Phsicochemical Treatment of Hazardous Wastes - Chapter 13 doc

13 Zero-Valent Iron 13.1 Introduction The use of zero-valent iron to reduce chlorinated hydrocarbons was first reported in patent literature by Sweeny and Fischer in 1972; however, Sweeny and Fischer never published their studies in peer reviewed journals, so their work was overlooked by the research community (Gillham and O’Hannesin, 1994). In the late 1980s, Reynolds observed that organics dis- appeared from the iron pipes used in his study on the corrosion of PVC and iron pipes by water contaminated with organics. Several years later, Gillham realized the potential of using the reduction ability of zero-valent iron for practical purposes and holds several patents for the application of zero- valent iron degradation of organic compounds (Wilson, 1995). Pump-and-treat and impermeable confinement are frequently used to degrade halogenated and nonhalogenated hydrocarbons in groundwater; however, these remediation techniques have limitations. For example, pump-and-treat only transfers contaminants to another media such as air stripping or activated carbon. In addition, the discharge of large volumes of water and the production of secondary waste may be costly. Also, the hydraulic characteristics of the aquifer may be adversely affected. Permits are required for discharge, and groundwater rights have to be purchased for the disposal of large volumes of treated groundwater, which may result in excessive operating costs (Cantrell and Kaplan, 1997). The use of a common alternative, biological degradation, has increased for remediation, but gain- ing an understanding of the biochemical pathways and associated by-prod- ucts involved, as well as developing effective strains of bacteria and managing the population of bacteria, can be difficult and the process has not yet been well defined (Gillham and O’Hannesin, 1994). Zero-valent iron is a promising in situ remediation technology for the degradation of many common pollutants, as it is comparatively inexpensive, does not restrict land use, and requires no energy for operating. Zero-valent iron has been successfully utilized to destroy trichloroethenes, chromate, chlorinated organics, and mixed wastes. It is capable of reducing and TX69272_C13.fm Page 491 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 492 Physicochemical Treatment of Hazardous Wastes dehalogenating a wide variety of halogenated hydrocarbons over wide con- centration ranges (Hardy and Gillham, 1996). In addition, iron is nontoxic and inexpensive (Gillham and O’Hannesin, 1994). It is easily oxidized by organic compounds, thereby reducing the contaminant without an addi- tional reactant. In addition to being highly reductive to many halogenated hydrocarbons, zero-valent iron can reduce highly mobile oxycations (UO 2 2+ ) and oxyanions (CrO 4 2– , MoO 4 2– , TcO 4 – ) into insoluble forms (Cantrell and Kaplan, 1997). This chapter presents the theory and application of zero-valent iron and includes the relevant in situ chemical/physical processes. To illustrate these in situ technologies, the basic mechanisms of adsorption reduction and oxi- dation processes are discussed for in situ treatment of (1) organic pollutants, (2) heavy metals, and (3) mixtures of organic and inorganic pollutants. The history of zero-valent iron, current applications, mechanisms and kinetics of the system, system improvements, and advantages and disadvantages for zero-valent iron are also discussed. 13.2 Fundamental Theory The reactions in zero-valent iron are heterogeneous due to the strong depen- dence of the reaction rate on the surface area of the iron (Burris et al., 1995). The surface reaction proceeds in four steps. First, the reactant undergoes mass transport from the groundwater to the iron surface (Matheson and Tratnyek, 1994). Second, the contaminant is absorbed onto the surface of the iron, where the chemical reaction occurs. Third, the reaction products desorb from the surface, which allows the site to become available for another reaction (Burris et al., 1996a). Finally, the products of the reaction return to the groundwater. Rate limitation could occur at any step. Where it may not be