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Laundry is the process to make sure that cloths are washed and cleaned again after it being used and become dirty. A product that included of laundry is usually like detergents, bleaches, builders, and metal chelating agents. Laundry wastewater contain in sewage. It releases large amounts of chemicals into wastewater. Toxic chemicals in this substance are surfactants, chlorine, organic peroxides, phosphates, alkalis, and glycol ethers.
Detergent is one of substance that contributes large amounts of laundry wastewater. Detergent is substance that comfortable to washing cloths as active cleaning agent. It is quite effective for the process of washing when put it in form of aqueous solution. There is two group contain in detergent which that hydrophobic (non polar in form of tail) and hydrophilic molecule (as polar head) which means they will attack a dirty on cloths during the
process of washing. The hydrophobic group is unable stand with stable structure and it is unfriendly with water molecule while hydrophilic is to like be cooperative with water. In this case, polar head is interact with hydrogen bonds and hydrophobic tails aggregate resulting in highly organized will form spherical structure that being called as micelles. The detergents are existed as single molecule at low concentration. The concentration increases micelles begin to form. The concentration at which micelles begin to form is known as the Critical Micelle Concentration (CMC) (a' Hartmann, 2006).
Figure 2.1: A detergent micelle formed with Sodium Dodecyl Sulfate molecules in an aqueous solution (left) or a non-aqueous solution (right) (Source: b' Hartmann, 2006).
In this view, two parameters for water quality will be learned as follow the scope of study. There is turbidity and pH. In Malaysia, the government declared that two of the characteristic are including in the parameters for standard water quality. They already stated at a scale based on classes.
Table 2.1: National Water Quality Standards for Malaysia based on two parameters (source: Department of Environment, 2001; 2002; 2003)
Turbidity is one of a principal physical characteristic in properties of water quality. As the simple explanation, it is the measure of relative clarity of liquid. It is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample (EPA Guidance Manual Turbidity Provisions, 1999). This turbidity is occurred because a present of suspended matter or impurities that interfere with the clarity of the water. These impurities that form the
turbidity probably include silt, whether organic or inorganic matter, clay, plankton and other microscopic organisms.
Distance is measure by Kilometre, the time is measure by hour, the degree of acidity or basicity of a solution is measure by pH unit (Frederick, 2003). pH is the measurement of a hydrogen ion concentration, [H+] where the ion is contain in an aqueous solution. The range of measure for pH is from 0 to 14 pH. If the solution is below than 7 pH, it will consider as acidic properties while above than 7 pH actually called basic properties which is also known as caustic or alkaline properties. Since the value exactly at the centre of measurement (7 pH), it is "neutral" which means neither acidic nor basic. pH is truly defined as the negative logarithm of [H+].
pH is importance for aquatic organisms to be within their water body because a certain range of pH is need for survival and also for optimal growth. Even each organism has suitable pH to live, the range of pH from 6.5 to 8.0 are being preferred for most aquatic organisms. Outside of this range, organisms become physiologically stressed (Kelly et al., 2004). Other than that, reproduction can also be impacted, and life of organisms is opened to die if the pH is not in the optimal range. The lower of pH measure can make a toxic elements and compound are released from sediments into the water. In this situation, aquatic animals will take the toxic. The availability of plant nutrients are influence by the change of pH meter too.
ADVANCED OXIDATION PROCESSS (AOPs)
Advanced Oxidation Processes (AOPs) is the chemical process by using oxidation. It is using hydroxyl radical, OHâ€¢ as oxidizing agent for treatment water. To generate OHâ€¢, There are several method based on AOPs. There are such as Ozone method, Photocatalysis, and Fenton reaction. AOPs have ability to oxidizing complex organic constituents that found in water which is to hard to biodegrade them (a' Metcalf and Eddy, 2004).
Table 2.2: The potential of oxidizing agents (source: Ozonia, 1997)
Electrochemical Oxidation Potential (EOP), Volt
EOP that relative to Chlorine, Cl
hydroxyl radical, OHâ€¢
Hydrogen Peroxide, H2O2
Chlorine dioxide, CO2
Based on Table 2.2, there are few various oxidizing agent that usually used in the degradation's process of water (b' Metcalf and Eddy, 2004). Hydroxyl radical is second highest of powerful oxidizing agent after Fluorine, Fl. Even though Fluorine is the powerful than hydroxyl radical, the cost is high and not commercial to be used in treating wastewater.
