Comparative study of arsenic removal by iron using electrocoagulation and chemical coagulation.

This research studied As(III) and As(V) removal during electrocoagulation (EC) in comparison with FeCl(3) chemical coagulation (CC). The study also attempted to verify chlorine production and the reported oxidation of As(III) during EC. Results showed that As(V) removal during batch EC was erratic at pH 6.5 and the removal was higher-than-expected based on the generation of ferrous iron (Fe(2+)) during EC. As(V) removal by batch EC was equal to or better than CC at pH 7.5 and 8.5, however soluble Fe(2+) was observed in the 0.2-µm membrane filtrate at pH 7.5 (10-45%), and is a cause for concern. Continuous steady-state operation of the EC unit confirmed the deleterious presence of soluble Fe(2+) in the treated water. The higher-than-expected As(V) removals during batch mode were presumed due to As(V) adsorption onto the iron rod oxyhydroxides surfaces prior to the attainment of steady-state operation. As(V) removal increased with decreasing pH during both CC and EC, however EC at pH 6.5 was anomalous because of erratic Fe(2+) oxidation. The best adsorption capacity was observed with CC at pH 6.5, while lower but similar adsorption capacities were observed at pH 7.5 and 8.5 with CC and EC. A comparison of As(III) adsorption showed better removals during EC compared with CC possibly due to a temporary pH increase during EC. In contrast to literature reports, As(III) oxidation was not observed during EC, and As(III) adsorption onto iron hydroxides during EC was only 5-30% that of As(V) adsorption. Also in contrast to literature, significant Cl(2) was not generated during EC, in fact, the rods actually produced a significant chlorine demand due to reduced iron oxides on the rod. Although Cl(2) generation and As(III) oxidation are possible using a graphite anode, a combination of graphite and iron rods in the same EC unit did not produce As(III) oxidation. However, a two-stage process (graphite anode followed by iron anode in separate chambers) was effective in As(III) oxidation and removal. The competing ions, silica and phosphate interfered with As(V) adsorption during both CC and EC. However, the degree of interference depends on the concentration and presence of other competing ions. In particular, the presence of silica lowered the effect of phosphate with increasing pH due to silica's own significant effect at high pHs.

Ferrous and ferric ion generation during iron electrocoagulation.

Our research on arsenate removal by iron electrocoagulation (EC) produced highly variable results, which appeared to be due to Fe2+ generation without subsequent oxidation to Fe3+. Because the environmental technology literature is contradictory with regard to the generation of ferric or ferrous ions during EC, the objective of this research was to establish the iron species generated during EC with iron anodes. Experimental results demonstrated that Fe2+, not Fe3+, was produced at the iron anode. Theoretical current efficiency was attained based on Fe2+ production with a clean iron rod, regardless of current, dissolved-oxygen (DO) level, or pH (6.5-8.5). The Fe2+ remaining after generation and mixing decreased with increasing pH and DO concentration due to rapid oxidation to Fe3+. At pH 8.5, Fe2+ was completely oxidized, which resulted in the desired Fe(OH)3(s)/ FeOOH(s), whereas, at pH 6.5 and 7.5, incomplete oxidation was observed, resulting in a mixture of soluble Fe2+ and insoluble Fe(OH)3(s)/FeOOH(s). When compared with Fe2+ chemical coagulation, a transient pH increase during EC led to faster Fe2+ oxidation. In summary, for EC in the pH 6.5-7.5 range and at low DO conditions, there is a likelihood of soluble Fe2+ species passing through a subsequentfiltration process resulting in secondary contamination and inefficient contaminant removals.

Perchlorate and nitrate treatment by ion exchange integrated with biological brine treatment.

Groundwater contaminated with perchlorate and nitrate was treated in a pilot plant using a commercially available ion exchange (IX) resin. Regenerant brine concentrate from the IX process, containing high perchlorate and nitrate, was treated biologically and the treated brine was reused in IX resin regeneration. The nitrate concentration of the feed water determined the exhaustion lifetime (i.e., regeneration frequency) of the resin; and the regeneration condition was determined by the perchlorate elution profile from the exhausted resin. The biological brine treatment system, using a salt-tolerant perchlorate- and nitrate-reducing culture, was housed in a sequencing batch reactor (SBR). The biological process consistently reduced perchlorate and nitrate concentrations in the spent brine to below the treatment goals of 500 microg ClO4(-)/L and 0.5mg NO3(-)-N/L determined by equilibrium multicomponent IX modeling. During 20 cycles of regeneration, the system consistently treated the drinking water to below the MCL of nitrate (10 mgNO3(-)-N/L) and the California Department of Health Services (CDHS) notification level of perchlorate (i.e., 6 microg/L). A conceptual cost analysis of the IX process estimated that perchlorate and nitrate treatment using the IX process with biological brine treatment to be approximately 20% less expensive than using the conventional IX with brine disposal.

Influence of sulfide (S2-) on preservation and speciation of inorganic arsenic in drinking water.

