Critical Review
Reuse, Treatment, and Discharge of the Concentrate of Pressure-Driven Membrane Processes BART VAN DER BRUGGEN,* LIESBETH LEJON, AND CARLO VANDECASTEELE Laboratory for Environmental Technology, Department of Chemical Engineering, University of Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium
Application of pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis) results in the generation of a large concentrated waste stream, the concentrate fraction, as a byproduct of the purification process. Treatment of the concentrate is a major hurdle for the implementation of pressure-driven membrane processes since the concentrate is usually unusable and has to be discharged or further treated. This paper reviews possibilities to treat or discharge the concentrate: (i) reuse, (ii) removal of contaminants, (iii) incineration, (iv) direct or indirect discharge in surface water, (v) direct or indirect discharge in groundwater, and (vi) discharge on a landfill. General guidelines are given for the choice of a proper method as a function of the origin and composition of the water treated. Next, the further treatment of the concentrates in four application areas of pressure-driven membrane processes (drinking water industry, leather industry, and membrane treatment of landfill leachates and of textile process waters) is discussed.
Introduction Pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration, reverse osmosis) separate a feed stream into a purified permeate fraction, which is usually the desired product, and a concentrated retentate fraction, the concentrate (sometimes denoted as retentate). Microfiltration (MF) membranes have pores ranging from 0.1 to 2 µm and operate at pressures below 5 bar; suspended solids or emulsified components and bacteria/protozoa larger than the pore size can be removed from the feed solution (1). Ultrafiltration (UF) membranes remove macromolecules with a molecular weight above 5000-100 000, depending on the membrane pore size. The operating pressure ranges from 2 to 8 bar. UF also acts as a disinfection barrier by removing bacteria as well as viruses (2). In nanofiltration (NF), the rejection is extended to organic compounds with a molecular weight above 200-500 (depending on the pore size) and to multivalent ions (due to the membrane charge); the operating pressure is 5-15 bar. In reverse osmosis (RO), pressures range from 50 to 100 bar (seawater desalination) or 15-50 bar (brackish water desalination and other applications). The membranes are dense and allow the retention of small organic compounds as well as ions (3); larger components may obviously also be removed but are usually removed in a MF/ UF pretreatment as they may cause severe fouling problems in spiral wound NF or RO units. * Corresponding author e-mail: bart.vanderbruggen@ cit.kuleuven.ac.be; telephone: +32 16 32.23.40; fax: +32 16 32.29.91. 10.1021/es0201754 CCC: $25.00 Published on Web 08/02/2003
2003 American Chemical Society
Advantages of pressure-driven membrane processes are obvious. The purified product, the permeate, usually has an outstanding quality; the processes are easy to operate, moderate temperatures can be applied, no chemicals are added, energy requirements are generally low, and scalingup or combination with other separation processes is easy due to the modular construction (4). Two issues can be identified as major drawbacks for the implementation of pressure-driven membrane processes: the risk of membrane fouling (which may require extensive pretreatment or chemical cleaning of the membranes and may possibly result in a short lifetime of the membranes), and the need for further treatment of the concentrate fraction. Membrane fouling is being studied in a systematic way by a number of research groups (5-8), but an overview of treatment methods for the concentrate is missing. This paper reviews the possibilities for reuse, further treatment, and discharge of the concentrate based on a characterization of the concentrate in terms of volume and composition and taking into account the application from which the concentrate origins. In this overview, four typical application areas for membrane technology are focused on: the drinking water industry, the leather industry, the treatment of leachates from landfills, and the textile industry. These application areas are among the most important for membrane processes; other application areas such as the paper mill industry (9, 10), the food industry (11-14) or the electroplating industry (15) might use a similar approach, although particular difficulties may arise in each case. Moreover, the concentrate rather than the permeate may be the desired product in some cases, especially in the food industry. Concentrates resulting from the application of pressure-driven membrane processes in organic solvents, as often found in the chemical and pharmaceutical industry (16, 17), are not included in this review.