the sole limitation, mass transport plays an essential role in the kinetics of dechlorination (Matheson and Tratnyek, 1994). Essentially, reduction of hazardous wastes by zero-valent iron is due to the beneficial corrosion of iron. This process takes advantage of the chemical reaction that occurs when iron is oxidized. The contaminant is the oxidant (Fairweather, 1996), while zero-valent iron is a strong reductant capable of dehalogenating several halogenated hydrocarbons (Kaplan et al., 1996). Commercial-grade iron and industrial scrap iron are sufficient to reduce chlorinated solvents (Matheson and Tratnyek, 1994). Although iron is actu- ally consumed during the reaction, it remains effective for a long period of time. For example, 1 kg of iron can dechlorinate chloromethane at a concen- tration of 1 mg/L and sufficiently treat 0.5 million liters of water (Gillham and O’Hannesin, 1994). The reductive reaction is slow under anaerobic conditions, because iron may be oxidized by oxygen. Chlorinated contaminants possess an oxidizing TX69272_C13.fm Page 492 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC Zero-Valent Iron 493 potential similar to that for oxygen (Tratnyek, 1996). When strong oxidizing compounds such as chlorinated contaminants are not present, the iron is spontaneously corroded in water (Matheson and Tratnyek, 1994). 13.2.1 Thermodynamics The reaction during reduction of organic pollutants by Fe 0 has two parts. The anodic half-reaction produces Fe 2+ from Fe 0 and causes the iron metal to corrode (Agrawal and Tratnyek, 1996): Fe 0 + 2H + Fe 2+ + H 2 (13.1) The cathodic half-reaction varies with the reactivity of the available electron acceptors, such as H + and H 2 O in aqueous solutions. If conditions are aerobic, the cathodic half reaction makes O 2 , which is the electron acceptor, and H 2 will not be produced (Agrawal and Tratnyek, 1996): Fe 0 + 2H 2 O Fe 2+ + H 2 + 2OH – (anaerobic corrosion) (13.2) Fe 0 + 2H 2 O + O 2 2Fe 2+ + 4OH – (aerobic conditions) (13.3) The redox pair formed from oxidizing the zero-valent iron has a reduction potential of –0.440 V; therefore, zero-valent iron can reduce hydrogen ions, carbonate, sulfate, nitrate, and oxygen, in addition to alkyl halides (Matheson and Tratnyek, 1994). Both Equation (13.2) and Equation (13.3) cause the pH to increase. Zero-valent iron and organic substrate can react with a net result of iron oxidation and reduction of the substrate. In such a reaction, iron acts as a reducing agent: Fe 0 Æ Fe 2+ + 2e – (13.4) The Pourbaix diagram shown in Figure 13.1 illustrates the thermodynamic stability of iron species in aqueous solutions of a few organic substrates. The relative position of each substrate shows that the reaction between iron and the corresponding organic is thermodynamically favorable. Three elemen- tary reactions involved in the reductive dechlorination of organic com- pounds are shown in Figure 13.2. 13.2.2 Kinetics While the reaction thermodynamics is important, the reaction kinetics is equally important in designing zero-valent iron system; furthermore, the ¨Ææ ¨Ææ ¨Ææ TX69272_C13.fm Page 493 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 494 Physicochemical Treatment of Hazardous Wastes reaction is a heterogeneous reaction. The kinetics of heterogeneous reactions involving Fe 0 is determined by the following factors: • The reaction rate constants from Equation (13.1) to Equation (13.3) • Physical processes on the surface of the catalyst (reducing agent), including its properties • Mass transfer limitations and diffusion effects • Sorption/desorption processes involving the substrate and avail- ability of active reaction sites on the iron surface • Fluid flow characteristics, including velocity, flow regime The mass transfer limitations have been shown to be less significant in regard to the kinetics of chlorinated aliphatics based on relatively slow rates of degradation (Scherer et al., 2000). On the other hand, nitroaromatics and azo dyes have higher reduction rates under which the diffusion and mass transfer effects may become reaction rate-limiting factors (Agrawal and Trat- nyek, 1996). Scherer et al. (2000) identified three steps that may impose