Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/72—Treatment of water, waste water, or sewage by oxidation
  • C02F1/722—Oxidation by peroxides
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/28—Treatment of water, waste water, or sewage by sorption
  • C02F1/283—Treatment of water, waste water, or sewage by sorption using coal, charred products, or inorganic mixtures containing them
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/72—Treatment of water, waste water, or sewage by oxidation
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
  • C02F2103/06—Contaminated groundwater or leachate
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2305/00—Use of specific compounds during water treatment
  • C02F2305/02—Specific form of oxidant
  • C02F2305/026—Fenton's reagent
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W10/00—Technologies for wastewater treatment
  • Y02W10/30—Wastewater or sewage treatment systems using renewable energies
  • Y02W10/37—Wastewater or sewage treatment systems using renewable energies using solar energy

Definitions

• the present invention is directed to a process for treating contaminated water using a combination of adsorption and oxidation.
• Blowes et al in U.S. Patents 5,362,394 and 5,514,279, disclose treating contaminated water by excavating a trench in the aquifer in the path of a contaminant plume, and placing a body of active material which causes the contaminant, by chemical reaction, to change its oxidation- reduction state and to precipitate harmlessly in the body of the material .
• This process merely involves flowing waste through the active material without concentrating the contaminant .
• Leachate generation is a potential limitation in surface soil application of hydrogen peroxide, resulting in the downward transport of contaminants. In soil slurry reactors, the treatment volume of contaminated soil is generally small, representing a limitation to the overall treatment process.
• Limited reaction kinetics is the condition in which low concentrations of the target compound limits the second- order oxidation reaction.
• the clean-up goal for the target compound in the ground water can be difficult to achieve.
• Exacerbating the issue are the numerous scavengers which effectively compete against low concentrations of target compound for hydroxyl radicals.
• adsorption using activated carbon and oxidation using the Fenton mechanism has been widely used separately in ground water remediation and wastewater treatment
• problems associated with oxidation in subsurface systems involve poor reaction kinetics, excessive scavenging and excessive non-productive hydrogen peroxide consuming reactions.
• Problems associated with adsorption in subsurface systems relate to exhausting the sorption capacity of carbon. To replace the carbon, it must be excavated and transported to a specialized facility for disposal. Long-term risks associated with this disposal is environmentally undesirable. If the carbon is reactivated rather than disposed of, additional costs are incurred in this reactivation.
• Enzymatic and manganese reactions with hydrogen peroxide can consume hydrogen peroxide in reactions which do not yield hydroxyl radicals ( cf . Table 1 and Figure 2) .
• Selection criteria for granulated activated carbon should, therefore, include low manganese content.
• the iron content of the granulated activated carbon can be increased to enhance the Fenton mechanism.
• the effect of the enzymatic reactions are relatively short term because hydrogen peroxide inhibits catalase enzyme activity via the formation of an intermediate enzyme-substrate compound (Nicholls and Schonbaum, 1963; Aggarwal et al, 1991). Summary of the Invention
• contaminated water includes any water, waste stream, or ground water which has compounds which adsorb and can be oxidized by the hydroxyl radical.
• contaminated water is treated by first adsorbing contaminants onto a suitable non-treated sorbent and subsequently oxidizing the compounds near the surface of the sorbent .
• contaminants are adsorbed onto activated carbon containing iron, or which has been amended with iron in solution, which concentrates contaminants from the contaminated water onto the reactive medium.
• Hydrogen peroxide or other oxidizing agent is then added, which reacts with iron to generate hydroxyl radicals which oxidize the adsorbed contaminants.
• the contaminants which can be treated by the process of the present invention are substances which can be oxidized by hydroxyl radicals through the Fenton mechanism.
• This process makes it possible to treat mixed wastes. For example, benzene, xylene, toluene, and halogenated compounds can be treated in the same waste stream, whereas conventional zero-valent iron treatment only involved dehalogenation.
• the adsorption/oxidation system of the present invention provides for adsorbing and oxidizing contaminants on the sorbent surface. This process is also much more efficient that conducting the Fenton reaction in bulk liquid.
• Adsorption immobilizes and concentrates the contaminants onto the iron-treated sorbent.