Characterization of Concentrates The volume and composition of the waste stream from pressure-driven membrane processes depends on the operation mode of the process. NF and RO are usually operated in cross-flow mode: the concentrate and the rinsing water used for intermediate forward-flushing are the main waste streams. MF and UF can be operated in cross-flow mode or in semi-dead-end mode: the wastewater mainly originates from the rinsing water used in the backwashing of the membranes. The composition of the retentate stream may be different in either case, mainly due to the size of the components removed in, for example, semi-dead-end UF as compared to cross-flow RO, but in both cases a retentate stream is obtained. Chemical cleaning of the membranes may be an additional source of wastewater. In this paper, the total wastewater volume resulting from the application of VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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pressure-driven membrane processes is denoted as the concentrate. The characteristics of the concentrate depend on the feedwater characteristics, the pretreatment, the membrane process used, the recovery, and the additional chemicals used (18, 19). The recovery is the permeate to feed volume ratio (REC ) Qp/Qf); a high recovery is aimed at in order to minimize the waste fraction and to maximize the volume of the desired product. The concentrate to feed volume ratio is 1-10% for MF and UF, 15-30% for NF, and 15-60% for RO. The largest concentrate fraction is obtained for desalination of seawater with high salinity (19). The composition of the concentrate is qualitatively identical to the feed composition, but the concentration is higher than in the feed for components rejected by the membrane. The concentration factor (CF, the ratio of the concentration of a component i in concentrate and feed) can be calculated from the mass balance for component i:
Cr,i )
(QfCf,i) - (QpCp,i) Qr
and thus
CF )
[
]
Cr,i Qf Cp,i ) 1 - REC Cf,i Qr Cf,i
where Q is the volumetric flow (L/h) and C is the concentration (mg/L); the subscripts r, f, p, and i refer to the concentrate (or retentate), the feed, the permeate, and the component used, respectively. Thus, CF depends on the rejection (R ) (Cf,i - Cp,i)/Cf,i) and on the recovery (REC). For component with a rejection of 100% (Cp,i ) 0), the CF can be calculated (16) as
CF )
1 1 - REC
For the estimation of the concentration of pollution parameters such as the COD (chemical oxygen demand) in the concentrate, approximate models can be used (21). The concentrate composition is largely determined by the pore size of the membrane. Concentrates from MF or UF contain suspended solids and colloidal particles, whereas NF or RO concentrates contain high concentrations of ions and small organic compounds. Suspended solids and colloidal particles should not be present in NF or RO concentrates if a proper pretreatment is applied. The addition of chemicals may also influence the characteristics of the concentrate. Antiscalants such as polyacrylates, polyacrylic acids, or polyphosphates are commonly added to the feedwater in order to avoid scaling of the membranes (i.e., the precipitation of calcium carbonate or calcium sulfate) (22, 23). Low concentrations (below 10 mg/ L) are usually applied. Furthermore, a pH correction (addition of sulfuric acid or hydrochloric acid) may be required. Both antiscalants and acids influence the chemical equilibrium of dissolved components; the former play a complexing role and curb the crystallization, whereas the latter shift the equilibrium to a higher concentration in the solution. Finally, biofouling can be avoided by disinfecting the feedwater prior to the membrane unit by UV treatment or by chlorination (24). The latter method may further complicate the process because chlorine damages most membranes and should preferably be removed again by, for example, NaHSO3 addition. Chemical cleaning for the removal of scaling, organic fouling, and biofouling from the membranes (25-27) results in a small additional waste stream. Chemicals can be acids (such as phosphoric acid or citric acid), bases (such as sodium 3734
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hydroxide), and complexing agents (such as EDTA, polyacrylates, sodium hexametaphosphate) or disinfectants (H2O2 and NaOCl). New cleaning methods avoiding the addition of chemicals seem promising (28). The composition of the concentrate can be improved or the volume can be decreased by optimizing the membrane process. Possible measures are as follows: the use of a proper pretreatment, the right choice of the membrane process and membrane, the proper choice of chemicals, and the operation at a high recovery. The volume of the concentrate, however, has an optimal value because the concentrations in the concentrate increase as the volume decreases, and an increase in the energy consumption, which is related to volume and concentration, increases the operational costs.
