pollution control

pollution control Pollutants in groundwaters, surface waters (rivers, lakes, and oceans), and the atmosphere are introduced into the environment as a result of many industrial activities. After release these pollutants can be affected by physical, chemical, and biological processes. Mathematical models that are used to attempt to predict the mobility of pollutants by accounting for all three processes are generally referred to as ‘fate and transport models’. The majority of processes that affect the fate and transport of elements can be described by rate equations which show that the rate of pollutant reduction is a first-order process. This means that the rate (R) is directly related to the concentration of the pollutant ([A]) times a rate constant (k): R = k[A]. Few processes are second-order, in which the rate of pollutant reduction also depends on the concentration of a second chemical ([B]) that is present in nature: R = k[A][B].

In this context, physical processes are transport processes; they include sorption, volatilization, diffusion, and advection (horizontal flow). Chemical processes are solubility (chemical equilibria), oxidation and reduction (redox), hydrolysis, hydration, and photolysis. Biological processes are primarily biotransformation, biodegradation, and bioaccumulation. It is important to understand the transport processes for toxins because of their effect on living things. We therefore also need to understand the toxicology of each pollutant.

Transport processes

Transport processes can result in the direct dilution of the contaminant as a result of retardation, diffusion, or dispersion.

Sorption refers to the interaction of dissolved constituents (solute) with mineral surfaces in groundwater. The solute is generally a charged ion which can be adsorbed to the surface (see adsorption), absorbed into the surface, or undergo ion exchange with ions in the mineral. Adsorption results from mineral surfaces having a surface charge in aqueous solutions, the charge being pH-dependent. In general, mineral sur-faces are positively charged in acidic waters (low pH) and negatively charged in alkaline waters (high pH). As a result, positively charged metal ions (e.g. Pb²⁺) adsorb to mineral surfaces at intermediate to high pH. When these ions diffuse away from the mineral surface into the structure of the mineral because of concentration gradients, the process is referred to as absorption, not to be confused with adsorption. Dissolved ions can also exchange with ions already present in the structure of a mineral. This process is generally referred to as ion exchange, and is particularly important for minerals such as zeolites and clay minerals which have loosely bound cations in channels or interlayers within their structure. These loosely bound ions generate charge balance with the negatively charged structural units composed of silica, aluminium, and oxygen. For minerals that have ion-exchange properties, the maximum amount of inter-layer or channel ion ex-change with dissolved solids is generally referred to as the ion-exchange capacity.

Sorption of inorganic ions is related to the concentration of the dissolved constituent, the surface area of the minerals, and the pH of the solution. Sorption is generally described by adsorption isotherms, which show the amount of a chemical adsorped as a function of pH or some other chemical variable.

Sorption of organic molecules is primarily on to organic compounds in sediments. Their sorption is hence related to the amount of organic material present and whether the organic compound is hydrophobic (has low water solubility) or hydrophylic (has high water solubility). Hydrophylic organic compounds have low adsorption coefficients and a low bioaccumulation factor (see biogeochemistry). It follows that hydrophobic organic compounds have high adsorption coefficients and high bioaccumulation factors.

Volatilization of organic chemicals from water to the atmosphere is particularly important for chemicals that have high vapour pressures and low water solubilities. Organic vapour production is experienced, for example, at filling stations, where obnoxious petrol odours are usually prevalent. In contrast, many of us are familiar with the pleasant smell of a good whisky or brandy, which also results from the volatilization of certain organic molecules.

Volatilization is a first-order rate process and is an important control on the composition of the upper layers of surface waters and groundwaters. Deep down within fluids or aquifers volatilization is not an important transport process.

Diffusion occurs when solutes move under the influence of thermal-kinetic energy down a concentration gradient. Diffusion continues until the concentration gradient is absent.

The diffusion constant varies as a function of temperature, falling by about 50 per cent upon cooling from 25 °C to 5 °C. Diffusion is a relatively slow process and is not of major importance unless water flow is extremely slow. Over geological timescales, however, diffusion can be an important process.