• Treatment involves adding an oxidizing agent to the surface or solution of the iron-treated sorbent, which produces hydroxyl radical as a reaction intermediate.
• the hydroxyl radical oxidizes the contaminants sorbed to the iron-treated sorbent.
• Figure 1 is a schematic of the adsorption/oxidation process of the present invention.
• Figure 1A shows the initial time and concentration and the flow (Q 0 ) of water containing contaminants at an initial concentration (C 0 ) into the sorbent/iron medium, which results in contaminant adsorption to the granulated activated carbon yielding an acceptable effluent concentration (C a ) .
• Figure IB shows that contaminants concentrate on the carbon/iron medium and reach an equilibrium concentration (X .
• Figure IC shows that hydrogen peroxide perfused into the carbon/iron medium initiates the Fenton mechanism. This results in the formation of hydroxyl radical, which oxidizes sorbed contaminants, thus decreasing the concentration of contaminants and regenerating the granulated activated carbon (X 2 ) .
• Figure ID shows cessation of hydrogen peroxide application and contaminated water continuing to flow through the reactive medium for another cycle.
• Figure 2 is a cross-sectional diagram of a hydraulic barrier and adsorption/oxidation treatment unit.
• Figure 3 is a plan-view diagram of a hydraulic barrier and adsorption/oxidation treatment unit.
• Figure 4 is a schematic of hydrogen peroxide reactions in soil slurry containing 2-chlorophenol .
• (a) is non-hydroxyl radical producing reactions;
• (b) is cycling between Fe(II) and Fe(III) oxidation states;
• (c) is production of hydroxyl radical via the Fenton reaction;
• (d) is competition between 2-chlorophenol and scavengers (S ⁇ ) for hydroxyl radical; and
• (e) is the reduction of Fe(III) via the superoxide radical .
• Three successive applications of 100 mL 0.9% hydrogen peroxide into 1 g granulated activated carbon with 2CP 35.4 g/kg.
• Contaminated water is treated by a combination of adsorption of organic compounds in the water onto a sorbent to concentrate the contaminants and subsequent oxidation on the sorbent.
• This process is illustrated schematically in Figure 1.
• Contaminated ground water flows (Q 0 ) through a granulated activated carbon bed where contaminates adsorb onto the carbon. This results in acceptable concentrations of contaminants (C A ) in the effluent.
• the purified water continues through the treatment unit and back into the aquifer ( Figure 1A) .
• Adsorption immobilizes and concentrates the contaminants onto the sorbent, which also contains iron which is capable of facilitating Fenton-driven oxidation reactions.
• an oxidant such as hydrogen peroxide
• an oxidant is injected, which reacts with iron, generating a strong oxidant, the hydroxyl radical.
• the hydroxyl radical oxidizes sorbed contaminants ( Figure ID) .
• the sorbent is treated in si tu , and the sorption capacity of the sorbent is regenerated ( Figure ID) .
• the application of oxidant is performed at appropriate intervals to maintain an acceptable concentration in the contaminated water passing through the reactive unit.
• the process of the present invention treats contaminated water by destroying the contaminants in si tu, providing an efficient and economical treatment option.
• the process can be used above ground or below ground.
• the water treatment system can be constructed entirely below grade and can be entirely gravity driven, all of which reduces operation and maintenance costs.
• the system can be easily monitored.
• a broad range of water contaminants, including halogenated solvents, polycyclic aromatic hydrocarbons, petroleum constituents, etc., have a sufficiently high reaction rate constant with hydroxyl radical and, therefore, are viable target compounds to be oxidized. This indicates that the proposed treatment technology has wide application. Oxidation
• Reaction 1 hydrogen peroxide reacts with Fe(II) to yield hydroxyl radical and Fe(III), as shown in Table 1, Reaction 1.
• the Fe(III) is reduced to Fe(II) via reaction with hydrogen peroxide, as shown in Table 1, Reaction 2.
• Reactions 1 and 2 cycle iron between the ferrous and the ferric oxidation states, producing hydroxyl radical continuously until the hydrogen peroxide is fully consumed. These reactions may involve either dissolved iron (homogeneous reactions) or solid phase iron oxides (heterogeneous reactions) .
