Chemistry based on bulk materials has primarily utilized the properties of adsorption, photo-catalysis, membrane separation, or biodegradation. Many excellent articles and books have appeared in the past describing the chemistry of each method in detail. A brief overview of such methodologies is presented here. It is important to realize that these approaches are relatively homogeneous, targeting many contaminants present in water. Thus, process (e.g., target pesticide removal) effi ciency decreases signifi cantly in the presence of competing species.
15.3.1 Surface Adsorption
The most commercially utilized material for surface adsorption is activated carbon, representing all the carbonaceous materials derived from things such as charcoal, husk, wood, coal, and so on. In all the various product forms, it presents a relatively high surface area (> 500 m 2 /g) due to the presence of macro-, meso-, and micro-porous structure. Availability of this high surface area and the inherent tendency of carbonaceous materials to absorb many organic residues lead to the excellent adsorption characteristics of activated
Figure 15.1 Scanning electron microscope (SEM) images of (a) the rough areas of the surface from a granular activated carbon, (b) linear accumulation pattern of the contaminants (phenolphthalein) on the edges of the rough surface. Reprinted with permission from B. Tansel et al. [7]. Copyright (2002) with permission from Elsevier.
(a) (b)
Savage_Ch15.indd 195
Savage_Ch15.indd 195 11/11/2008 3:43:40 PM11/11/2008 3:43:40 PM
carbon ( Fig. 15.1 ). Surface activation through thermal heating or chemical activation through the use of acids/bases can further increase its absorption capacities. The adsorption of pesticides on activated carbon is driven through two major factors: the nature of the porous structure and the presence of competing organic species in water. The nature of the porous structure can be modifi ed to a large extent for suiting the molecule of interest, for example, the size of pesticide molecules ranges anywhere between 0.5 and 2 nm whereas the size of natural organic matter varies between 1 and 100 nm. Although activated carbon is one of the most versatile materials for water purifi cation, its use suff ers from the inherent disadvantage of the necessity to reprocess the adsorbent and to dispose of residuals in an environmentally responsible manner.
15.3.2 Biological Degradation
The inspiration for biological degradation originates from the Nature. Nature has been doing it for many decades during which time pesticide residues were not observed in drinking water, and biological degradation is by far the most effi cient route of environmental clean-up. The mechanism followed by the Nature is the biodegradation of pesticides using microorganisms such as bacteria and fungi [ 8 ]. It is suggested that the metabolism of pesticides usually involves a three-phase process as described in Table 15.2 . The metabolism of pesticides is dependent on a multitude of factors: environmental conditions (temperature, moisture, pH, etc.), nature of pesticide (hydrophilicity, functional groups, etc.), and metabolism of microorganisms.
Table 15.2 Summary of the Three Phases of Pesticide Metabolism [ 9 ]
Characteristics Initial properties
Phase I Phase II Phase III
Reactions Parent compound
Oxidation, hydrolysis, reduction
Intra- or extracellular conjugation to xylose, methyl, or acetyl groups
Secondary conjugation or incorporation into
biopolymers Solubility Lipophilic Amphophilic Hydrophilic Hydrophilic or
insoluble Phytotoxicity Toxic Modifi ed or less
toxic
Greatly reduced or nontoxic
Nontoxic Mobility Selective Modifi ed or
reduced
Limited or immobile
Immobile Bioavailability Readily
absorbed in GI tract of animals
Readily absorbed in GI tract of animals
Less absorption Limited absorption or unavailable
Savage_Ch15.indd 196
Savage_Ch15.indd 196 11/11/2008 3:43:41 PM11/11/2008 3:43:41 PM
15.3.3 Membrane Filtration
With the advent of smaller pore size membranes in the early 1970s and their comprehensive use for fi ltration of contaminated water from diff erent sources (i.e., wastewater, domestic water, seawater, etc.), attempts were made to utilize the low pore size and charge properties of membranes for fi ltering pesticides from water. In principle, membrane-based fi ltration of pesticides depends on three parameters:
Size distribution of the membrane pores and contaminant species: The retention capacity of a membrane for a contaminant molecule is measured through the fraction of pores on the membrane that are smaller than the molecule.
Membrane surface charge along with the dipole moment of the contaminant species: Molecules having a dipole can easily orient to have favorable charge interaction between the membrane and the molecule leading to its permeation.
Correspondingly, the retention capacity of a membrane is seriously hampered in case of the presence of a competing polar contaminant in water.
Surface adsorption capacity of the membrane: The adsorption characteristics of organic matter on membrane surfaces are governed by a variety of factors—
organic matter concentration, its nature and mass distribution, calcium ion concentration, and physical and chemical properties of the membranes. Usually the membrane surfaces are negatively charged, which implies anions are repelled whereas cations are attracted toward it, leading to the formation of an electrical double layer at a membrane surface.
Overall, in spite of its ability to remove a broad spectrum of organic and inorganic contaminants, the membrane fi ltration technique is usually aff ected by permeate fl ux deterioration caused by blockage in the membrane transport pores, originating from adsorption of contaminants on membrane surfaces ( Fig. 15.2 ).
~50 μm
~50 μm
(a) (b)
Figure 15.2 Scanning electron microscope micrographs of NF-270 membrane surfaces (a) clean and (b) treated with 10 mg/L humic acids. Reprinted with permission from K.V. Plakas et al. [10]. Copyright (2006) with permission from Elsevier.
Savage_Ch15.indd 197
Savage_Ch15.indd 197 11/11/2008 3:43:41 PM11/11/2008 3:43:41 PM