Physical charge separation

Electrically driven processes, such as ED/EDR and CDI, have been proposed to treat concentrate [11—14]. Both of these processes work by imposing an electric field that creates a physical separation between the cation and anions, thereby preventing scale-prone ion pairs from forming. Both of these processes will be discussed in general terms.

4.3.1 Electrodialysis/electrodialysis reversal

Both ED and EDR have the potential to operate at very high water recoveries. In the late 1980s, EDR was demonstrated for desalting 5000 mg/L total dissolved solids (TDS) groundwater at 94% water recovery [54]. EDR has also been applied to reclaim 8000 mg/L TDS RO concentrate to achieve RO-EDR recoveries of up to 96% [55]. However, RO is often preferable over ED and EDR, based not only on the rejection of both ionizable and nonionizable components, but also from an energy usage perspective. However, in highly sulfonated waters such as RO concentrate, ED/EDR may offer a decided advantage over RO through its ability to achieve high water recoveries without incurring scaling.

Fig. 6 shows a conventional ED configuration whereby alternating cationic and anionic transfer membranes selectively remove charged, soluble ionic constituents in the presence of an electric field [56]. Traditional ED and EDR membranes are manufactured by mixing either a cation exchange or anion exchange resin with a polymer. The membranes allow for the passage of like-charged ions, while blocking the passage of water and oppositely charged ions. Several research teams are working on improving ED/EDR membranes through development of low-resistivity nanoporous and block-polymer ED membranes that operate at much lower energy consumption than traditional ED membranes ([57]; http:// www.sandia.gov/water/projects/desal2.htm). The philosophy behind these new membranes was that it would take less energy to push ions through nanoengineered materials than through a solid functionalized polymer — the standard membrane material used for conventional ED.

Moreover, by functionalizing the membranes with chemical groups having high affinities for targeted species (e.g., a nitrate binding group for nitrate), it may be possible to generate an ion-selective membrane [58,59]. Preliminary results by Lawrence Livermore National Laboratory using nanocomposite membranes indicate that significantly smaller voltage is needed to drive the same ion flux through the nanoporous membranes, indicating higher energy efficiency, although the permselectivity is not as great as with the commercial membranes [60]. A technology with lower overall energy use coupled with ion selectivity would greatly lower the costs of treating contaminated water supplies. •••-

Example

EDR experiments were conducted using a 2.0 gpm EDR unit operating in both batch- and continuous-operation modes. The EDR unit contained 2 electrical stages, 4 hydraulic stages, and 120 cell pairs. For batch-mode tests, approximately 100 gal of primary RO concentrate and primary RO concentrate post-ICD and MF (see Section 3.1) were processed through the EDR unit with the concentrate being discarded and the product serving as the feed for the next pass. For continuous-mode testing, primary RO concentrate was run through the EDR unit at 50% water recovery. The purpose of these tests were to evaluate (1) would natural organic matter (NOM) and antiscalant carryover from the primary RO negatively impact EDR membrane performance and (2) could 95% total system water recovery be achieved using RO-EDR?

Fig. 7 provides the conductivity data over time for both the (a) continuous-mode (b) and batch-mode EDR experiments. Fig. 7a demonstrates that EDR membranes exposed to unaltered RO concentrate showed no adverse fouling after 8.5 days of operation. Therefore, in this instance, the concentrated NOM and antiscalant in the RO concentrate did not lead to immediate fouling, though longer term testing may prove otherwise. Fig. 7b and Table 2 show that upon successive treatment of the concentrate, the final salinity can be tailored for an intended end use. However, nonionizable species, such as SiO2 and NOM, are retained in the diluate, or product water, and a polishing step may be needed to use the water for potable purposes. Research by Sethi et al. [25], using the same CRW primary RO concentrate and a bench-scale ED apparatus, confirmed that up to 80% recovery of the concentrate was possible, though further processing of the diluate was needed prior to the water being put back into production. -•

8000

6000

i 4000

2000

8000

6000

2000

O Feed □ Product A Reject

ufaDD^nrfpn

100 150

Hour

O Feed □ Product A Reject

Min.