• Reactions 1 and 2 indicate that the overall Fenton mechanism is acid generating. pH affects hydrogen peroxide stability (Schumb et al, 1955) and iron solubility. Oxidation efficiency is optimum under acidic conditions (Watts et al, 1991) . In any oxidation system involving Fenton-derived hydroxyl radical, pH should be monitored, and steps taken to mitigate acidic conditions.
• a similar reaction involving the hydrogen peroxide oxidation/reduction cycling of Mn 2+ and MnOOH(s) is thermodynamically favorable (Pardiek et al, 1992) and kinetically fast, but does not yield hydroxyl radical, as shown in Table 1, Reactions 7 and 8.
• Naturally-occurring soil microorganisms contain enzymatic catalysts, such as catalase and peroxidase, which also readily decompose hydrogen peroxide without producing the hydroxyl radical .
• the reactions between manganese or enzymatic catalysts and hydrogen peroxide reduce the amount of hydrogen peroxide available for Fenton reactions.
• the non-target chemical species scavenge hydroxyl radical which would otherwise oxidize the target contaminants.
• Hydrogen peroxide is generally present at high concentrations in Fenton systems and has a moderate reaction rate constant (2.7 x 10 7 L/mol-s; Buxton et al, 1988) and, therefore, is responsible for scavenging a significant fraction of hydroxyl radical produced in Fenton systems.
• Oxygen is a significant byproduct of reactions involving hydrogen peroxide in soils or aquifers. Reaction 6 in Table 1 indicates that 0 2 ⁇ reacts with Fe(III) to yield 0 2 .
• the rapid rate of degradation of high concentrations of hydrogen peroxide, in conjunction with the relatively low solubility of dissolved oxygen, the formation of bubbles, i.e., oxygen gas is certain.
• the formation of gaseous oxygen in porous media may result in gas blockage of fluid flow.
• any type of sorbent may be used in the process of the present invention, depending upon the contaminants to be removed from the water.
• the criteria for the sorbent are that it be capable of concentrating the contaminant sought to be treated/removed, and that it provide iron in some form for the Fenton mechanism during oxidation of the contaminant.
• the primary role of granulated activated carbon, as of any sorbent, in the process of the present invention is to immobilize and concentrate target compounds on the same surface on which the hydroxyl radical is produced. Subsequently, the target compounds on or near the surface of the sorbent are oxidized.
• sorbents for use in the process of the present invention include ion exchange resins, both anionic, cationic, or both, zeolites and other molecular sieves, alumina, silica, silicates, aluminum phosphates, and the like.
• ion exchange resins both anionic, cationic, or both
• zeolites and other molecular sieves alumina, silica, silicates, aluminum phosphates, and the like.
• adsorbent is effective in adsorbing and concentrating a particular contaminant.
• Granulated activated carbon is a preferred sorbent for removing organic compounds from waste streams .
• the pore size distributions and surface chemistry for a given granulated activated carbon are directly related to the starting raw material and the activation conditions. pH and concentration of transition metals in the carbon vary and, therefore, affect the reactivity of oxidants in granulated activated carbons.
• the oxidant can be any conventional oxidizing agent that works through the Fenton mechanism for oxidizing contaminants. While hydrogen peroxide has been illustrated in the specific examples, any other oxidizing agent that produces hydroxyl radicals in the presence of iron can be used, including ozone, permanganate salts, persulfate salts, and the like. Iron can be added to the sorbent to enhance the
• the amount of iron will affect the ability to carry out Fenton reactions and, therefore, the iron concentration of the sorbent can be optimized.
• the concentration of iron may be adjusted so that the density of iron sites (i.e., the spatial distribution of hydroxyl radical production sites) is similar to the density of sorption sites on the sorbent to assure the spatial probability of hydroxyl radical and contaminant interaction.
• One method of iron attachment to a sorbent involves raising the pH using a sodium hydroxide solution to precipitate ferric iron in the pores of the sorbent.
• the use of other forms of iron and the use of chelators and ligand agents can be used to attach iron to sorbent surfaces.
• a solution of iron and oxidant can be perfused through the sorbent to oxidize contaminants adsorbed thereto.