Possibilities for Concentrate Processing General Description of Methods. Because the composition of the concentrate from pressure-driven membrane processes depends on the application, the further environmental fate of the concentrate may be extremely unpredictable; a large variation in possibilities for reuse, further treatment, or discharge exists. Cost factors and legal aspects also play an important role (see Supporting Information). Generally, all methods for concentrate processing can be classified into one of the following categories: (i) reuse, (ii) further treatment by removal of contaminants, (iii) incineration, (iv) direct or indirect discharge in surface water, (v) direct or indirect discharge in groundwater, and (vi) landfilling. Table 1 gives an overview of possible processes in each category. Reuse is the most attractive option but is only applicable in few cases where the concentrated fraction is actually the desired product, such as in the food industry (e.g., dairy products, starch processing). The permeate is then a side product, which can be reused as a rinsing water or discharged. If reuse of the concentrate is not possible, further treatment may be necessary before discharge. Two options for further treatment can be distinguished (Table 1): (a) water removal from the concentrate and (b) removal of specific components by a proper choice of a selective treatment method. The first option leads to a sludge or solid waste that is subsequently reused (if possible), landfilled (if necessary after solidification/ stabilization or a similar pretreatment to avoid leaching of contaminants), or incinerated in a rotating kiln furnace (hazardous waste) or a grate furnace (nonhazardous waste). The second option leads to a (treated) wastewater that has to be reused (if possible) or discharged in surface water (direct or indirect via sewage systems) or in groundwater. Other factors than the volume and composition that have to be taken into account for selecting a proper treatment process are (20, 29) as follows: legal requirements such as permits and conditions; cost of further treatment; local factors such as the proximity and size of a wastewater treatment plant, the presence of surface water or open land, soil characteristics, and geological structure; flexibility of the disposal method in case of an expansion of the existing plant; and public acceptance. Concentrates in the Drinking Water Industry. The first large applications of membrane technology were found in the drinking water industry, and to date the drinking water industry is still the most important application area for membrane processes. In the mid-1990s, a survey on the use of membrane techniques in the drinking water industry in the United States was conducted (30) for installations with a capacity above 25 000 gal/d (95 m3/d): 73% of the 137 installations were RO installations for desalination of brackish water, 11% were NF installations, another 11% were electrodialysis installations, and the remaining 5% were RO plants for seawater desalination. In 48% of the installations, the concentrate was discharged in surface water; in 23%, the concentrate was treated in a wastewater treatment plant; in
TABLE 1. Overview of Possibilities for Reuse, Further Treatment, and Discharge of the Concentrate category reuse (29, 36, 37, 50) further treatment (32, 34, 47, 48, 64-67)
incineration discharge in surface water (40-42) discharge in groundwater (33, 44) landfilling (57, 58)
processes as the desired product (e.g., concentrated food products) as fertilizer, for soil improvement, as fuel production of salts and other minerals concentration by water removal: thermal (e.g., evaporation, distillation) other (e.g., electrodialysis, settling) removal of specific compounds: activated sludge: organic compounds oxidation processes: (electro)chemical, photooxidation adsorption/ion exchange rotating kiln furnace (hazardous waste) grate furnace (nonhazardous waste) direct discharge indirect discharge via sewage system application on the land: irrigation, evaporation ponds deep injection in the soil as solid waste after pretreatment with additional treatment (e.g., tabilization/solidification) without additional treatment as liquid waste
13%, the concentrate was reused on the land for, for example, irrigation; in 10%, the concentrate was discharged to groundwater by deep injection; and in the remaining 6%, the concentrate was discharged to evaporation ponds. Desalination of brackish water and seawater by RO is among the oldest applications of membrane technology. Typical recoveries in RO are 35-50% for seawater desalination and 70-90% for brackish water desalination (31). The permeate has a total dissolved solids (TDS) concentration of 100-600 mg/L (32). The composition of the concentrate highly depends on the composition of the feedwater, but all concentrates have a high TDS up to 70 000 mg/L. The brine can be reused for irrigation of halophytes (33), plants that tolerate a salinity of up to 35 000 mg/L, and can be used for production of oil seeds or grains, or as leaders, for landscaping or as a habitat for animals. Halophytes only take up