2.4.1 Fenton reagent
The Fenton reagent has two ions that can be react with organic to process of treatment. There is Ferrous, Fe (II) and hydrogen peroxide, H2O2 that consist in the system of Fenton under acidic condition. Fenton is the methods that effective for the destruction of any kind of waste such toxic waste, laundry waste and non-biodegrable. This method is more effective and also more suitable rather than secondary biological treatment (Chen & Pignatello, 1997). Other than that, Fenton is also use to treat a variety of wastes such as the textile industry, chemical manufacturing, and fuel terminals (Polish, 2005).
Basically, Fenton is one of method treatment of advanced oxidation process (AOP) because it treats the water by Hydroxyl radical through the oxidation process. There is another method of oxidation using the Fenton but it is advanced step of Fenton to treat the waste water. It is called Electro-Fenton Process (EFP). This method is considers to be include in electrochemical method or electrolysis method. EFP likes to be the adding of electrochemical apparatus and cell to conduct this technique. The research about EFP has been done by Elodie (2003) which are to degrade azo dye, electro Fenton process is efficient for the process.
2.4.2 Hydroxyl radical formation
Fenton's reagent is known as a precursor of the hydroxyl radical OHâ€¢ a highly oxidizing agent. Fenton that contain both of substances is reacted each other to form hydroxyl radical which means it should follow a certain mechanism. The mechanism is called hydroxyl mechanism. Two years after Fenton's death the hydroxyl mechanism was mentioned by Haber and Willstatter (1931) in a paper on radical chain mechanism. This mechanism that be shown is a first step reaction of Fenton chemistry. The step reaction of this mechanism include in radical mechanism. The other mechanism is non-radical mechanism. Following Barb et. al (1951), the steps of free radical mechanism following by these:
Fe2+ + H2O2 Fe3+ + OH- + OHâ€¢ (1)
Fe2+ as catalysis reacts with hydrogen peroxide to generate hydroxyl radicals.
OHâ€¢ + H2O2 HO2â€¢ + H2O (2)
The hydroxyl radical has the tendency to react with Hydrogen Peroxide to produce hydroperoxyl radical. Hydroperoxyl radical is weaker oxidizing agent compared to hydroxyl radicals (a' Mohajeri et al, 2010).
Fe3+ + HO2â€¢ Fe2+ + H+ + O2 (3)
Fe2+ will regenerate through this reaction. By the way, the other reaction for regeneration of Fe2+ will be discussed in detail in next section.
Fe2+ + HO2â€¢ Fe3+ + HO2- (4)
Fe2+ possibly can react with hydroperoxyl radical to produce an amount of Fe3+.
Fe2+ + OHâ€¢ Fe3+ + OH- (5)
Fe2+ will react with hydroxyl radicals which that amount of Fe3+ is produce again.
Based on equation (1), interaction between Fe2+ and H2O2 are producing OHâ€¢ radicals. OHâ€¢ radical has a great oxidizing ability to degrade toxic organic contaminants (Walling, 1975). OHâ€¢ radical will react with organic substrates that contain in waste water. The degradation of organic compound are faster than usual if the amount of Fe2+ is present in large amounts because it will produce more OHâ€¢ radical.
2.4.3 Regeneration of Fe(II)
Fe(II) also regenerated through the reaction of Fe(III) with H2O2 in the so-called Fenton-like-reaction. The process regeneration of Fe2+ is occurred when Fe3+ and HO2â€¢ radical are react each other as the mentioned in equation (2). By the way, the fully reactions to generated Fe2+ through Fenton-like-reaction follow by this:
H2O2 + Fe3+ FeOOH2+ + H+ (6)
FeOOH2+ HO2â€¢ + Fe2+ (7)
HO2â€¢ + Fe2+ HO2- + Fe3+ (8)
HO2â€¢ + Fe3+ H+ + O2 + Fe2+ (9)
Three reactions can regenerate Fe2+. Actually, the equation (9) and (3) are the same. And also for equation (8) and (4) are similar equation. In this case, Fe2+ will oxidize to Fe3+ through any reaction can be occurred such as in equation (1), equation (4) or (8) and equation (5). But, to regeneration of Fe2+, it can happen when the amount of Fe3+ reacts with Hydroperoxyl radical.
2.4.4 Reaction between hydroxyl radicals and organic matter.
After the process to produce hydroxyl radicals, the radical will degrade organic matter in laundry wastewater:
RH + OHâ€¢ Râ€¢ + H2O (10)
Râ€¢ + Fe3+ R+ + Fe2+ (11)
Based on all the equations, the Fenton process is efficient if the hydroxyl radical that produce in large amount. By the way, to produce the radical, it is demands and requires a high concentration Fe2+ because one molecule of Fe2+ is consumed to produce the each of hydroxyl radical (Rein, 2001).