis of removal of Cr(VI) to sub-partper-billion concentrations needs further exploration to determine the level of treatment that will be required to meet potential regulatory limits. Several treatment technologies are currently being evaluated. The following treatment methods will be summarized in this literature review, and further explored in the case studies in this research: 1. Coagulation/Precipitation/Filtration 2. Ion Exchange 3. Membrane Treatment 4. Adsorption 5. Electrochemical technologies The following section provides a brief overview of each of these technologies. For comparison, their relative effectiveness is summarized in Table 3, along with some of the issues that need to be resolved or considered for each treatment technology. Coagulation/Precipitation/Filtration Reduction Assisted Cr(VI) reduction with various chemicals such as ferrous salts, zero valent iron, and sodium bisulfite have been researched in great detail specifically for the removal of Cr(VI). The mechanism is to reduce Cr(VI) to Cr(III) and then co-precipitate less-soluble Cr(III) with Fe(III) oxyhydroxides or hydroxides. Zero valent iron (Fe 0 ) is commonly used as permeable reactive barriers in in-situ treatment of groundwater sources contaminated with Cr(VI) (Fruchter (2002)). Although several studies have observed different rates of reaction (Wilkin et al (2005), Melitas et al (2001), Ponder et al (2000), Liu et al (2008)), it has been noted that the rate greatly depends on Fe o concentration, ph, and Fe o type. Considerable Cr(VI) removal efficiency (around 99%) has been observed in groundwater remediation applications (Wilkin et al (2005). The effect of ph in the removal is pronounced, with higher ph levels (ph>8.0) shown to decrease the removal (Alowitz and Scherer (2002)). Ferrous iron has been studied for it is effectiveness in reduction of Cr(VI) to Cr(III) and subsequent precipitation as Cr(III) hydroxides (Eary and Ral (1988), Qin et al (2005), Burge and Hug (1997), Lee and Hering (2003), Brandhuber et al (2004)). Fe(II) oxidation by Cr(VI) requires approximately a 3:1 molar ratio of Cr(VI) to Fe(II) at ph 6.5 yielding Cr(III) Fe(III) hydroxide precipitates (Qin et al (2005)). The process that achieves the removal is known as Reduction/Coagulation/Filtration or Reduction/Precipitation/Filtration (RPF). It has been observed that ph, temperature, ionic strength and

solution composition such as dissolved oxygen, phosphates, humic and fulvic acids, affect Cr(VI) removal (Seldak and Chan (1997), Pettine et al (1998)). However, rapid reduction and precipitation have been observed by many researchers (Eary and Rai (1998), Qin et al (2005), Buerge and Hug (1997), Lee and Hering (2003)). It has been noted that between ph 2 and 10, the stoichiometric reduction of Cr(VI) is successfully achieved, however, under conditions of higher ph and the presence of phosphates, non stoichiometric reduction occurs due to dissolved oxygen competition (Eary and Rai (1998)). Another study observed minimum reaction rate at ph 4, increasing above and below that value (Buerge and Hug (1997)). Higher filtration rates, dissolved oxygen concentration and high filtration ph (above 7.5) have also been found to negatively affect the removal of Cr(III) precipitates (Qin et al (2005)). Song et al (2013) successfully removed low level Cr(VI) contamination to sub-ppb levels with the RPF process. Trace levels of chromium in lime used at a softening plant were contributing approximately 0.4 ppb Cr(VI) to the treated water. With a combination of ferrous iron and ferric salt that was used for coagulation, Cr(VI) removal to below 0.1 ppb was achieved without compromising TOC and turbidity removal. This study was the first to demonstrate that Cr(VI) removal at sub-ppb level is achievable and a combination of coagulants for reduction and precipitation can potentially provide a low cost alternative (relative to large capital improvements for new treatment equipment) for drinking water providers. There may be costs associated with retrofitting for the reduction step. Overall, the results of RPF studies have been promising and the level of process understanding is relatively high. RPF provides a potential alternative for Cr(VI) removal at conventional plants without having to change treatment processes substantially; however further evaluation of the RPF approach is needed to determine whether Cr(VI) removal will be commensurate with the possible regulatory MCLs similar to the California