a fraction of the saline water, so that contamination of groundwater layers is possible; the local geology has to be fit therefore for this reuse application. A drainage system, alternatives for brine removal, and storage facilities for periods with high rainfall help to overcome this risk. In other cases, evaporation may be necessary to reduce the concentrate volume in order to allow reuse or discharge of the concentrate (34). Evaporation is expensive, but the cost for transporting the brine to the nearest discharge location is high as well (32). Furthermore, the concentrated brine can be useful for the production of salt and other minerals (29), and, if the NaCl crystals are dissolved in pure water, it can be useful for the production of NaOH, Cl2, and H2 by electrolysis. A simple evaporation technique is storage in evaporation ponds where the water gradually evaporates (35). This method is used in semi-arid areas in the Mid-East, Australia, or the United States where evaporation rates are high, land is sufficiently available, and annual rainfall is low. These evaporation ponds often also serve for the production of brine shrimp. A more general method is the further treatment of the concentrate by distillation techniques such as multi-stage flash evaporation (MSF) and multi-effect distillation (MED). The combination of RO with these techniques may have a synergetic effect: the RO feedwater can be preheated by using it as a cooling water for the distillation, which increases the RO performance; furthermore, freshwater recovery is higher, and the remaining brine can be used for salt production (36, 37). The combination of RO and distillation is an important evolution, and optimization of this hybrid process (by, for example, replacing the first RO unit in a two-step process by
a NF unit) might lead to a significant improvement of the desalination process. Scaling problems at elevated salinity can be solved by the seeding technique: crystals are added to the feed, which act as crystallization cores in the solution so that crystallization on the material of the plant is avoided (38). Distillation can be replaced by electrodialysis if the salinity is low or an integrated membrane operation for seawater desalination including UF or NF, a membrane contactor, a RO unit, and a membrane crystallizer can be applied (39). This option is very challenging as it would reduce the waste fraction to an absolute minimum; therefore, further research on these processes is of high importance. If reuse of the concentrate is not aimed at, the most straightforward method for further processing of the concentrate is discharge in surface waters with high salinity on the condition that the plant is near a sea, ocean, or estuarium (40, 41). To avoid degradation of the surface water quality, an additional treatment of the concentrate may be required. Regulations, however, often take a local “mixing zone” into account where the water quality standards for nontoxic compounds can be exceeded. The assumption is that a variation of the water salinity below 1000 mg/L (the average natural variation of salinity in oceans) does not have an influence on the aquatic ecosystem. Obviously, additives such as biocides and antiscalants should also be taken into account. Local conditions have to be considered in estimating the effect of the brine discharge (41, 42). If the effect on the aquatic system is too large or the salinity of the brine is too high, mixing with other waste streams (rinsing water, municipal wastewater) may be a solution. For desalination plants not near a saline water body, discharge in the sewage system can also be considered. However, this is not often possible because of the large volumes to be discharged and the negative effect of this large and saline stream on the operation and the effluent quality of the wastewater treatment plant (18, 20, 43). Injection of the concentrate into the soil is another option (44). The location for injection has to be carefully chosen so that no risk for groundwater contamination occurs. Injection pipes for brine disposal need to have an additional liner to prevent corrosion; and monitoring of the soil around the location of injection is necessary. For the production of drinking water from surface water or groundwater, MF and UF are the most commonly used membrane processes, although the number of NF applications is also increasing. The use of MF and UF on a large scale only started in the beginning of the 1990s, so that the VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Recycling of Cr(III) from tanning baths in the leather industry by using a combination of ultrafiltration (UF) and nanofiltration (NF). experience with concentrate streams is still limited (45). The concentrate or rinsing water from MF or UF is however easier to treat than the one of RO or NF: the ion concentrations in the concentrate are not increased, and the only difference between feedwater and concentrate is a higher concentration of suspended solids and colloids (46). The most often used processing methods are (44) recycling of the rinsing water or concentrate to the feed; discharge in surface