limitations on the reduction rates. The formation of precursor complex on active metal sites can be rate limited by the number of reaction sites. Burris et al. (1996) suggested that the hydrophobicity of the contaminant may significantly affect the sorption rate of substrates. Scherer et al. (2000) illus- FIGURE 13.1 Eh–pH diagram (or Pourbaix diagram) showing equilibria with water, iron, and common environmental contaminants including perchloroethene (PCE), nitrobenzene (ArNO 2 ), and chromate (Cr[VI]). Hematite ( a -Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) are assumed to be the controlling phases for iron speciation. The stability lines for the reduction of nitrobenzene (ArNO 2 ) to (ArNH 2 ), Cr(VI) to Cr(III), and PCE to TCE are superimposed to show the instability of Fe 0 in the presence of these contaminants. (From Scherer, M.M. et al., CRC Crit. Rev. Environ. Sci. Technol. , 30(3), 363–411, 2000. With permission.) Pe 17 11 5 -1 -7 -1 4 Eh (V versus SHE) 1.0 0.5 1. 0 0.0 -0.5 0 2 4 6 8 1 0 12 14 Fe 0 Fe 3 O 4 (s) H 2 H 2 O Fe 2+ Fe 3+ Cr(VI) Cr(III) PCE TCE O 2 H 2 O ArNO 2 ArNH 2 Fe 2 O 3 (s) TX69272_C13.fm Page 494 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC Zero-Valent Iron 495 trated that the transfer of electrons from the surface of the reducing agent to the substrate could affect the rates by the three major mechanisms shown in Figure 13.3: • Electron transfer from bare iron metal exposed by pitting of the oxide layer, while the pitting mechanism involves localized corrosion and possible catalytic dissolution pathways • Electron transfer from conduction bands in the oxide layer • Electron transfer from adsorbed or lattice Fe(II) surface area, express- ing reduction of a sorbed or lattice surface site FIGURE 13.2 Scheme showing proposed pathways for reductive dehalogenation in Fe 0 –H 2 O systems: (A) direct electron transfer from iron metal at the metal surface; (B) reduction by Fe 2+ , which results from corrosion of metal; (C) catalyzed hydrogenolysis by the H 2 that is formed by reduction of H 2 O during anaerobic corrosion. Stoichiometries are shown. (From Matheson, L.J. and Trat- nyek, P.G., Environ. Sci. Technol. , 28, 2045–2053, 1994. With permission.) Fe 3+ RCl + H + RH + CL - TX69272_C13.fm Page 495 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 496 Physicochemical Treatment of Hazardous Wastes The physical properties of the iron metal are, therefore, important factors associated with mass transfer limitations of the electron transfer processes and reaction-limiting steps. 13.2.3 Adsorption As a contaminant moves through soil and groundwater, chemical processes will affect both contaminant concentration and overall hydrogeochemistry (Schoonen, 1998) of the system. Different adsorption mechanisms cause pol- lutants to adsorb onto the soil, volatilize, precipitate, and be part of the oxidation–reduction processes. Adsorption is loosely described as a process in which chemicals partition from a solution phase into or onto the surfaces of solid-phase materials. Adsorption at particle surfaces tends to retard con- taminant movement in soil and groundwater. FIGURE 13.3 Conceptual models of electron transfer (ET) mechanisms at Fe 0 –oxide–water interface: (A) ET from bare iron metal exposed by pitting of the oxide layer; (B) ET from conduction bands in the oxide layer; (C) ET from adsorbed or lattice Fe(II) surface sites. (From Scherer, M.M. et al., CRC Crit. Rev. Environ. Sci. Technol. , 30(3), 363–411, 2000. With permission.) TX69272_C13.fm Page 496 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC Zero-Valent Iron 497 The three types of adsorption are (1) physical, (2) chemical, and (3) exchange adsorption. Especially important to the success of in situ treatment by Fe 0 are the soil characteristics, which affect soil sorptive behavior such as mineralogy, permeability, porosity texture, surface qualities, and pH. Phys- ical adsorption is due to van der Waal’s forces between molecules where the adsorbed molecule is not fixed on the solid surface but is free to move over the surface and may condense and form several superimposed layers. An important characteristic of physical adsorption is its