SAB 4973: HAZARDOUS WASTE TREATMENT TECHNOLOGIES
Technologies Chemical methods Coagulation, flocculation, combined with flotation and filtration, precipitation, ion exchange, electroflotation, electrokinetic coagulation. Physical methods Membrane-filtration processes (nanofiltration, reverse osmosis, electrodialysis,...) and adsorption techniques. Biological treatments Biodegradation methods such as fungal decolorization, microbial degradation, adsorption by (living or dead) microbial biomass and bioremediation systems
Adsorbents Adsorption techniques employing solid sorbents are widely used to remove certain classes of chemical pollutants from waters, especially those that are practically unaffected by conventional biological wastewater treatments. However, amongst all the sorbent materials proposed, activated carbon is the most popular for the removal of pollutants from wastewater They must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption. The adsorbents must also have a distinct pore structure which enables fast transport of the gaseous vapors.
Most industrial adsorbents fall into one of three classes: Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as Silica gel and Zeolites. Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite. Polymer-based compounds - Are polar or non-polar functional groups in a porous polymer matrix.
Silica gel prepared by the coagulation of colloidal silicic acid results in the formation of porous and noncrystalline granules of different sizes. It shows a higher surface area as compared to alumina, which ranges from 250 to 900 m2/g. silica is expensive adsorbent prepared by the reaction between from sodium silicate and acetic acid
Zeolites natural or synthetic crystalline aluminosilicates which have a repeating pore network and release water at high temperature. Zeolites are polar in nature. Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others.
Alumina Aluminium oxide (Al 2 O 3 ), a synthetic porous crystalline gel, which is available in the form of granules of different sizes having surface area ranging from 200 to 300 m 2 /g The most common form of crystalline alumina is known as corundum, a octahedral crystalline.
Activated carbon is the oldest adsorbent known and is usually prepared from coal, coconut shells, lignite, wood etc., using one of the two basic activation methods: physical and chemical is a highly porous, amorphous solid consisting of micro crystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it is reacts with oxygen at moderate temperatures (over 300 °C).
Biodegradation/bioremediation The chemical breakdown of materials by living organisms in environment. Organic material can be degraded aerobically with oxygen, or anaerobically, without oxygen. The process depends on certain microorganisms, such as bacteria, yeast, and fungi.
Biodegradation factors of polymer Polymer structure Polymer morphology Effects of radiation Molecular weight
Polymer structure Natural macromolecules, e.g. protein, cellulose, and starch are generally degraded in biological systems by hydrolysis followed by oxidation.
Biodegradability Since most enzyme-catalyzed reactions occur in aqueous media, the hydrophilic–hydrophobic character of synthetic polymers greatly affects their biodegradabilities. A polymer containing both hydrophobic and hydrophilic segments seems to have a higher biodegradability than those polymers containing either hydrophobic or hydrophilic structures only. the flexible aliphatic polyesters are readily degraded by biological systems, the more rigid aromatic polymer compound is generally considered to be bioinert.
Polymer morphology One of the principal differences between biopolymer and synthetic polymers is that biopolymer do not have equivalent repeating units along the chains. This regularity enhances crystallization, making the hydrolyzable groups inaccessible to enzymes. It was reasoned that synthetic polymers with long repeating units would be less likely to crystallize and thus might be biodegradable.
Effects of radiation Photolysis with UV light and the γ-ray irradiation of polymers generate radicals and/or ions that often lead to cleavage and crosslinking. Oxidation also occurs, complicating the situation, since exposure to light is seldom in the absence of oxygen.
Molecular weight Low molecular weight hydrocarbons, however, can be degraded by microbes. Plastics remain relatively immune to microbial attack as long as their molecular weight remains high.
Aerobic biodegradation pathways of aromatic compounds in bacteria and fungi
Methods of biodegradation Under appropriate conditions of moisture, temperature, and oxygen availability, biodegradation is a relatively rapid process Two types of microorganisms are of particular interest in the biodegradation of natural and synthetic polymers: bacteria and fungi.
Type of fungi for biodegradation White rot mushroom Brown rot mushroom Soft rot mushroom
White rot mushroom White rot fungi can degrade all cell wall components, including lignin. They often cause a bleaching of normal wood coloration. Their ability to metabolize large amounts of lignin in wood is unique among microorganisms.
Brown rot mushroom Brown-rot mushroom depolymerase cellulose rapidly during incipient stages of wood colonization. Considerable losses in wood strength occur very early in the decay process, often before decay characteristics are visually evident.
Brown-rot mushroom commonly cause decay in living trees, downed timber and wood used in buildings. Cell wall carbohydrates are degraded extensively during decay leaving a modified, lignin-rich substrate. Scanning electron micrograph of brown-rotted wood. Only slight pressure causes the wood cell walls to crumble into minute fragments.
Soft rot fungi Fungi that cause soft-rot are taxonomically classified in the subdivisions, Ascomycota and Deuteromycota. However, soft rots can occur in dry environments and may be macroscopically similar to brown rot.
Soft rot in wood often appears brown and can be confused with decay caused by brown rot fungi. Soft rot is different from other types of wood decay. Chains of cavities are produced inside the cell wall. This micrograph taken of a section from soft-rotted wood and viewed with a light microscope shows cavities within the cell walls.
Two distinct types of soft rot are currently recognized. Type 1 is characterized by longitudinal cavities formed within the secondary wall of wood cells and Type 2 used to describe an erosion of the entire secondary wall. The middle lamella is not degraded (in contrast to cell wall erosion by white-rot fungi), but may be modified in advanced stages of decay.
Different of white and brown rot White rot fungi, found in the wood of deciduous trees, first attack the lignin of wood. Once the lignin is digested, the fungi destroy cellulose and other major parts of cells. The partially decayed wood with residual cellulose is off-white in color, hence the name "white rot fungi." Brown rot fungi, found in conifers, damage the cellulose first but do very little, if any, damage to the lignin. The name "brown rot fungi" came about because infected wood becomes dark reddish-brown to golden in color.
White Rot mushroom Degradation System: Three types of extracellular enzymes are produced by white rot fungi that are non- selective yet effective in attacking lignin. These are often referred to as Lignin Modifying Enzymes (LMEs)/ligninolytic enzymes, and they are Lignin Peroxidase (LiP), Manganese- Dependent Peroxidase (MnP) and Laccase (Lac).
Lignin peroxidase LiP: Not all white rot fungi produce LiP, but it is a key component for the fungi that are being investigated for use. LiP oxidises methoxyl groups on aromatic rings (R-O-CH 3 ), and can work on substrates with quite high redox potentials.
Manganese peroxidase MnP is another enzymes containing peroxidase, and uses H 2 O 2 to catalyse oxidation of Mn² + to Mn³ +, this in turn oxidises phenolic substrates. Although similar in action to LiP, it does not have the same ability to oxidize substances with higher redox potentials.
Laccase Laccase is a multi copper oxidase which has the ability to oxidise phenolic compounds. In the presence of oxygen, it converts phenolic compounds into quinone radicals and then further converts them to quinones. It also produces some co- substrates which can be useful for degradation.