Phsicochemical Treatment of Hazardous Wastes - Chapter 2 potx

Organic pollutants are further classified as halogenated/nonh-alogenated volatile organic compounds VOCs and halogenated/nonhalo-genated semivolatile organic compounds SVOCs.. Environmen

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of over 40,000 organic chemicals used by various industries in the UnitedStates has resulted in a broad range of hazardous waste problems Theamount of industrial wastewater containing these recalcitrant pollutants isincreasing significantly Many of these chemicals are resistant to degradationand pose a potential health threat to the human population In addition,many hazardous waste materials are recalcitrant, normally nonbiodegrad-able, and even toxic to microorganisms; therefore, physicochemical treatmenttechniques are better alternatives than biological treatment

Since 1970, one of the most widespread industrial practices has been thehalogenation of organics, which produces organic solvents, pesticides, chlo-rofluorocarbons (CFCs), and polychlorinated biphenyls (PCBs) The haloge-nation of hydrocarbons yields compounds of lower flammability, higherdensity, higher viscosity, and improved solvent properties compared to non-halogenated solvents For example, 46.5% of chlorine gas used in the UnitedStates was for the production of chlorinated organic compounds Classified

as derivatives of aliphatic hydrocarbons, halogenated solvents have beenused extensively in a number of industrial processes, and over 400,000 tons

of halogenated solvents are used annually for metal cleaning Due to theirhigher density, high water solubility, and low degradability, chlorinatedsolvents are extremely mobile in groundwater

Chlorinated solvents are commonly used in the manufacturing of cides Carbon tetrachloride is a commercial product widely used in theUnited States for dry cleaning, metal degreasing, and fire extinguishers It

pesti-is also used for the production of CFCs and grain fumigation Methylenechloride is commonly used in paint stripping, for which it is mixed withalcohols, acids, and amines Methylene chloride is also used in the extraction

of caffeine from coffee and other beverages As a result, chlorinated organic

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24 Physicochemical Treatment of Hazardous Wastes

pollutants can still be found at most contaminated sites because of highresistance to biodegradation under natural environmental conditions.The most common nonhalogenated solvents are a group of petroleumdistillates, aliphatic and aromatic hydrocarbons, alcohols, ketones, esters,and ethers Nonhalogenated solvents have a number of industrial uses, such

as cold cleaning, which includes metal degreasing, parts cleaning, and paintstripping They have also been used as carriers for paints, varnishes, andprinting inks Prior to the RCRA, waste solvents were disposed of in landfills,sewers, and soil pits After the Resource Conservation and Recovery Act(RCRA), they had to be disposed of through solvent recycling, fuel blending,and incineration Hydrocarbons such as benzene, toluene, xylenes, and anumber of alkylbenzenes and low-molecular-weight ketones are importantclasses of nonhalogenated solvents Ketones consist of R and R¢ alkyl groupslinked to a keto or carbonyl group Other miscellaneous nonhalogenatedsolvents include glycols, such as ethylene glycol and propylene glycol;ethers, such as dimethyl ethers; and amines, such as isopropylamine

2.2 Classification of Hazardous Pollutants

In general, hazardous wastes are classified as organics, heavy metals, andradioactive Organic pollutants are further classified as halogenated/nonh-alogenated volatile organic compounds (VOCs) and halogenated/nonhalo-genated semivolatile organic compounds (SVOCs) These classifications helpfacilitate the selection of remediation technologies according to the treatabil-ity of each class in a specific contaminated media:

• Halogenated volatile organic compounds (HVOCs) — Halogenatedorganic compounds contain molecules of chlorine, fluorine, bro-mine, and/or iodine The nature of the halogen bond and the halo-gen itself can significantly affect the performance of a specifictreatment technology HVOCs are difficult to treat

• Halogenated semivolatile organic compounds (SVOCs) — HalogenatedSVOCs may also contain molecules of chlorine, bromine, iodine,and/or fluorine The degree of volatilization from halogenatedSVOCs is much less than for HVOCs The most common types ofhalogenated SVOCs include polychlorinated biphenyl (PCBs), pen-tachlorophenol (PCP), and hexachlorobenzene

• Nonhalogenated volatile organic compounds (nonhalogenated VOCs) —Nonhalogenated compounds do not have a halogen attached tothem Common types of nonhalogenated VOCs include acetone,styrene, and methanol

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Environmental Hazardous Wastes 25

• Nonhalogenated semivolatile organic compounds (nonhalogenated SVOCs)

— Nonhalogenated SVOCs do not contain halogens The degree ofvolatilization from nonhalogenated SVOCs is much less than fornonhalogenated VOCs The most common types of nonhalogenatedSVOCs include pyrene, fluorene, and dibenzofuran