reversibility. On the other hand, chemical adsorption is a result of much stronger forces with a layer forming, usually of one molecule thickness, where the molecules do not move. It is normally not reversible and must be removed by heat. The exchange adsorption and ion exchange process involves adsorption by elec- trical attraction between the adsorbate and the surface (Rulkens, 1998). Adsorption may occur in a combination of three possible mechanisms: hydrophobic expulsion, electrostatic attraction, and complexation. Most non- polar compounds, such as various organics, adsorb by this mechanism, and the degree of partitioning is correlated to the octanol/water partitioning coefficient, K ow . Polar substrates such as various metals sorb via electrostatic attraction and complexation. Table 13.1 shows the typical sorption mecha- nisms and typical examples. Sorption isotherm curves are graphical relationships showing the parti- tioning between solid and liquid form where mass adsorbed per unit mass of dry solids ( S ) is plotted against the concentration ( C ) of the constituent in solution. K is the sorption equilibrium constant; N is a constant describing the intensity of sorption. The linear sorption isotherm can be expressed as follows: S = K d C N (13.5) TABLE 13.1 Sorption Mechanisms in Soils Mechanism Other Terminology Examples Hydrophobic expulsion Partitioning Nonpolar organics (e.g., PCBs, PAHs) Electrostatic attraction Outer-sphere Nonspecific Physisorption Physical Ion exchange Some anions (e.g., NO 3– ) Alkali and alkaline earth metals (Ba 2+ , Ca 2+ ) Complexation reaction Inner sphere Specific Chemisorption Chemical Ligand exchange Transition metals (e.g., Cu 2+ , Pb 2+ , CrO 4 2– ) Source: Adapted from Scherer, M.M. et al., CRC Crit. Rev. Environ. Sci. Technol. , 30(3), 363–411, 2000. TX69272_C13.fm Page 497 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 498 Physicochemical Treatment of Hazardous Wastes The Freundlich isotherm can be described as: S = K d C 1/ N (13.6) The Langmuir isotherm is used to describe single-layer adsorption based on the concept that a solid surface possesses a finite number of sorption sites. When these active sites are filled, the site will no longer sorb solute from the solution: S = K a d • G max (13.7) where a = absorption constant related to the binding energy (L/mg), and G max = maximum amount that can be adsorbed by the solid (mg/kg). Because adsorption isotherms are equilibrium equations, the rate at which the material is adsorbed has to be studied in terms of chemical affinities, pH, solubility, hydrophobicity, and many other physical and chemical char- acteristics. Because organic nonpolar compounds have stronger attraction to organic matter than to mineral content, the amount of adsorption of an organic contaminant is more dependent on the organic content of the soil. The adsorption partition coefficient is generally used to determine this adsorp- tion amount, as it is empirically related to the organic fraction of the soil ( f oc ), and the normalized partition coefficient K oc can be expressed as follows: K p = K oc f oc (13.8) The amount of adsorption is also dependent on the moisture content of the soil, as water competes for adsorption sites, as illustrated in Figure 13.4. In a zero-valent iron system, pollutants will be retained at the Fe surface. Movement of metals into other environmental compartments (i.e., ground- water, surface water, or the atmosphere) should be minimal as long as the retention capacity of Fe is not exceeded. The extent of movement of a pollutant in the zero-valent iron system is intimately related to the solution pH, the surface chemistry of the Fe, the specific properties of pollutants, and the associated waste matrix. The retention mechanisms for pollutants include adsorption of the pollutant by the Fe surfaces and precipitation. The retention of cationic metals by Fe has been correlated with Fe proper- ties such as pH, redox potential, surface area, cation exchange capacity, organic matter content, clay content, iron and manganese oxide content, and carbonate content. Anion retention has also been correlated with pH, iron and manganese oxide content, and