When dissolved solids are carried along with the flowing groundwater, the process is called advective transport or convection. The amount of solute that is transported is a function of its concentration in the groundwater and the quantity of the groundwater flowing. Groundwater can move at rates that are both greater and less than the average linear velocity. This is caused by differences in pore size, the tortuosity of pore channels, and the differential velocity of fluid across pores caused by drag resulting from the roughness of pore surfaces. Differences in velocity cause mixing to occur along the flow-path. This mixing is called mechanical dispersion, and it results in dilution of the solute at the advancing edge of the flow. The magnitude of dispersion is largest in the longitutinal direction of the flow; the dispersion in the transverse direction of the flow is roughly 10 per cent of the longitudinal dispersion. Dispersion is usually described with the advection–dispersion equation. This equation describes the change in concentration with respect to time in one, two, or three dimensions. In general, dispersion calculations do not take account of any retardation processes and are thus worst-case calculations. Reactive transport modelling is a relatively new area of research, which, because of its complexity, is actively pursued by few research groups at present.

Chemical processes

Chemical processes involve a variety of chemical reactions which affect the concentration of dissolved constitutents along the flow-path. Minerals dissolve in water in a similar way to sugar in a cup of coffee or tea. However, mineral solubility is, in general, very low under the near-neutral pH conditions typical of natural aquatic systems. The amount of a mineral that can dissolve (its solubility) depends on the pH of the water, the temperature, the pressure, and, for elements that have more than one oxidation state (e.g. iron, Fe²⁺ and Fe³⁺), on the amount of oxygen in the water. Mineral solubilities are known from experiments from which thermodynamic data have been derived. In order to use these data it is necessary to assume that the system has reached equilibrium. This takes a very long time (years or decades) at ambient temperatures, but a very short time at high temperatures (a matter of seconds or hours at 1000 °C). Polluted waters at ambient temperatures may not have reached thermodynamic equilibrium. However, by assuming equilibrium conditions it is possible to predict how the system will react in the future; such calculations are therefore of great value for the geochemist when evaluating the effect of chemical reactions on the fate of pollutants in groundwater.

Evaluating chemical equilibria in groundwater systems is done either in terms of solubility or by inferring an alteration from a primary mineral (unstable at ambient conditions) to a secondary mineral (stable at ambient conditions). These mineralogical reactions, whether solubility or alteration reactions, are generally described as proton (H⁺) transfer reactions. Oxidation–reduction reactions are generally referred to as electron (e⁻) transfer. The solubility of calcite (calcium carbonate, CaCO₃) can be simply described by the following reaction:CaCO₃(s) + 2H⁺ = Ca²⁺ + H₂CO₃(aq).

This reaction shows that in dissolving CaCO₃ there is a transfer of 2H⁺ for each Ca²⁺ and that the solubility is dependent on pH (H⁺ concentration).

Similarly, the weathering of feldspar (KAlSi₃O₈) to form kaolinite (Al₂Si₂O₅(OH)₄), one of the most common weathering reactions on the Earth's surface, can be described by 2KAlSi₃O₈ + 2H⁺+ 9H₂O = Al₂Si₂O₅(OH)₄ + 2K⁺ + 4H₄SiO₄(aq).


This reaction also requires the transfer of protons, that is, one H⁺ for each K⁺. Such reactions can be illustrated on activity–activity diagrams on which are plotted the effective concentration (activity) of dissolved ions.

Oxidation and reduction reactions can be described either in terms of oxygen transfer or of electron transfer. For example, the oxidation of the mineral magnetite, (Fe(II)Fe(III)₂O₄), to form the mineral haematite, (Fe(III)₂O₃), can be represented by the equation2Fe₃O₄ + 0.5 O₂ = 3Fe₂O₃

or by2Fe₃O₄ + H₂O = 3Fe₂O₃ + 2H⁺ + 2e⁻.