Each class of the aforementioned organic pollutants may include hundreds

of substituted compounds For example, chlorinated benzenes may includeone hexachlorobenzene, a pentachlorobenzene, three dichlorobenzenes, andthree trichlorobenzenes Table 2.1 lists 100 priority pollutants classified bythe USEPA

Halogenated and nonhalogenated VOCs are found in many products such

as gasoline, paints, paint thinners, and solvents that are used for dry cleaningand metal degreasing Furthermore, halogenated and nonhalogenatedSVOCs also have the same properties and behaviors as VOCs These com-pounds are typically used in liquid form and readily evaporate They maycause adverse effects on the environment and human health through con-taminated soil and/or groundwater

Table 2.2 lists the hydrophobic and electronic properties of substituents.The most common substituents include a wide variety of electron-withdraw-ing substituents, such as –F, –Cl, –Br, –I, –NO2, –SO3H, –CN, and –COOH,and electron-donating substituents, such as –CH3, –C2H5, –NH2, –OH, and

halogenated/nonhalo-genated VOCs and SVOCs or non-VOCs

2.3 Sources of Hazardous Waste

According to Toxic Release Inventory (TRI) reports, a total of 2.58 billionpounds of releases occurred in 1997, with two industries reporting more thanhalf of that total The chemical manufacturing industry was responsible for797.5 million pounds of the total releases, and the primary metal industryreported a use of 694.7 million pounds These amounts represented 30.9%and 27.0% the TRI, as illustrated in Figure 2.1

The chemical manufacturing industry ranked as the number one source

of hazardous waste, with 742.6 million pounds of on-site releases, 342.2million pounds of air emissions, 106 million pounds of surface water dis-charges, and 215.8 million pounds of underground injection The chemicalsaccounting for the largest amounts of underground injection by chemicalmanufacturing facilities were nitrate (40.6 million pounds) and ammonia(29.0 million pounds) compounds The primary metal industry ranked sec-ond, with on-site releases of 405.9 million pounds The paper product sectorranked third, with on-site releases of 228.8 million pounds, consisting mostly

of air emissions of 193.8 million pounds

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USEPA Priority Pollutants

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Note: Log P is the partition coefficient; smand sp are Hammett sigma constants at the meta and para positions, respectively; F and R are polar and

resonance constants, respectively, proposed by Swain and Lupton (1968).

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The five different pathways for the release of chemical waste products inthe environment are:

• Fugitive air refers to the release of chemicals into the air from on-siteequipment leaks, evaporative losses from surface impoundmentsand spills, and building ventilation systems

• Stack air refers to the release of chemicals into the air through site stacks, vents, ducts, pipes, or any confined air stream

on-• Water refers to the release of chemicals into rivers, lakes, streams,oceans, and other bodies of surface water from all discharge points

at the facility This category includes the release from on-site water treatment systems, open trenches, and stormwater runoff

waste-• Underground refers to underground releases and is defined as theinjection of fluids into on-site subsurface wells for the purpose ofwaste disposal

• Land refers to the release of chemicals onto the land Land releasesinclude landfills, land treatment, and surface impoundment

Prior to establishment of the RCRA in 1976, most disposals occurredthrough methods that were considered easiest for maintenance and indus-trial personnel As a result, organic pollutants found their way into theenvironment through different pathways For example, liquid wastes con-taining lubricating oils and other petroleum residues were commonly dis-carded onto soil and unpaved roads by soil spreading Substances such asgasoline, heating oil, and jet fuels leak out of rusted and corroded under-ground storage tanks (USTs) Based on a 1991 survey covering 1.6 millionactive and closed tanks, gasoline and diesel tanks represented 62 and 20%,respectively, of the total USTs, respectively The distribution of USTs hasprobably changed somewhat, as approximately 600,000 tanks have beensealed between 1991 to 1995 The substances stored in RCRA-regulated tanksare shown in Figure 2.2 Because gasoline and diesel fuels account for themajority of the USTs, these substances pose the greatest threat to ground-water due to leakage of the tanks