redox potential. In addition to Fe properties, consideration must be given to the type of pollutants, their concentration, the competing ions, and the complexing ligands. Transport of metals associated with various wastes may be enhanced due to the following reasons (Puls et al., 1995): TX69272_C13.fm Page 498 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC Zero-Valent Iron 499 • Transportation caused by metal association with mobile colloidal size particles • Formation of metal organic and inorganic complexes that do not sorb to soil solid surfaces • Competition with other constituents of waste, both organic and inor- ganic, for sorption sites • Deceased availability of surface sites caused by the presence of a complex waste matrix The available surface area of the iron has the greatest effect on the reaction rate (Johnson et al., 1996). The iron is commonly treated with hydrochloric acid prior to the experiment to accelerate dechlorination and improve repro- ducibility. By cleaning the metal surface, the passive oxide layer is broken off, the surface area is increased by etching and pitting, and the density of highly reactive sites is increased (Agrawal and Tratnyek, 1996). Because of FIGURE 13.4 Adsorption mechanism for nonpolar organic species. (Adapted from Semer, R. and Reddy, K.R., J. Haz. Mat. , 57, 209–230, 1998.) TX69272_C13.fm Page 499 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 500 Physicochemical Treatment of Hazardous Wastes the wide range of waste characteristics and various ways in which metals can be adsorbed to the Fe surface, the extent of pollutant retention by a soil is specific to the type of site, soil, and waste involved. 13.2.4 Halogenated Hydrocarbons Dechlorination is a surface reaction with the zero-valent iron serving as the electron donor. When there is a proton donor, such as water, chlorinated compounds will be dehalogenated. The reaction kinetics depends upon the mass transfer to the surface of the iron, the available surface area, and the condition of the surface. The reaction is pseudo first order, and direct contact with the surface of the iron is required for degradation to take place (Gillham and O’Hannesin, 1994). The basic equation for dechlorination by iron metal is as follows: Fe 0 + RX + H + Fe 2+ + RH + Cl – (13.9) The reduction potentials for various alkyl halides range from +0.5 to +1.5 V; therefore, when Fe 0 serves as an electron donor, the reaction is thermo- dynamically favorable. Because three reductants are present in the treatment system (Fe 0 , H 2 , and Fe 2+ ), three possible pathways exist. Equation (13.9) represents the oxidation of Fe 0 by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994): 2Fe 2+ + RX + H + Æ 2Fe 3+ + RH + X – (13.10) H 2 + RX Æ RH + H + + X – (13.11) If all three pathways are not possible, then reactions will be limited. Addi- tional limitations may occur if the reaction is aerobic because Fe 3+ could be produced by further oxidation of Fe 2+ and cause precipitation of iron oxides (Helland et al., 1995). The end products such as ferrous chloride and ferric oxide are not capable of reducing chlorinated compounds (Gillham and O’Hannesin, 1994). Burris et al. (1995) state that Fe 2+ and H 2 do not have an effect on degradation. Hardy and Gillham (1996) have suggested the possi- bility that degradation may be due to a catalytic reaction utilizing hydrogen produced from the reduction of water. In developing and improving the performance of the zero-valent iron technique in the field, detail mechanisms are important and critical for a specific site. The dominant process is the ¨Ææ TX69272_C13.fm Page 500 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC [...]... -1 .3 3 -1 .4 5 4 2 -1 .5 -1 .6 1 6 -1 .7 7 -1 .8 -1 .9 -2 -1 .9 8 -1 .8 -1 .7 -1 .6 -1 .5 -1 .4 -1 .3 -1 .2 logk (Predicted) FIGURE 13. 5 Plot of observed log k values vs those predicted by the PLS model (From Liu, Y et al., Chemosphere, 50, 1275–1279, 2003 With permission.) © 2004 by CRC Press LLC TX69272_C13.fm Page 519 Tuesday, November 11, 2003 12:32 PM Zero-Valent Iron 519 13. 