water; and discharge to a wastewater treatment plant. Because the concentration of suspended solids is limited (e.g., in Flanders, 60 mg/L for discharge in surface water and 1000 mg/L for discharge to a wastewater treatment plant), a reduction is usually necessary. Settling can be applied or an additional MF/UF unit used. In either case, sludge is obtained that has to be dehydrated naturally (in drying beds) or mechanically (by using e.g. a belt filter press, a vacuum filter, or a centrifuge). The sludge can then be used as a fertilizer (after aerobic or anaerobic stabilization, aerobic or anaerobic composting, thermal drying, or addition of lime), as a cofuel in addition to coal, or it can be incinerated with energy recuperation. NF or RO concentrates are more difficult to treat because of the increased concentrations of salts and small organic compounds. If these concentrations are not excessive, the concentrate can be ultrafiltrated (47); the UF permeate can, for example, be discharged in surface water; whereas the UF concentrate can be used for irrigation or discharged to the sewage system. If a NF or RO unit is used for nitrate removal, biological denitrification of the concentrate may offer a solution, although the feasibility for concentrates was not yet proven. A combination of ion exchange, electrodialysis and evaporation can also be used (48); Ca2+ and Mg2+ are removed by ion exchange in order to prevent scaling in electrodialysis; electrodialysis results in a further increase of ion concentrations; and evaporation yields a crystalline material that can be landfilled. Concentrates from the Leather Industry. An important application of NF in the tanning industry is the recycling of chromium from exhausted chromium baths (49-52). These baths are indispensable in the tanning process in order to avoid rotting of the animal skins. Chromium sulfate (Cr2(SO4)3) is often used because it offers a good mechanical and 3736
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hydrothermal resistance and a high penetration rate in the skin; furthermore, it is highly suitable for dyeing. The chromium salts are added in excess so that 30-40% of the initial Cr(III) dose remains in the tanning bath. A combination of UF and NF can then be used to recycle the tanning baths (Figure 1). UF results in a reduction of suspended solids and fat; the concentrate is brought back to the equalization tank. The UF permeate is brought to a NF unit, where Cr(III) is concentrated; the concentrate can directly be reused for retanning baths or further concentrated by precipitation (at high pH, addition of NaOH required) and dissolution (in a concentrated sulfuric acid solution). The latter method results in a solution with a high Cr2O3 concentration that can be used for the preparation of new tanning baths. The NF permeate contains a high chloride concentration because chlorides (monovalent) are almost not retained by nanofiltration. This is an advantage when the permeate is reused in pickle baths applied in leather processing (saving in chemicals to be added). An alternative use that has been reported is irrigation in agriculture. Both concentrate and permeate fraction can thus directly be reused in the process (category 1 in Table 1). A typical composition of the UF feed, concentrate, and permeate and the NF permeate and concentrate is given in Table 2 (50). No discharge is needed, apart from a small fraction of sludge and fat from the equalization tank. Alternatives for the recycling of chromium are precipitation-dissolution and adsorption on smectite or activated clay. The former method results in a lower quality product, whereas the latter method is difficult to implement because of the very specific process conditions, which often leads to new waste streams (53). Concentrates from Membrane Treatment of Landfill Leachates. Landfill leachates are a complex mixture of organic and inorganic components with a significant variation in volume and composition: the volume depends on the local climatic conditions, and the composition is determined by the composition of the waste material on the landfill and the age of the landfill (54). Landfill leachates have to be collected and treated in order to avoid contamination of local groundwater layers. The variations in volume and composition hamper an efficient classical treatment (aerobic or
TABLE 2. Typical Composition of the UF Feed, Concentrate and Permeate, and NF Permeate and Concentrate Obtained for Application of Pressure-Driven Membrane Processes for Cr(III) Recovery in the Leather Industry (50) parameter
UF feed
UF conc
UF permeate ) NF feed
NF conc
NF permeate
pH TSSa (mg/L) CODb (mg/L) Cl- (mg/L) SO42- (mg/L) Cr (mg/L) NH4+-N (mg/L) org-N (mg/L) Fe (mg/L) Ca (mg/L) Mn (mg/L) Al (mg/L) Mg (mg/L) oil, fat (mg/L)
3.7 612 5 960 11 136 26 137 4 343 422 250 24 1 100 2.2 91 867 116
3.7 428 6 413 11 098 27 239 8 269 420 301 29 948 2.5 97 822 148
4.1 154 5 126 10 844 27 966 2 729 367 165 32 1 086 2.4 96 870
4.0 370 7 641 7 390 83 455 9 285 720 209 81 1 367 6.3 259 6 162
4.0 28 3 315 13 190 10 550 30 320 98 8 12 0.4 5 60
a
Total suspended solids.
b
Chemical oxygen demand.