These reactions are thus dependent on the amount of oxygen in the system, which can also be represented by the dependence of pH (activity of H⁺) and the pe (activity of e⁻). In this respect the solubility of minerals can be shown on pe–pH diagrams which describe the solubility of the minerals as a function of pe and pH.

It will be apparent from the preceding discussion that minerals dissolve in water and the mineralogy of the groundwater reservoir will be reflected by the water composition. The composition of the groundwater in turn plays an import-ant role in determining the concentration of pollutants in groundwater.

The effect of chemical equilibrium reactions can be calculated where thermodynamic data are known. These data are generally commonly available for inorganic species. However, the effects of organic ligands (molecules or ions bonded together) on the amounts of dissolved metals (or metal– organic complexes) in water are still poorly characterized and are therefore the subject of active research.

Oxidation of organic chemicals occurs as a result of reaction with oxidants formed during photochemical processes in natural waters. These oxidants include singlet oxygen and alkylperoxyl radicals (monovalent aliphatic hydrocarbon radicals containing O₂₋ groups). The oxidation process in thus a second-order rate process.

Reduction in anaerobic environments occurs by both inorganic and biological processes. An example of such a reduction process is the replacement of a chlorine atom for a hydrogen atom on industrially produced organochlorine chemicals. Rate expressions for this process are still poorly defined for organic chemicals. Oxidation and reduction of inorganic chemicals depend solely on the amount of oxygen in the system.

Hydration is important for some organic chemicals. In this process carbonyl compounds form hydrates which have different properties from those of the parent (unhydrated chemical + water = hydrated chemical). Rate expressions for this process are not well known at present.

Hydrolysis of organic compounds results in the introduction of a hydroxyl group (−OH) into the chemical structure, commonly with the loss of a functional group (−X), RX + H₂O = ROH + H⁺ + X⁻.

This process is a first-order rate reaction. Some organic chemicals show a pH-dependent elimination reaction. These are second-order rate processes in which the hydrolysis reaction is dependent on the concentrations of H⁺ or OH⁻.

Photolysis is important for organic chemicals in surface waters and in the atmosphere. It occurs when chemicals are bombarded by light of wavelengths of more than 290 nm (ozone filters out shorter wavelengths). Photochemical transformations can occur by direct or indirect photolysis.

Direct photolysis takes place if the chemical absorbs light and then undergoes a transformation reaction from an ‘excited state’. These transformations include rearrangement, dissociation, and oxidation. The rate of a photolysis reaction is a first-order process.

Indirect photolysis occurs when substances naturally present in aquatic environments absorb sunlight to form excited chemical species or radicals, which then react with the pollutant chemical. Indirect photolysis is thus a second-order rate process.

Biological processes

Biological processes involve enzyme-catalysed transformation of chemicals and the build-up of chemicals in the food chain.

Organisms require energy, carbon, and other fundamental inputs from the environment for their growth and maintenance. In life they manufacture enzymes which may transform or biodegrade contaminants that have been introduced into the environment. By far the most important biodegradation processes in aquatic and soil environments are carried out by microbes.

The biodegradation rate is a function of microbial biomass and the concentration of the chemical under given environmental conditions. Micro-organisms use the chemical substrate (the substance acted upon by an enzyme) as an energy source. In doing so the biomass is increased. Biodegradation rates, are therefore a function of cell growth rate. If an organic compound is used by a micro-organism as a sole source of carbon, the specific growth rate of the organism is a function of the concentration of the compound.

In the environment, where cell concentration, X, is relatively large, and pollutant concentration is low, microbial populations will not change significantly when the chemical is consumed. The degradation rate under these conditions is thus a ‘pseudo’-first-order process. The first-order rate equation can be used under conditions where micro-organisms become acclimatized to the chemical and can actively use it. The acclimatization period is required to induce the organisms to produce necessary enzymes, to develop biodegradation organisms by mutation, to increase the number of microbes to substantial levels, and to use the diauxic (other readily metabolized) substrates. This acclimatization period cannot generally be ignored when a chemical is newly introduced into an uncontaminated environment.