5 Engineering Applications One of. .. Dichloromethane Hexachloroethane 1,1,1-Trichloroethane 1,1,2-Trichloroethane 1,1-Dichloroethane Tetrachloroethene Trichloroethene cis-1,2-Dichloroethene trans-1,2-Dichloroethene 1,1-Dichloroethene Vinyl chloride 1,2,3-Trichloropropane 1,2-Dichloropropane Benzene Toluene Ethylbenzene Hexachlorobutadiene 1,2-Dibromoethane Freon 113 N-nitrosodimethylamine Source: USEPA, EPA/600/R-98/125, U.S Environmental Protection... et al., 1996) Table 13. 5 shows the effect of pH on the pseudo first-order rate constants and half-lives of DBCP with or without mixing Table 13. 6 indicates that sulfate and nitrite ions do not significantly affect the pseudo first-order rate constants and half-lives of DBCP 13. 3.1.3 Polychlorobiphenyls Degradation of polychlorobiphenyls (PCBs) by zero-valent iron requires temperatures of 400°C (Grittini... are provided in Table 13. 3 © 2004 by CRC Press LLC TX69272_C13.fm Page 504 Tuesday, November 11, 2003 12:32 PM 504 Physicochemical Treatment of Hazardous Wastes TABLE 13. 3 t1/2 Values for Selected Halogenated Aliphatics Compound t1/2 (min) Perchloroethylene Tetrachloroethene Trichloroethene 1,1-Dichloroethene trans-1,2-Dichloroethene cis-1,2-Dichloroethene Vinyl chloride 17.9 — 13. 6 40.0 55.0 432.0... mass balance of nearly 80% The hypothesized reaction (Equation 13. 35) consisted of the following steps: NO2– + 4H+ + 3e– Æ 0.5 N2 (gas) + 2H2O © 2004 by CRC Press LLC (13. 33) TX69272_C13.fm Page 510 Tuesday, November 11, 2003 12:32 PM 510 Physicochemical Treatment of Hazardous Wastes 0.5N2 + 4H+ + 3e– Æ NH4+ (13. 34) NO2– + 8H+ + 6e– Æ NH4+ + 2H2O (13. 35) Thus, it was observed that the first-order rate... 2Cl– (13. 44) 2Fe(OH) 2 + HOCl + H2O ´ 2Fe(OH)3 + H+ + Cl– (13. 45) Addition of Equation (13. 44) and Equation (13. 45) gives the following equation: 2Fe + 3HOCl + 3H2O ´ 2Fe(OH)3 + 3Cl– + 3H+ (13. 46) At higher pH, Reaction (13. 44) and Reaction (13. 45) occur as follows: 2Fe + 2OCl– + 2H2O ´ 2Fe(OH)2 + 2Cl– (13. 47) 2Fe(OH)2 + OCl– + H2O ´ 2Fe(OH)3 + Cl– (13. 48) Thus, in this case, addition of Equation (13. 47)... 1,2,4Trichlorobenzene 2-Chlorophenal 0.0215 0.962 8 7 3-Chlorophenal 0.0165 0.986 11 8 4-Chlorophenal 0.0112 0.953 9 Source: Liu, Y et al., Chemosphere, 50, 1275–1279, 2003 With permission © 2004 by CRC Press LLC TX69272_C13.fm Page 518 Tuesday, November 11, 2003 12:32 PM 518 Physicochemical Treatment of Hazardous Wastes TABLE 13. 13 Observed and Predicted First-Order Rate Constants of Reductive Dehalogenation... 33.0 0.24 No decline 0 .13 19.2 4.4 5.3 Source: Gillham, R.W and O’Hannesin, S.F., Ground Water, 32(6), 958–967, 1994 With permission © 2004 by CRC Press LLC TX69272_C13.fm Page 506 Tuesday, November 11, 2003 12:32 PM 506 Physicochemical Treatment of Hazardous Wastes TABLE 13. 5 Effect of Ions on Pseudo First-Order Rate Constants and Half-Lives of DBCP Transformation Using 0.1-M HEPES Buffer Solution... 5.6 ¥ 106 Hg2+ + 2Cl– ´ HgCl2(aq); K2 = 1.7 ¥ 1 013 © 2004 by CRC Press LLC (13. 38) (13. 39) TX69272_C13.fm Page 514 Tuesday, November 11, 2003 12:32 PM 514 Physicochemical Treatment of Hazardous Wastes Hg2+ + 2Cl– ´ HgCl3–; K3 = 1.2 ¥ 1014 (13. 40) Hg2+ + 4Cl– ´ HgCl42–; K4 = 1.2 ¥ 1015 (13. 41) When simplified, the predominant reaction during the deposition of mercury from a solution containing chloride... 0.1-M MES 0.1-M HEPES 0.1-M Tricine 0.1-M CHES 7 6 7 8 9 k (hr–1) (Mild Shaking) t1/2 (hr) (No Shaking) t1/2 (hr) (Mild Shaking) 0.099 0.341 0.393 0.259 — 0 .132 0.482 0. 513 — — 6.99 2.03 1.76 2.67 — 5.23 1.44 1.35 — — Source: Siantar, D.P et al., Water Res., 30(10), 2315–2322, 1996 With permission TABLE 13. 6 Effect of Sulfate and Nitrite Ions on Pseudo First-Order Rate Constants and Half-Lives of DBCP . 1,1-Dichloroethane Ethenes Tetrachloroethene Trichloroethene cis-1,2-Dichloroethene trans-1,2-Dichloroethene 1,1-Dichloroethene Vinyl chloride Propanes 1,2,3-Trichloropropane 1,2-Dichloropropane. the pseudo first-order rate constants and half-lives of DBCP. 13. 3.1.3 Polychlorobiphenyls Degradation of polychlorobiphenyls (PCBs) by zero-valent iron requires tem- peratures of 400°C (Grittini. H + RH + CL - TX69272_C13.fm Page 495 Tuesday, November 11, 2003 12:32 PM © 2004 by CRC Press LLC 496 Physicochemical Treatment of Hazardous Wastes The physical properties of the iron