FIGURE 2. Schematic representation of a possible treatment of landfill leachates used in Mechernich, Germany (59). anaerobic biological methods; physicochemical treatment such as chemical precipitation, oxidation with ozone, hydrogen peroxide, or UV, and activated carbon adsorption) so that membrane technology offers the best solution (5558). The membrane system that can be applied is a dual-step RO (59), preceded by a cartridge filtration or a biological treatment. The permeate of the second RO unit has low concentrations of organic and inorganic contaminants and can be discharged in surface water or in groundwater layers. The concentrate contains high concentrations of organic and inorganic compounds and has to be further treated. An example is the treatment system used for leachate of a landfill in Mechernich, Germany (59), schematically represented in Figure 2. The total capacity is 150 m3/d; the first RO unit uses tubular membranes in order to avoid fouling; the second RO unit uses spiral wound modules. The concentrate from the second RO is brought back to the inlet of the first RO unit.
The concentrate from the first RO unit is evaporated in two stages, upon which the residue is dried in a fluidized bed. The dried material is landfilled; the distillate from the evaporation is brought back to the second RO unit. This scheme can be simplified for relatively new landfills where the leachate is relatively biodegradable: the concentrate can then directly be re-injected into the landfill. During the recirculation, the quality of the concentrate will improve, and an equilibrium between leaching and biodegradation will be reached for the organic fraction (57, 58). However, this method is not usuable for older landfills and for leachates containing a large inorganic fraction. Evaporation and drying is then necessary and may be followed by a solidification, so that a material is obtained with low water permeability and with low leaching of, for example, heavy metals, which can be landfilled without any additional environmental risk. An alternative for re-injection in the case of leachates with a high organic content is incineration. This option allows to extract energy from the waste material. The incineration has to be carried out in a rotating kiln furnace, which allows a better incineration than a grate furnace and is thus more suitable for hazardous materials. Concentrates from the Textile Industry. RO or NF can be used for the treatment of exhausted dye baths in the textile industry (60-63). These baths contain high concentrations of organic components (dyes and additives such as antifoam agents) and inorganic components (salts, used to regulate the rate of dye fixing on the textile). The permeate can usually be reused as a process water for dyeing or rinsing; the concentrate is a highly colored waste stream with high concentrations of organic and inorganic compounds. Recycling of dyes or other chemicals is generally not possible because the composition of the concentrate is too complex and because the components are often modified by, for example, hydrolysis. Direct discharge is also not possible given the composition of the concentrate. Color can be removed from the concentrate in an activated sludge system. However, the removal efficiency should be expected to be low because many dyes are not biodegradable. In the case of end-of-pipe membrane filtration after biological treatment, a further biological degradation of the concentrate cannot be obtained. By applying an oxidation reaction on the concentrate, further biological degradation may be possible. A useful but expensive method is ozone or hydrogen peroxide dosage; the use of Fenton reagent (hydrogen peroxide in combination with Fe2+ as a catalyst) has proven to be effective (64). Similarly, the usefulness of electrochemical oxidation for color removal from concentrate streams in the textile industry was demonstrated (65); the cost of this process is also high. A third oxidation method is photocatalytic oxidation with TiO2 as a catalyst (66). Degradation of organic compounds is efficient and cheaper than for other methods; the major problem is the recuperation of the catalyst. MF might be a solution to this problem. For further biological degradation, an anaerobic system should be considered. Anaerobic degradation is very efficient a wide range of dyes, including azo dyes. Breaking of the azo bond can be easily obtained in this way, and the effluent may be of a sufficient quality for discharge in surface water or sewage system. As a polishing step, adsorption on activated carbon is very efficient but only useful for waste streams with small, specific contaminations given the cost of regeneration of the activated carbon columns. In this view, bio-decoloring by microorganisms offers more perspectives for the future (67). A combination of adsorption and enzymatic degradation results in a very efficient and cost-effective color removal. The application possibilities for textile wastewater or conVOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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centrate streams are large; however, the research in this field is still ongoing.
Supporting Information Available The legal framework of concentrates processing including additional references and a table. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review August 23, 2002. Revised manuscript received June 26, 2003. Accepted July 2, 2003. ES0201754