• adsorption/oxidation process described above is, thus, used in systems in which contaminated water can be diverted through a sorbent/iron treatment unit. Details of an adsorption/oxidation treatment process are provided below in the context of a hydraulic barrier in conjunction with a carbon/iron treatment unit, but this example is for illustrative purposes only and is not limiting of the invention.
• This treatment process can also be used in above- ground treatment systems and be constructed in existing or planned containment systems to serve as a pressure release mechanism to improve hydraulic control.
• a containment system or hydraulic barrier can be designed to leak while meeting stringent ground water quality criteria.
• Flow blockage through the granulated carbon/iron medium may be a limitation of the process.
• the reaction product oxygen (Reactions 6, 8 and 9 in Table 1) will result in gas formation which may fill the void spaces and inhibit water flow.
• This problem can be avoided using an upflow regimen allowing gaseous oxygen bubbles to rise in the carbon/iron unit and escape into the headspace of the unit or distribution gallery and into the air.
• Manganese oxide may be used to ensure that all hydrogen peroxide is consumed after it leaves the reactive media. This step minimizes the introduction of hydrogen peroxide into the distribution gallery. Biofouling may occur because of the high surface area and substrate concentration associated with the granulated activated carbon.
• Limited treatment volume, hydroxyl radical scavenging, low reaction kinetics, and non-productive hydrogen peroxide consumption may reduce the effectiveness of hydrogen peroxide application in soil slurry reactors.
• These limitations are minimized in the treatment system of the present invention. For example, contaminants partition from the ground water onto the carbon/iron medium, thus achieving stringent treatment criteria. Through this process, contaminants are concentrated on the carbon/iron reactive medium which enhances reaction kinetics. Further, hydrogen peroxide is applied in a scavenger-reduced solution to minimize the role of scavengers in the treatment unit . Flow blockage and pH reduction may also result, but design options can be implemented to minimize these potential limitations. Sorption/Oxidation System Design
• Cross-section and plan-view diagrams of a hydraulic barrier and treatment unit according to the present invention illustrate the hydraulic and treatment components of the proposed system, as shown in Figures 2 and 3.
• Contaminated water flowed into a gravel-filled collection gallery 20 and was directed through the carbon/iron treatment medium 21.
• the ground water then passed through the hydraulic barrier 22 via a pipe 23 and back into the aquifer, through a gravel filled distribution gallery 24.
• the collection and distribution galleries facilitate water flow since head loss is minimized in gravel relative to the head loss through the porous medium.
• the combined head loss through the alternative flow regimen must be less than the head loss of the original flow regimen to ensure continuity and to minimize ground water flow stagnation.
• the equivalent porous medium of the alternative flow regimen is comprised of the collection/distribution galleries, pipe flow, and treatment unit.
• the influence of the hydraulic barrier, collection/distribution galleries and treatment unit on the water gradient, flow pattern and capture zone must be evaluated on a site-specific basis.
• ground water flow in the pipe through the hydraulic barrier provides minimal disturbance to the wall . Since the system can be constructed below ground and is gravity driven, it is not subject to freezing or power outages (O'Brien et al, 1997) .
• the granulated activated carbon/iron medium can be readily accessed for sampling or replenishment if necessary.
• the granulated activated carbon is regenerated in si tu, and replenishment may be unnecessary.
• the granulated activated carbon/iron medium can be slurried, pumped and remixed in the treatment unit if recycling of the granulated activated carbon/iron medium is determined to be beneficial.
• Contaminants adsorbed to the carbon/iron medium were oxidized via hydrogen peroxide perfusion at selected intervals .
• Hydrogen peroxide was introduced by gravity into the system through a port at the surface which leads to a slotted distribution header 25 at the bottom of the carbon/iron unit.
• Gaseous oxygen formed in the carbon/iron unit will rise because of buoyancy and will escape into the headspace of the reactor unit, where it is vented into the atmosphere.
• the particle size of the activated carbon should be large enough to ensure mobility of gas bubbles in the carbon/iron medium and to minimize head loss. Assuming oxygen bubbles are diverted to the distribution gallery, an open chamber can be designed which will allow separation of bubbles and water. Passive gas capture and venting designs may also be used. It is undesirable to introduce hydrogen peroxide into the gravel-filled distribution gallery, since hydrogen peroxide decomposition and oxygen blockage may result.