PHG. Conventional The removal of chromium using coagulation and precipitation without reduction has demonstrated limited effectiveness with respect to Cr(VI) removal. Low Cr(VI) removal rates (<30%) have been observed with iron-based coagulants, even with high Fe(III) concentrations (Lee and Hering, 2003). This alternative is not favorable due to the limited effectiveness of the metal coagulants, the potential re-oxidation of Cr(III) to Cr(VI) during backwash water recycling and poor settleability performance (McNeill et al (2012), Sharma et al (2008)). Ion Exchange Ion exchange is one of the EPA recommended treatment techniques for the removal of chromium, and it has shown promising results for Cr(VI) (Sharma et al, 2008). Cation exchangers are effective in the removal of Cr(III) (Sharma et al (2008)). Weak base anion (WBA) and strong base anion (SBA) resins have been used in several Cr(VI) removal studies (Brandhuber et al (2004), McGuire et al (2007)). Several factors such as solution ph and competing ions have been found to affect the removal efficiency of ion exchange resins, and additional study is required to resolve these issues with respect to Cr(VI) removal to meet potential sub-part-per-billion regulation. SBA resins for Cr(VI) removal have successfully demonstrated removal of more than 95% of Cr(VI) (Brandhuber et al. (2004)). In a recent study by McGuire et al, (McGuire et al (2007)) bench and pilot scale experiments were performed at the City of Glendale, CA with WBAs. WBAs showed more capacity than SBAs for Cr(VI) and were able to remove Cr(VI) to an average of 30 ug/l to <5 ug/l. The major difference between these two types of resins is that SBAs can be regenerated whereas WBAs are only single-pass resins. While brine disposal is employed for SBAs, ph adjusted resin disposal is required for WBA. Brine and resin disposal are serious considerations for implementation of these technologies, and could represent large operational expenditures for utilities in the long run. Studies have also shown that acidic conditions enhanced chromate removal for both cation and anion resins. A lower limit or around ph 3 has been observed, where no additional removal is observed with

decreasing ph (Sengupta and Clifford (1986), Rengaraj (2003)). It has also been noted that the presence of chloride negatively affects the removal capacity of the resins (Sengupta and Clifford (1986)). Performance of ion exchange resins has been proven and there is a good process understanding for the removal mechanisms. The main limitations of this technique are regular regeneration for SBA, resin disposal for WBA, concentrate disposal, potential fouling, ph adjustment, and competing ions in the water (McNeill et al (2012), Sharma et al(2008)). Additional study is needed to resolve issues associated with competing ions, especially with respect to the presence of high chloride concentrations. Membrane Treatment Membrane technology (reverse osmosis (RO) and nanofiltration (NF)) has an important role in the removal of inorganic contaminants, and has shown promising results for Cr(VI) removal (Brandhuber et al 2004, Yoon et al (2009), Hafiane et at (2000), Muthukrishnan and Guha (2007)). The major factors that influence removal have been found to be ph, surface membrane charge, conductivity and initial Cr(VI) concentration (Yoon et al (2009), Brandhuber et al. (2004)). NF and RO have been more successful in the removal of Cr(VI) than ultra-filtration (UF) due to their smaller pore size. They can be integrated into existing water treatment plant processes with relative ease (Brandhuber et al (2004)) due to their relatively small footprint. Regardless of the membrane type, studies have shown that rejection of chromate decreases with increasing conductivity and increases with increasing ph (Yoon et al (2009), Hafiane et at (2000), Muthukrishnan and Guha (2007)). The rejection of low and high initial solution chromate concentrations is impacted by solution ph: Lower rejection occurs at higher concentrations for alkaline ph and higher rejection occurs at lower concentrations at acidic ph. This trend has been attributed to chromium speciation at different ph levels. Size exclusion and electrostatic repulsion have been found to be the major mechanisms that affect chromium rejection (Yoon et al (2009)). Lately inorganic membranes are also getting attention from researchers due to their high chemical and thermal stability (Owlad et al (2009)). Ceramic membrane combined