Dennis Clifford

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 silicas own significant effect at high pHs.

Biological denitrification of spent regenerant brine using a sequencing batch reactor

The successful development of a laboratory-scale sequencing batch reactor (SBR) denitrification process to treat and reuse nitrate ion-exchange brine is described. Whereas previous research has focused on the use of a continuous upflow sludge blanket reactor (USBR) to denitrify 0.36 N NaHCO3 or 0.17 N NaCl brine containing up to 700 mg NO3-N l−1, this work examines the feasibility of using an SBR to denitrify 0.5 N NaCl brine containing up to 835 mg NO3-N l−1. The influence of salt concentration on denitrification rate was investigated with artificial brine at concentrations of 0.25 and 0.5 N when methanol was the sole exogenous carbon source. After acclimation, the denitrification rate in 0.5 N NaCl was only 10% lower than in the no-salt control reactor. The effect of mass ratio (R) of methanol to nitrate-nitrogen on denitrification rate and residual TOC in the denitrified SBR effluent was studied in the range of 2.2–3.2. At the optimum methanol-to-nitrate-nitrogen ratio of 2.7, the time for >95% denitrification was 8 h. In one set of runs, actual spent regenerant was reused 15 times; each time it was denitrified in the SBR, filtered and compensated with NaCl before reuse. The research indicates that a denitrifying batch reactor provides simple operation, reliable effluent quality and compatibility with the inherent batch operation of the ion-exchange process.

The determinants of divalent/monovalent selectivity in anion exchangers

Abstract This research demonstrates that it is the distance of fixed-charge separation in the resin which is the primary determinant of divalent/monovalent selectivity. Anion resins, particularly acrylics and epoxies, with closely spaced N-containing (amine) functional groups are inherently divalent-ion selective. This is because the uptake of a divalent anion, e.g. sulfate, requires the presence of two closely-spaced positive charges. Results from this study of 30 commercially-available, strong- and weak-base, anion resins indicate that in order to bring positive charges into close proximity within a resin one can: (1) incorporate the amine functional groups into the polymer chains as opposed to having them pendant on the chains; (2) minimize the size and number of organic (“R”) groups attached to the N atom, i.e., minimize the size of the amine; and (3) maximize the resin flexibility, i.e., its ability to reorient to satisfy divalent counterions, by minimizing the degree of crosslinking. The distance-of-charge-separation theory is not restricted to divalent anion exchangers but also applies to cation exchangers and to polyvalent ions in general.

A survey of lead battery recycling sites and soil remediation processes

Abstract This paper is intended to provide some background information on lead battery recycling sites (LBRSs) in the U.S. Included in the report are (a) a discussion of the regulations and guidelines governing lead in soil; (b) a description of typical LBRS operations; (c) a listing of the 47 sites studied by the EPA under CERCLA; (d) a discussion of a potential problem associated with the widely used remediation technique of solidification/stabilization; and (e) an overview of some innovative treatment technologies that may be used at LBRSs

The Treatability of Perchlorate in Groundwater Using Ion-Exchange Technology

The study of the interaction of ion-exchange resins with the perchlorate ion is not a recent development. There are numerous reports in the ion-exchange literature of the 1950s related to the behavior of various anions and the new synthetic ion-exchange resins of that time period.1–6 The perchlorate ion was among many different anions tested for the construction of affinity sequences in an attempt to determine the basis for selectivity. From these tests is was shown that the affinity for the perchlorate ion by these early resins was very high. The nonpolar, hydrophobic matrix of these resins provided an environment that the nonpolar, hydrophobic perchlorate ion found much preferable to that of the aquatic environment.