Generally, H2SO4, HNO3, HCl or the combination of ethylenediaminetetraacetate with acetic acid (EDTA-HAc) have been used to preserve arsenite and arsenate species prior to analysis. When these acidic preservatives are added in sulfidic water, instantaneous precipitation of poorly crystalline orpiment, As2S3(am), occurs, thereby lowering the total arsenic, As(Tot), analysis. A new method for the determination of As(Tot) was developed in which acid-preserved sulfidic water samples were oxidized with NaOCl, converting As2S3(am) and thioarsenic species to arsenate. A new method was also developed for the separation of uncharged arsenite and charged thioarsenic species in fresh, unpreserved sulfidic water by adsorbing the charged thioarsenic species while allowing uncharged arsenite to pass through a strong-base resin unhindered. The adsorbed thioarsenic species could be eluted efficiently with 0.16 M NaOCl solution.

Preservation of inorganic arsenic species in groundwater.

The objective of this research was to develop a robust preservation method for stabilizing inorganic As(IlI/V) species in synthetic and actual groundwaters. Ethylenediaminetetraacetic acid (EDTA), H2SO4, H3PO4, and EDTA-acetic acid (HAc) were evaluated in synthetic groundwater containing 3 mg/L Fe(ll) in the pH range 6.5-8.4 and Eh range -100 to +200 mV at room temperature. In the absence of strong UV light, only EDTA-HAc was found to be an effective preservative under all the experimental conditions tested. A total of 89 samples (including 16 samples in triplicate) from 55 drinking waterwells in Minnesota, California, and North Dakota were preserved with a combination of EDTA-HAc and speciated to evaluate its effectiveness for preserving inorganic arsenic species in actual groundwater samples. The preserved and field-speciated samples were repeatedly speciated and analyzed in the laboratory for up to 85 days after collection. Field-speciated As(lll) concentrations were compared with the As(lll) concentrations in the preserved samples. The results were highly correlated (slope 0.9773, R2 = 0.9986), which indicates that during sample transportation and storage the distribution of arsenic species did not change for samples preserved with EDTA-HAc.

Virus removal by iron coagulation-microfiltration.

This study was undertaken to determine virus removal efficiency by iron coagulation followed by microfiltration (MF) in water treatment using the MS2 bacteriophage (approximately 25 nm diameter) as a tracer virus. Results from these bench-scale studies were used to propose a mechanism for virus removal by iron coagulation-MF. Ferric chloride was used as coagulant, and the dosages were 0, 2, 5, and 10 mg/L as Fe(III) with pH adjusted during mixing to 6.3, 7.3 and 8.3. In the absence of iron-coagulation and with less than 2 mg/L Fe, MF alone achieved less than a 0.5 log removal of MS2 virus. However, iron-coagulation pretreatment dramatically improved virus removal, especially in the 5-10 mg/L Fe dose range, ultimately achieving more than 4-log removal at pH 6.3 with 10-mg/L Fe dose. For the 5 and 10 mg/L Fe dosages, decreasing pH in the 8.3-6.3 range resulted in significantly greater virus removal. For 0 and 2 mg/L iron dosages, decreasing pH in the 8.3-6.3 range also improved virus removal, but to a lesser extent. The experimental data indicates negatively charged MS2 viruses first adsorbed onto the positively charged iron oxyhydroxide floc particles before being removed by MF. MS2 viruses were not inactivated in iron or aluminum coagulation as evidenced by the fact that their concentrations before and after coagulation, settling, and re-suspension of the coagulated sludge were not statistically different.

Comparison of electrocoagulation and chemical coagulation pretreatment for enhanced virus removal using microfiltration membranes.

This research studied virus removal by iron electrocoagulation (EC) followed by microfiltration (MF) in water treatment using the MS2 bacteriophage as a tracer virus. In the absence of EC, MF alone achieved less than a 0.5-log removal of MS2 virus, but, as the iron-coagulant dosage increased, the log virus removal increased dramatically. More than 4-log virus removal, as required by the Surface Water Treatment Rule, was achieved with 6-9 mg/L Fe(3+). The experimental data indicated that at lower iron dosages and pH (< approximately 8 mgFe/L and pH 6.3 and 7.3) negatively charged MS2 viruses first adsorbed onto the positively charged iron hydroxide floc particles before being removed by MF. At higher iron dosages and pH (> approximately 9 mgFe/L and pH 8.3), virus removal was attributed predominantly to enmeshment and subsequent removal by MF. Additionally, the experimental data showed no obvious influence of ionic strength in the natural water range of 10(-7)-10(-2)M on MS2 virus removal by EC-MF. Finally, EC pretreatment significantly outperformed chemical coagulation pretreatment for virus removal. The proposed mechanism for this improved performance by EC is that locally higher iron and virus concentrations and locally lower pH near the anode improved MS2 enmeshment by iron flocs as well as adsorption of MS2 viruses onto the iron floc particles.