with adsorption/ultrafiltration removal technology was able to remove about 0.5 ppm Cr(III) down to 10 ppb (Pagana et al (2008)). In general, membrane removal is well understood and can be incorporated into an existing plant with relative ease compared to other major retrofits. There are several issues that need to be considered such as overall cost, treatment of reject water, and potential fouling (McNeill et al (2012), Sharma et al (2008)). Adsorption Adsorption is a process where a contaminant is removed from the aqueous phase and accumulates at the solid-liquid interface of an adsorbent. Generally, concentration, ph, salinity, temperature, and solution composition affect the efficiency and adsorption capacity of the adsorbents (Owland et al (2009)). Various adsorbent materials have been studied by researchers to test the chromium removal efficiency including; different activated carbons, biological materials, zeolites, chitosan, nut shells, clay, peat moss, industrial waste, rice, and wool (Babel and Kurniawan (2003), Mohan and Pittman (2006), Owland et al (2009), Dakiky et al (2002)). All these potential absorbents have a varying degree of adsorption capacities ranging from 0.57 mg/g for bentonite clay to 273 mg/g for chitosan to 145 mg/g for Filtrasorb GAC (Babel and Kurniawan (2003). Studies have shown that increased ph and salinity impact adsorption capacities unfavorably (Di Natale et al (2007), Han et al (2000)). Adsorption capacity can also be hindered by interfering species such as sulfate, phosphate, and dissolved organic carbon. Activated carbon, in particular, shows promising treatment effectiveness. The removal mechanism is believed to be two-fold: adsorption onto the surface and Cr(VI) reduction to Cr(III) in the presence of activated carbon. There may be an additional benefit, as reactivation may increase adsorption capacity. Han et al (2000) compared regenerated versus virgin carbon adsorption capacities in their studies and found that the carbons regenerated with potassium phosphate and sulfuric acid showed approximately three times more capacity than that of virgin carbons in the removal of chromium.

Adsorption mechanisms for chromium removal process are not fully understood. Effectiveness is still being studied for many adsorbents at bench-scale level. The major drawbacks of this treatment alternative are poor performance at ph 7-9 (which entails addition of ph adjustment chemicals to a treatment stream), expense and effectiveness of carbon regeneration (as applicable), and poor selectivity of adsorbents (Mohan and Pittman (2006), McNeill et al (2012). Electrochemical Removal Electrochemical treatment using electrodes and electrolytes is a newer approach than the alternatives discussed above (Owlad et al (2009)). It has been mostly studied in the industrial wastewater area. The major factors affecting the efficiency of chromium removal are ph, influent concentration and the charge applied (Golub and Oren (1989)). Up to 98% chromium removal has been observed in acidic conditions and also with higher charge (Rana et al, 2004). This technology has been studied at bench-scale level and at high chromium concentrations (2-100 ppm) (Rana et al. (2004), Golub and Oren (1989)). This approach has not been studied at sub ppb levels that are encountered in drinking water. RESULTS Evaluations for treatment technologies and associated costs have been completed for several utilities through various studies (McGuire et al. (2007), Blute (2011), McNeill et al (2012)). While there are certain advantages to each treatment alternative, there are also issues to consider and these are listed in Table 3. RFP treatment has been shown to achieve the Cr(VI) removal to sub ppb levels commensurate with potential regulation. Alternatives such as membranes and ion exchange resins are also proven technologies, but likely entail higher operating costs. Adsorption and electrochemical removal technologies have shown effective removal of chromium in preliminary studies, but substantial research is needed to optimize treatment to achieve consistent removal to sub-part-per-billion Cr(VI) concentrations. Table 3. Summary of Hexavalent Chromium Treatment Options for Drinking Water Treatment Technology Impact of Lower Chromium Issues to Consider MCL (20 ug/l => 5 ug/l) Strong Base Anion Exchange (SBA) Weak Base Anion Exchange (WBA) Membrane Reduction/Precipitation/ Filtration Adsorption with Activated Carbon Frequent resin regeneration high volumes