Figure 7 EDR treatment conductivity data of primary RO concentrate run at (a) steady-state (continuous-mode) and (b) in batch-mode (b). Dashed lines indicate separate passes though EDR unit.

4.3.2 Capacitive deionization

CDI is a novel technology for removing ionic species from aqueous solutions. This electrochemical process is conducted at ambient conditions and low voltages (e.g., 1V) and requires no high-pressure pumps, membranes, distillation columns, or thermal heaters. CDI is an electrosorption process that acts as a "flowthrough" capacitor. In principle, an aqueous solution containing dissolved solids (e.g., NaCl, CaCO3, and CaSO4) is passed between matching pairs of carbon aerogel electrodes. Ionic species, such as sodium and chloride, are held at the charged electrode surfaces (Fig. 8) and

Table 2 Batch-mode electrodialysis reversal (EDR) water quality data

Primary RO feed

Primary RO concentrate

Final EDR diluate

Conductivity (ms/cm)

1228

5684

759

Total hardness (mg/L as CaCO3)

337

2220

104

Total alkalinity (mg/L as CaCO3)

76

490

135

Ca (mg/L)

80

560

30

Ba (mg/L)

116

774

19

SO4 (mg/L)

349

2240

85

SiO2 (mg/L)

7.1

29.4

29.4

Carbon Aerogel Negative Electrode

Brackish Water

Figure 8 Graphical representation of capacitive deionization process.

Carbon Aerogel Negative Electrode

Brackish Water

Treated Water

Positive Electrode

Figure 8 Graphical representation of capacitive deionization process.

are temporarily removed from the solution. The solution is continually deionized (purified) as it passes through successive electrode pairs.

Electrode materials for CDI have included porous carbon [61,62], carbon cloth [12,32,63,64], carbon nanotubes [65], and carbon aerogels [63,66,67], with carbon aerogels making up the bulk of the recent research effort. Carbon aerogels are unique, porous materials consisting of interconnected, uniform carbonaceous particles (3—30 nm) with small (< 50 nm) interstitial pores [66]. This structure leads to high density, a high specific surface area of 400-800 m2/g low hydraulic resistance, and an exceptional electrical conductivity of ~100S/cm. The aerogel chemical composition, microstructure, and physical properties can be controlled at the nanometer scale, giving rise to unique electrical properties.

Several separation mechanisms may be controlling ion uptake. Typically, nonreducible and nonoxidizable ions, anions, and cations are removed from solution by the imposed electric field via electrostatic attraction (without charge transfer) within the electrode/electrolyte interface. Large polyvalent oxyanions, heavy metals, and colloidal impurities may be removed by a combination of physisorption, chemisorption, electrodeposition, electrophoresis, and double-layer charging, with charge transfer possibly taking place [68]. After the electrodes become saturated with salt or impurities, the electrodes are regenerated by electrical discharge, allowing the captured salt ions to be released into a relatively small, concentrated purge stream.

Example

Limited studies have been done using CDI with natural waters [69,70]. While these studies were not conducted on high-TDS RO concentrate, they are illustrative of the problems associated with CDI in this regard. The limitations of CDI to act as a concentrate minimization technology are threefold: (1) CDI preferentially removes monovalent ions over divalent ions; (2) the limited sorption capacity of the carbon aerogel electrodes; and (3) natural organic matter easily fouls the high surface area electrodes. Fig. 9 shows the percentage removal of various ions from a blend of Colorado River water and California state project water. These data show that in a competitive environment (i.e., when multiple ions of varying valences are present), the sorption of the divalent species are limited. This finding is important in that, for RO concentrate, divalent ions control the scaling potential of the water and ultimately the water recovery of the system. Therefore, by not removing the divalent ions, the solubility of scale-prone salts within the CDI stack is decreased with the reduction of solution ionic strength, ultimately leading to scale formation. -•

Despite claims of sorption capacities of up to 80 mg TDS/g of aerogel, real-world applications have only achieved ~8mg TDS/g of aerogel [71]. This finding is caused by the fact that ion selectivity is based on ionic hydrated radius [69]. As such, only pores greater than 20 nm in diameter are available as sorption sites to allow for electric double-layer formation [72].

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