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Before 1976, waste was usually poured into surface storage areas such aspits, ponds, and lagoons The waste then disappeared by seeping throughthe soil in the pits At sanitary landfills, which were designed to acceptnewspapers, cans, bottles, and other household waste, liquid hazardouswastes were often disposed of in drums and, in some cases, poured directlyinto the landfills, and the waste often migrated to surface and ground waters.Drum storage areas, where waste chemicals were stored in 55-gallon drums,were often located on loading docks, concrete pads, or at other temporarystorage areas until the wastes could be disposed of The drums that werestored in this manner eventually corroded and leaked, causing chemicalreleases that seeped into the underlying soil and groundwater Uncontrolledincineration, in which hazardous waste such as chlorophenols and PCBscombusted, has sometimes resulted in incomplete combustion, formation ofmore toxic products in the ash, and emission of hazardous air pollutants Typical sources of contamination from the contaminated sites of the Depart-ment of Defense (DOD) are shown in Figure 2.3 The most prevalent contam-inants in groundwater are VOCs and metals, which appear in 74 and 59% ofthe DOD groundwater sites, respectively SVOCs and metals were more con-sistent across different media than were VOCs SVOCs were found in 31 to 43%

of the sites, and metals were found in 59 to 80% of the sites Fuels were found

at fewer than 22% of all sites, a figure that may reflect the reporting of BTEX(benzene, toluene, ethylbenzene, and xylene) constituents of fuels and petro-leum under VOCs Figure 2.3 also shows the major contaminant groups bymedia and DOD component The most frequently occurring group, metals, isfound at 69% of all sites, followed by VOCs at 65% and SVOCs at 43% VOCsand metals are found at most sites of all branches of the service, except at Armysites, where VOCs account for only 41% of the sites

FIGURE 2.2

Distribution of underground storage tanks (From USEPA, National Survey of Underground age Tanks, U.S Environmental Agency, Office of Underground Storage Tanks, Washington, D.C., Spring, 1991.

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2.4 Contaminated Media of Hazardous Wastes

Because hazardous wastes have different physicochemical properties(vapor pressure and solubility), contaminated media may contain incon-sistently distributed pollutants The distribution depends on environmen-tal conditions and is a function of the properties of the chemical In general,contaminated media are classified as: (1) soil, (2) air, (3) sludge and sedi-ments, and (4) groundwater, and each type may contain pollutants indifferent phases:

• Gaseous phase — Contaminants are present as vapors in saturatedzones

• Solid phase — Contaminants in liquid form are adsorbed in soil

• Aqueous phase —Contaminants are dissolved into pore water ing to their solubility in both saturated and unsaturated zones

accord-• Immiscible phase —Contaminants are present as nonaqueous-phaseliquids (NAPLs), primarily in unsaturated zones

Quantitative distributions of organic pollutants in different phases largelydepend on their physical properties: the solubility constants in water (Kow)and soil (Koc) and Henry’s constant Figure 2.4 illustrates their distribution

Disposal Area

Disposal Pit/Dry Well

Site Type

VOCs Metals SVOCs Fuels Explosives Other

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2.4.1 Groundwater

Groundwater can be contaminated with water-soluble substances found inoverlying soil BTEX and fuel oxygenates, such as methyltertiary butyl ether(MTBE), can leak from USTs and contaminate groundwater (Suidan et al.,2002) These organic chemicals are known to be carcinogenic in nature andpose a great threat to human health, specifically to the 53% of the U.S.population who depend on groundwater as their drinking water resource.Concentrations of organic pollutants in groundwater largely depend upontheir solubility in water Water solubility is the maximal concentration of acompound that can be dissolved in water at a given temperature The dis-solution of an organic compound into water mainly depends upon both thestructure and size of an organic pollutant, and three factors are involved:Van der Waals forces, hydrogen bonding, and dipole–dipole interactions.Because water is a highly polar solvent, polar organic pollutants tend to havehigher water solubility In addition, substituents will also affect the solubility

of a given class of organic compounds:

• The presence of substituents such as hydroxyl (–OH) and amino

hydrocar-• As the number of carbons increases in any given organic class, thesolubility decreases due to high polarity

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• Double bonds have little effect on the water solubility of their responding compounds

cor-• The larger a polycyclic aromatic hydrocarbon (PAH) is, the lesssolubility the PAH will have due to the large and nonpolar planarmolecular structure

Table 2.3 lists the solubility constants of typical organic pollutants and theirranges

2.4.2 Soil

Contaminants in soil can partition between the soil and air, soil and water,and soil and solids; however, the physical structure and chemical composi-tion of surface and subsurface soil are highly variable Compared to aqueoussystems, soil is a complex heterogeneous media composed of solid, liquid,and gaseous phases The four major components of soil are the inorganic(mineral) fraction, organic matter, water, and air Soil consists of 50% porespace, which is occupied by air and water; 45% minerals; and 5% organicmatter Figure 2.5 illustrates a typical soil structure that is important toconsider for remediation