of brine requiring disposal Large capacity treatment system required; less by-pass flow Frequent resin replacement Large capacity treatment system required; less by-pass flow. Large capacity treatment system required; less by-pass flow Greater water loss Residual disposal Performance at sub ppb levels Additional chemical requirements Increased solids production Not yet demonstrated for lower chromium levels Unknown cost implications Disposal of Cr-containing brine Co-occurring ion competitive effects Regeneration Large ph changes needed Cost and availability of resin Disposal of resin Leaching of resin Disposal of Cr containing concentrate. Water loss may be unacceptable Membrane fouling Capital cost Complex system Large footprint Possible cost associated with retrofit Poor selectivity Performance depends on carbon Expense and effectiveness of

Electrochemical Precipitation Learning curve Not demonstrated for lower chromium levels regeneration Sludge Production Production of loose deposit CONCLUSIONS The regulatory future of hexavalent chromium remains uncertain. After the current toxicological review in finalized, U.S. EPA might decide to regulate hexavalent chromium as a separate MCL, and/or make changes to existing total chromium regulation. If development of a potential MCL for Cr(VI) should follow the precedent established with the California PHG, regulation at sub-part-per-billion concentrations is a likelihood that must be considered. There are many aspects of hexavalent chromium occurrence and treatment that need further evaluation before the feasibility and effectiveness of such an MCL can be fully evaluated. Resolution of the following issues will be of key importance in the near future: 1. Sources of Cr(VI) are not well understood. Natural occurrence, and contributions of Cr(VI) either via treatment processes or drinking water distribution need clarification to inform monitoring approaches and control strategies. 2. The removal efficiency and cost of available treatment technologies has not been adequately evaluated with respect to potential sub-part-per-billion MCLs for Cr(VI). Treatment evaluation relates directly to the feasibility of establishing a given MCL. 3. Oxidation of Cr(III) to Cr(VI) in re-dox conditions typical of drinking water distribution systems is thermodynamically probable, but has not been adequately explored or observed in-situ. Greater understanding is needed to determine where best to monitor Cr(VI) in drinking water systems, and whether Cr(VI) would best be regulated in a manner similar to disinfection by-products. REFERENCES Alowitz, M.J., and Scherer, M.M., 2002. Kinetics of Nitrate, Nitrite, and Cr(VI) Reduction by Iron Metal. Environ. Sci. Technol., 34(3):299-306 Babel, S., and Kurniawan T.A., 2003. Low-Cost Adsorbents for Heavy Metals Uptake From Contaminated Water: A Review. Jour. of Hazardous Mat., B27:219-243 Blute N., 2011. Cr(VI) Treatment Options and Cost. Water Research Foundation Tech Transfer Workshop on Hexavalent Chromium. Brandhuber P., Frey M., McGuire M.J., Chao P., Seidel C., Amy G., McNeill L., and Banerjee K., 2004. Low Level Hexavalent Chromium Treatment Options: Bench-Scale Evaluation. Water Research Foundation Report #91042, Denver, CO. Buerge, I.J., and Hug, S.J., 1997. Kinetics and ph Dependence of Chromium(VI) Reduction by Iron(II). Environ. Sci. Technol.,31(5), 1426-1432 CDPH Website, http://www.cdph.ca.gov/certlic/drinkingwater/pages/chromium6.aspx, February 2013. Dakiky, M., Khamis M., Manassara A., Mere eb M., 2002. Selective Adsorption of Chromium (VI) in Industrial Wastewater using Low-Cost Abundantly Available Adsorbents. Advances in Environmental Research, 6:533-540. Di Natale F., Lancia A., Molino Al. Musmarra O., 2007. Removal of Chromium Ions from Aqueous Solutions by Adsorption on Activated Carbon and Char. Jour. of Hazardous Mat.,145:381-390. Eary, L.E., and Rai, D., 1988. Chromate Removal from Aqueous Wastes by Reduction with Ferrous Ion. Environ. Sci. Technol.,22(8):972-977 Fruchter, J., 2002. In Situ Treatment of Chromium-Contaminated Groundwater. Environ. Sci. Technol.,36(23): 464A-472A Golub, D., and Oren, Y., 1989. Removal of Chromium From Aqueous Solutions by Treatment with Porous Carbon Electrodes: Electrochemical Principles. Jour. of Applied Electrochemistry, 19:311-316 Hafiane A., Lemordant D., and Dhahbi, M., 2000. Removal of Hexavalent Chromium by Nanofiltration. Desalination, 130:305-312

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