Some pollutants may be biotransformed only when another organic compound is present to serve as a carbon energy source. This phenomenon is known as co-metabolism and is a second-order rate process. Mathematical expressions for such transformations are still poorly defined.

Bioaccumulation of chemicals in living species is especially important for hydrophobic chemicals which can be partitioned into fat and lipid tissues. Bioaccumulation also occurs where inorganic chemicals are partitioned into bones (which are composed of the mineral hydroxyapatite) and bone marrow. Bioaccumulation is usually evaluated in terms of the bioconcentration factor (BCF), which is equal to the concentration of the chemical in tissue (C_t on a dry weight basis) normalized to the concentration of the chemical in water (C_w); BCF = C_t/C_w.

Bioconcentration data are complicated by the fact that the concentration of a chemical is usually higher in the fatty tissues of the species than in leaner tissues. The rate of uptake and the time for attainment of equilibration in various organs (and species) also depends on the route of uptake (e.g. whether it is part of the diet or is absorbed through the skin). There is a correlation between the BCF and how hydrophobic the organic chemical is.

Environmental fate and transport

In evaluating the most important fate process for a given contaminant it is important to pinpoint the processes that have the shortest half-lives (t_½), that is, the time required for the removal of one-half of the initial concentration of the chemical. As for radioactive decay, the half-life of a chemical can be calculated if the rate constants (k₁, k₂, k₃, etc.) are known: t_½ = 0.693/(k₁+k₂+k₃).

When considering the fate and transport of contaminants in the environment, each pollutant has to be evaluated individually. The heavy metal lead and the lubrication fluids known as PCBs (polychrorinated biphenyls) provide good examples. Lead is an important pollutant in mining areas where sulphide minerals (such as galena, PbS) are mined, and in many industrial contexts such as metallurgy, paints, and permanent magnets. PCBs were the miracle fluids, used widely in the western world in transformers, turbines, and vacuum pumps for their high thermal stability and dielectric properties. They were also used as plasticizers in paints, plastics, resins, inks, etc. PCBs were used until it became known that they are both highly toxic to most forms of life and are very stable in the environment.

The dominant mechanism controlling the fate of lead is adsorption. Precipitation of lead sulphate (PbSO₄), lead carbonate (PbCO₃), and lead sulphide (PbS) minerals and bioaccumulation are also important. At low pH values, adsorption and precipitation are not nearly as effective in removing lead from solutions as at higher pH. Lead is therefore much more mobile in acidic waters (such as mine drainage) than at higher pH values. In alkaline and near-neutral waters, removal of lead by adsorption and precipitation occurs relatively quickly. Lead causes genetic changes, foetal malfunctions, and bowel cancer in humans. It is well known that lead causes irreversible retardation in the development of children, especially those exposed to leaded car exhausts. Lead accumulates in bone, because it can partition for calcium in the bone-forming mineral apatite.

Adsorption and volatilization dominate the environmental dynamics of PCBs in natural waters. PCBs do, however, adsorb only to organic compounds within groundwater reservoirs. PCBs can be desorbed from sediments, causing a continuous source of contamination. Dissolved PCBs move with the bulk water flow. PCBs cause growth inihibition and reductions in population in a range of species from algae to birds. In humans PCBs cause cancer of the kidneys and the liver and are known to be mutagens (i.e. capable of changing inheritable characteristics), teratogens (i.e. capable of causing malformation), and act as hormone mimickers.

K. Vala Ragnarsdottir

Bibliography

Moore, J. W. and and Ramamoorthy, S. (1984) Heavy metals in natural waters. Springer-Verlag, New York.
Moore, J. W. and and Ramamoorthy, S. (1984) Organic chemicals in natural waters. Springer-Verlag, New York.