• a layer of manganese oxide ore or manganese-rich granulated activated carbon can be installed above the carbon/iron bed to rapidly decompose the remaining hydrogen peroxide and ensure that no hydrogen peroxide will be introduced into the distribution gallery.
• Laboratory results indicate that the Fenton mechanism is an acid-generating process. This is consistent with Reactions 1-2 shown in Table 1, which indicate a net production of hydrogen ion. Acid production may be problematic, and its control at field scale may be advantageous.
• Different approaches can be used to control the pH in the oxidation system. A layer of limestone placed on top of the sorbent/iron unit and/or in the distribution gallery will neutralize a low pH solution. Another pH control method uses an automated pH-stat.
• This system comprises continuous pH measurement and adjustment of pH using an acceptable source of base, such as sodium hydroxide. It is important to note that oxidant perfusion into the treatment unit occurs infrequently, and the volume of water relative to the volume of water between oxidation events is small. Therefore, pH control may be necessary only when the treatment unit is undergoing oxidation. One option, of course, is simply to remove the solution containing spent oxidant for disposal or treatment elsewhere.
• Monitoring treatment performance involves an upgradient well in the influent area and one downgradient well in the effluent area. A well 26 in the upper treatment unit could be useful for obtaining ground water quality data on the treatment unit. Specifically, monitoring for the contaminant provides information on breakthrough of the carbon/iron unit and indicates when oxidation is required.
• monitoring chlorides concentration in the treatment bed using monitoring well 27 during an oxidation treatment is useful in determining when oxidation is complete. For example, assuming the treatment bed is operated in batches or continuously, chloride concentration would eventually diminish as sorbed contaminants are oxidized. This simple monitoring system can be used to verify that the water quality leaving the treatment unit satisfies ground water quality cleanup goals.
• Oxidation of sorbed contaminant occurs when an oxidant is perfused through the sorbent/iron unit. This is accomplished by, for example, introducing hydrogen peroxide into a port at the surface. The hydrogen peroxide then flows downward and out of a slotted distribution header and into the sorbent/iron media, as shown in Figures 2 and 3. Perfusing hydrogen peroxide can be effected either in continuous flow or in batch modes.
• the sorption/oxidation treatment system can be designed as two parallel units. For example, when hydrogen peroxide is perfused through one sorbent/iron unit, contaminants in the water can be treated via the second sorbent/iron unit.
• This provides the flexibility of operating one unit in a sorption mode and one unit in an oxidation/standby mode to ensure complete use of hydrogen peroxide.
• Other design configurations are also possible, such as series, batch, or continuous.
• the design options are also applicable to above-ground treatment systems .
• the frequency at which oxidant is applied and the concentration depends on the mass loading rate, the mass of sorbent and treatment efficiency.
• the treatment efficiency depends on numerous parameters, including pH, hydrogen peroxide concentration, iron concentration, contact time, scavenging, non-productive oxidant degradation reactions, reaction rate constants, concentration of target compounds, etc. These parameters vary significantly from site to site, and the frequency necessarily reflects such variability.
• a ground water plume comprised of contaminants, such as halogenated volatiles, polycyclic aromatic hydrocarbons, and fuel compounds (BTEX) , can be treated together.
• Significant process control can be achieved in the system, including concentration and hydraulic retention time of oxidant and type of sorbent (particle size, oxidant reactivity, manganese content, contaminant sorption, iron concentration, etc.).
• Performance monitoring can be simplified since ground water wells can be placed directly in the collection and distribution galleries for pre-treatment and post-treatment evaluation, respectively.
• contaminated ground water flows (Q 0 ) through a granulated activated carbon (GAC) bed and contaminants adsorb onto the carbon, with resulting acceptable concentrations thereof (C A ) in the effluent. Subsequently, the purified water continues through the treatment unit and back into the aquifer ( Figure 1A) .
• GAC granulated activated carbon
• Adsorption immobilizes and concentrates the contaminant onto the GAC, which also contains iron capable of facilitating Fenton-driven oxidation reactions.