The behavior of polyprotic anions in ion-exchange resins

After observing that pH dropped dramatically when chloride-form strong-base anion resins were contacted with neutral and alkaline solutions of bicarbonate and arsenate anions, three representative strong-base resins were tested for their ability to produce pH changes. It was discovered that all three chloride-form resins could convert HCO3−− to CO32−, and H2AsO4− to HAsO42− with the expulsion of protons, a decrease in pH, and an increase in the ionic concentration of the aqueous phase. For bicarbonate which can give up and take up protons, the following reaction was observed: 2R4N+Cl− + HCO−3 ⇔ (R4N+)2CO3 + Cl− + H+Cl− Depending on the local concentration of excess bicarbonate, the HCl produced either remained as HCl or reacted to form carbonic acid in a reaction which tended to buffer the system. The resin characteristics related to its preference for divalent ions and its ability to produce higher-valent anions by proton expulsion were (a) appropriate functional group charge spacing, (b) ability to reorient functional groups to satisfy polyvalent ions, and (c) presence of hydrogen-bonding hydrophilic groups such as carbonyl and hydroxyl. IkA-458, the resin that was clearly superior in its ability to produce higher-valent ions, exhibits the highest divalent selectivity, is hydrophilic, and carries its quaternary amine functional groups at the end of a long flexible pendant arm containing an amide bond with hydrogen-bonding potential. Implications of the experimental findings as applied to arsenate removal from water are discussed.

Anion exchange studies of lead-EDTA complexes

Abstract This research with anion exchange resins demonstrates the possibility of removing chelated anionic complexes from water. The affinity of various commercially available polystyrene and acrylic resins for the Pb-EDTA complex was established. The effects of pH, competing ions, resin matrix, functionality, and porosity were studied.

Chromate ion-exchange process at alkaline pH

Abstract The chromate ion-exchange recovery process is always carried out at acidic pH because of its much higher chromate removal capacity at acidic pH as opposed to alkaline pH. However, acidic pH operation always gives rise to early, gradual chromate breakthrough during conventional fixed-bed column runs. Possibility of alkaline pH operation, which gives self-sharpening type chromate breakthrough during column runs, has been studied in detail. A new polystyrene matrix resin with relatively high degree of crosslinking was found to have high chromate selectivity at alkaline pH. This enhanced chromate selectivity is due to the resins increased hydrophobicity. Particularly at low ionic strength of cooling water, alkaline pH operation with this new resin may offer higher chromate selectivity compared to conventional acidic pH operation.

Fluidized bed reactor for the biological treatment of ion-exchange brine containing perchlorate and nitrate

The removal of perchlorate and nitrate from contaminated drinking water using regenerable ion-exchange processes produces a high salt brine (3-10% NaCl) laden with high concentrations of perchlorate and nitrate. This bench-scale research describes the operation of acetate-fed granular activated carbon (GAC) based fluidized bed reactors (FBR) for perchlorate-only, and combined nitrate and perchlorate removal from synthetic brine (6% NaCl). The GAC was inoculated with a salt-tolerant culture developed by the authors and used previously in batch systems. An FBR was an effective design for perchlorate reduction and exhibited first-order degradation kinetics with respect to perchlorate concentrations. Nitrate was also removed by the organisms in the column and had no negative effects on the removal of perchlorate using the FBR design. However, at higher concentrations of nitrate the FBR was more difficult to operate due to loss of carbon and biomass from the formation of nitrogen bubbles and the high recycle flow rates needed.

Thermal regeneration of powdered activated carbon (pac) and pac-biological sludge mixtures

Abstract A temperature-programmed graphite microfurnace apparatus with mechanical stirring is described for the thermal regeneration of powdered activated carbon (PAC) alone or PAC-biological sludge mixtures from the PAC enhancement of the activated sludge process. Three PACs with widely differing BET areas were evaluated to determine the effects of regeneration on their physical and chemical properties. PAC weight % recoveries were in the range of 60–80%, and the recovery of wastewater (filtered mixed liquor) adsorption capacity regularly exceeded 100% for all carbons. In one case, 222% recovery was achieved. Regeneration always reduced BET area but the effects on I 2 number, pore size distribution and particle size were mixed. Compared to the regeneration of PAC alone, the presence of 40–50% biological sludge was clearly detrimental to the regeneration process. Nevertheless, essentially complete regeneration could be achieved with biomass present by adding a 20-min 400°C, N 2 -purge-gas charring step to the usual 20-min, 825°C, H 2 O-N 2 -CO 2 purge-gas reactivation step. Unfortunately, charring the biomass produced a regenerated material containing 27–34% inerts including carbonaceous char and mineral ash. This black inert material was quite insoluble in dilute HCl and had no useful adsorption capacity. The controlled application of H 2 O or trace amounts of O 2 during charring is suggested for the minimization of detrimental char but further research is needed.