The available treatment data show large variations in treatment cies Some of the important considerations relevant to the selection of treat-ment technologies for VOCs include the following:

efficien-• Particle size distribution refers to the distribution of particles in thesoil matrix In general, the three types of soil are sand, clay, andloam Sand is soil composed of at least 70% sand; clay is soil con-sisting of at least 35% clay; and loam soil contains equal weights ofsand, clay, and silt Particular size or soil texture can affect thetreatability of contaminated soil in two ways The potential reactionsites are primarily limited to the surface of particles The surface-to-volume ratio has a major impact on the nature and rate of reactionsbetween the particle and the contaminant; therefore, larger sand-sized particles are less reactive than smaller clay-sized particles,particularly if reactions may occur between the sheets of clay min-erals

• Cation exchange capacity is defined as the quantity of cations sorbedper mass of soil and is expressed as milliequivalents of positivecharge per 100 g of soil The high cation-exchange capacity of clayminerals enables them to be more reactive with manycontaminants than the minerals’ characteristic of organic mattercomprised of sand-sized particles Thus, the relatively large surfacearea and the high cation exchange capacity make clays and organicmatter more difficult to treat than sands and silts

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• Porosity is defined as the percentage of soil occupied by pore space

and is an important parameter related to the transport, retardation,

and mass transfer of pollutants in groundwater

• Volumetric water content is the fraction of the soil pores that are filled

with water

• Bulk density of soil is the soil weight per unit volume, including water

and voids It is used in converting weight to volume in the mass

balance calculations

• Particle density is the specific gravity of a soil, which is important in

soil washing and in the determination of the setting velocity of

suspended soil in flocculation and sedimentation processes

Source: Watts, R.J., Hazardous Wastes: Sources, Pathways, Receptors, John Wiley & Sons, New

York, 1997 With permission.

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• Permeability and hydraulic conductivity are the controlling factors in

the effectiveness of in situ treatment technologies The ability of soil

flushing fluids to contact and remove contaminants can be reduced

by low soil permeability Low permeability can lessen the

volatiliza-tion of VOCs in soil vapor extracvolatiliza-tion or limit the effectiveness of in

situ vitrification by slowing vapor releases

• Organic content is usually divided into two categories: humic and

nonhumic Nonhumic chemicals are unaltered amino acids,

carbo-hydrates, fats, and other chemicals that are present in soil as a

consequence of living organisms Humic materials are formed by

microbial mediated reactions They contain polymerized phenols

with carboxic, carbonyl, ester, and methoxy groups It is one of the

most important properties influencing the transport of hazardous

compounds

FIGURE 2.5

A soil profile, illustrating the traditional O, A, B, and C horizons and some further subdivisions

of them (From Greenland, D.J and Hayes M.H.B., Eds., The Chemistry of Soil Constituents, John

Wiley & Sons, New York, 1978 With permission.)

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Among these factors, organic content determines the soil distribution

coef-ficient and is an important parameter that quantifies the distribution of

organic pollutants in soil The soil distribution coefficient (Kd) is the ratio

between the mass of contaminant sorbed by soil (mg/g) to the mass of

contaminant in the aqueous phase (mg/mL), or:

(2.1)

It can be experimentally determined by dividing the equilibrium

concen-tration of organic pollutant adsorbed on soil by its equilibrium concenconcen-tration

in water after 24 hours of mixing Because the sorption of organic pollutants

onto soil is almost exclusively onto the organic fraction due to the

hydro-phobicity of the organic pollutant, Koc, the soil adsorption coefficient of

organic pollutants due to organic content in soil is defined as follows:

(2.2)

Therefore, Kd can be calculated by normalizing Koc according to the following

equation:

(2.3)

where Kd is the soil distribution coefficient; Koc is the soil adsorption

coeffi-cient of organic pollutants due to organic content in soil; and f oc is the weight

fraction of organic content in soil (%)

In practice, the above equation is valid only if the organic carbon content

is the primary sorbent, for organic pollutants with molecular weights less

than 400, and when the organic pollutant does not have special-function

groups that could promote ion exchange or complexation In other words,

when hydrophobic partition is the sole adsorption mechanism, Koc is strongly

water partition coefficient is defined as follows:

(2.4)

Table 2.4 lists the Kow for different hazardous organic pollutants To avoid

expensive experimental tests when determining Kd, Koc can be estimated by

Kow using correlations equations reported in the literature, as shown in Table

2.5

K Mass of contaminant sorbed by soilMass of contaminant in aqueous phase

d=

Mass of contaminant in aqueous phase (mg / mL)

Kd=Koc¥f oc

Concentration in aqueous phase (mg / L)

ow=

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TABLE 2.4

Octanol/Water Partition Coefficients for Common Hazardous Compounds

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