• hydrogen peroxide Prior to breakthrough of contaminants from the reactive GAC ( Figure IB) , hydrogen peroxide is injected, which reacts with the iron in the GAC, generating the hydroxyl radical, a strong oxidant, which oxidizes sorbed contaminants ( Figure IC) .
• the GAC is treated in si tu, and the sorption capacity of the GAC is regenerated ( Figure ID) . Hydrogen peroxide is applied at appropriate intervals to maintain an acceptable concentration in the water passing through the reactive unit.
• H 2 0 2 has been used to generate -OH (rxns 1, 2) and oxidize undesirable contaminants in soils and aquifers (Watts et al, 1993; Ravikumar and Gurol , 1994; Yeh and Novak, 1995) .
• the Fenton mechanism involves either dissolved Fe in homogeneous reactions or solid phase Fe in heterogeneous reactions. In either case, H 2 0 2 cycles Fe between oxidation states yielding • OH and other byproducts (rxns 1, 2) .
• the overall Fenton mechanism is acid generating. H 2 0 2 stability increases with decreasing pH in Fenton systems and oxidation efficiency is optimum under acidic conditions (Watts et al, 1991) .
• Performance evaluation of Fenton-based remediation is generally determined from the rate of disappearance of a target analyte or the appearance of a decomposition product
• Chloride ion (CI " ) is a byproduct of the radical-mediated oxidation of chlorinated compounds, and its measurement is a reliable indicator of contaminant transformation. However, transformation of either the parent chlorinated compound or oxidation byproducts may proceed without simultaneous CI " release (Getoff and Solar, 1986) . Also, complete CI " release does not assure mineralization of the parent compound as indicated by intermediates measured in such systems.
• Limitations to Fenton-based oxidation may include excessive H 2 0 2 decomposition and the associated non-productive reactions (those that do not result in -OH production), excessive scavenging of -OH, insufficient Fe, and limited reaction rate kinetics (Huling et al, 1998a; 1998b) .
• H 2 0 2 decomposition and the associated non-productive reactions such as excessive H 2 0 2 decomposition and the associated non-productive reactions (those that do not result in -OH production), excessive scavenging of -OH, insufficient Fe, and limited reaction rate kinetics (Huling et al, 1998a; 1998b) .
• oxidation/reduction cycling of Mn(II) and MnOOH(s) by H 2 0 2 does not yield -OH.
• These non-productive H 2 0 2 reactions reduce the amount of H 2 0 2 available for Fenton reactions and can be a significant source of treatment inefficiency.
• Non-target chemical species present in soil and ground water, both naturally occurring (N0 3 “ , S0 4 2” , Cl “ , HP0 4 2” , HC0 3 “ , C0 3 “2 ) (Buxton et al , 1988; Pignatello, 1992; Lipczynska-Kochany et al, 1995) (rxn
• H 2 0 2 is generally present at high concentrations in Fenton systems and has a moderate reaction rate constant and, therefore, is responsible for scavenging a significant fraction of • OH produced in Fenton systems (Huling et al, 1998a) .
• Reaction rates between • OH and target compounds may be limited simply due to low concentrations. For example,
• 2-chlorophenol was selected as the target contaminant because it is relatively non-volatile, soluble, conveniently analyzed and has a published reaction rate constant with hydroxyl radical.
• the reaction of hydroxyl radical with 2-chlorophenol (rxn 3; Figure 4) yields a phenyl radical which has several resonance structures and may react with 2-chlorophenol to release chloride ion (Getoff and Solar, 1986) .
• 2-chlorophenol + • OH ⁇ reaction products k 10 1.2 x lO ⁇ M ⁇ s "1 . Hydroxyl radical species react with phenols and related compounds preferentially on the ortho- and para- positions and less on the meta- and iso-sites.
• the phenoxyl radical has several resonance structures which can lead to the formation of different products. Direct hydrogen- abstraction via hydroxyl radical is also possible.
• the first stage of oxidation of chlorophenols leads to the formation of various hydroxy benzenes (phenol, catechol, resorcinol, hydroquinone and hydroxyhydroquinone) (Getoff and Solar, 1986) .
• Other reactions which follow the addition and abstraction reactions include disproportionation and dimerization .
• Chloride ion is a byproduct of the radical-mediated oxidation of chlorinated compounds, and its presence is an indicator that the compound was transformed. Transformation of either the parent chlorinated compound or oxidation byproducts may proceed without simultaneous release of chloride ion. Further, complete release of chloride ion does not ensure mineralization of the parent compound. Nevertheless, chloride ion production is a reliable indicator of contaminant transformation. In the examples given herein, production of chloride ion was used as a diagnostic to quantify hydroxyl radical mediated transformation reaction of 2-chlorophenol.
• Granulated activated carbon is widely used and is successful in facilitating the removal of organic compounds from waste streams.
• Granulated activated carbon has a high surface area, about 800-1000 m 2 /g, low bulk density, high porosity, high sorption capacity, is available in a large range of particle sizes, is produced from different sources of raw material, and its activation varies between manufacturers.
• the performance of granulated activated carbon varies in chemical and physical characteristics and in sorption performance. More specifically, the following characteristics vary: (1) reactivity with hydrogen peroxide; (2) sorption isotherm;
• Reaction of an oxidant with granulated activated carbon can be attributed to different mechanisms, including:
• the reactors used were 125 mL Ehlermeyer flasks containing 1.0 gram granulated activated carbon and 40 mL 6 mM solution of 2-chlorophenol.
• the reactors were placed on an orbital shaker table for 24 hours, which allowed complete (> 99%) sorption of 2-chlorophenol.
• the granulated activated carbon slurry was decanted, and the solutions were analyzed for chloride ion, 2-chlorophenol, and total iron (Fe ⁇ ) .
• the remaining granulated activated carbon was amended with 100 mL hydrogen peroxide, of 0.7%, 0.9%, 1.2%, or 7.2%, w/w concentration, or deionized water in three successive applications, unless otherwise noted.
• the reactors were wrapped in foil to prevent photodecay, covered with parafilm to minimize volatile losses and evaporation, and placed on an orbital shaker table at 100 rpm.
• the granulated activated carbon slurry was decanted, and the solution was analyzed for hydrogen peroxide, chloride ion, 2- chlorophenol , and Fe ⁇ . Control reactors containing granulated activated carbon and hydrogen peroxide, but not 2- chlorophenol, were used to measure background chloride ion.
• the pH of 2-chlorophenol control solutions was adjusted to between 2 and 7 and the solutions were analyzed for 2- chlorophenol. These data indicated no transformation.
• the granulated activated carbon slurry pH was measured by placing a pH probe (Orion Sure-Flow ROSS Combination pH) into the slurry for five minutes to instrument stabilization. Samples were collected by pipetting 1.5 mL from a completely mixed suspension and filtered using a Gelman 0.2 ⁇ m filter which stopped all reactions and removed colloidal particles interfering with subsequent analyses . Hydrogen peroxide was measured immediately, and 2-chlorophenol subsamples were stored at 4°C for analysis when the experiment was completed. The 2- chlorophenol was obtained from Aldrich Chemical .
• EXP3 granulated activated carbon is commercially available bituminous-based carbon obtained from Calgon Chemical Corp. (Pittsburgh, PA) .
• EXP4 granulated activated carbon was derived from the same stock of Bakers carbon but was activated differently to minimize degradation of hydrogen peroxide (Rich Hayden, personal communication, 1997) . Two additional granulated activated carbons were obtained from
• the slurry of granulated activated carbon and iron was filtered and rinsed with deionized water through a number 35 sieve which retained > 99.99% of the granulated activated carbon.
• the granulated activated carbon was air dried and placed into 40 mL glass vials until used. Representative samples of the granulated activated carbon/iron stock were analyzed by inductively coupled argon plasma after metals were extracted from the granulated activated carbon by digesting a 0.25 gram sample in 40 mL of 19% nitric acid for 40 minutes in a microwave oven at 150°C and 145 psia.
• Check standards, blanks, duplicates, and spikes were run with each sample set, and the analytical quality was found to be in control.
• Hydrogen peroxide was analyzed using a modified peroxytitanic acid calorimetric procedure.
• Filtered granulated activated carbon slurry samples in triplicate were prepared in a similar manner. Absorbance of the hydrogen peroxide-titanium sulfate mixture was measured at 407 nm (A 407 ) using a
• Transformation products of 2-chlorophenol on the granulated activated carbon were identified using gas chromatography and mass spectroscopy .
• This analysis involved a derivatization technique using N-methyl-N- [ (tert-butyldimethyl) silyl] tri- fluoroacetaminde from Aldrich Chemical Co., Milwaukee, WI , which yielded tert-butyl-dimethylsilyl ethers and esters
• the injection and transfer oven temperatures were 275°C.
• the treatment ratio, T.R. was calculated as the ratio of moles of contaminant oxidized to the number of moles H 2 0 2 consumed over the same time frame (i.e., ⁇ C1 " / ⁇ H 2 0 2 ) .
• the number of moles of 2-chlorophenol oxidized was assumed to be stoichiometrically 1:1 to chloride ion measured in solution and corrected for background.
• the low initial slurry pH in Fe amended reactors is attributed to the acidity associated with FeS0 4 - 7H 2 0 used to alter the Fe content of the GAC.
• the pH decline with time may be attributed to different mechanisms: acid production associated with the Fenton mechanism; hydrogen ion release from the oxidation of 2CP; and production of acidic compounds, such as carboxylic acids. Since the solubility of ferrous Fe is inversely proportional to pH, some Fe may become soluble (mobile) under acidic conditions.
• Fe ⁇ measurec in unfiltered slurry samples (i.e., soluble or solid phase) containing GAC with 24.0 or 5500 Fe ⁇ mg/Kg was ⁇ 1.0 %, and with 9790 and 12050 Fe ⁇ mg/Kg, was 3 and 3.5 %, respectively.
• H 2 0 2 concentrations (0.94, 2.1, 7.1% w/w) in conjunction with Calgon Chemical Corp.
• GAC EXP4 low Fe
• the degradation rate of H 2 0 2 conformed to pseudo-first order degradation kinetics and half-lives increased with increasing H 2 0 2 concentration (Table 4) .
• the overall H 2 0 2 degradation rate decreased 60-78% with increasing H 2 0 2 application to the GAC.
• the decrease in H 2 0 2 degradation rate may be partially attributed to the decrease in pH; however, the precise mechanism is unknown. Table 4
• Controls were used to differentiate compounds extracted from GAC not attributed to 2CP or its oxidation products. Carbonate and unknown nitrogen derivatives (CND) were extracted from the GAC indicating background compounds (Table 6) . 2CP and CND were found in the GAC where 2CP was applied. Extraction of the Fe amended GAC yielded a tBDMS sulfate derivative (SD) in addition to 2CP and CND. Under oxidizing conditions, several organic acid byproducts were measured. The most abundant were confirmed to be oxalic and maleic acids, while minor acids were identified as malonic and fumaric.
• CND Carbonate and unknown nitrogen derivatives
• Sequential adsorption/oxidation was evaluated by adsorbing 2CP to the GAC (Calgon Chemical Corp. EXP4 , 5500 mg/Kg Fe) in three successive events using similar procedures (volume, concentration, equilibrium time) , and oxidizing the GAC suspension between sorption events.
• H 2 0 2 (100 mL, 0.59 M) was applied twice to the GAC (119 g/Kg 2CP) and 62% of the Cl " from the sorbed 2CP was recovered.
• 2CP was re-adsorbed (90 g/Kg 2CP) , H 2 0 2 applied (100 mL, 2.9 M) and 125% of the Cl " as 2CP re-amended to the GAC was recovered.
• Contaminants in water are adsorbed onto sorbents and oxidized in the presence of iron, which may be present on the sorbent or added with the oxidant, via Fenton-driven reactions.
• the selection of sorbent affects treatment effectiveness, since the concentration of oxidant reactants, such as iron and manganese, varies between manufacturers of granulated activated carbon.
• the iron content of the sorbent can be altered to enhance the Fenton-driven oxidation reactions.
• the rate and extent of oxidation depends on oxidant concentration, which affects hydroxyl radical concentration and scavenging. The efficiency of oxidation increases with increased contaminant concentration on the surface of the sorbent .

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