**2.6.3 Elimination of natural organic matter**

Humic substances (HS) can be generally defined as a class of naturally existing biogenic heterogeneous organic substances that can be further classified as being yellow-brown and having high molecular weights (MacCarthy, 2001). HS can also be defined as the fraction of filtered water that adsorb on XAD-8 resin (a non-ionic polymeric adsorbent) at pH 2 (Obernosterer & Herndl, 2000). They are the major components of the dissolved organic carbon (DOC) pool in surface water (marine waters and fresh waters) and sub-surface or ground waters. They are often said to be a main factor that lead to yellowish-brown colour in the water bodies (Schmitt-Kopplin, 1998). The concentration of the HS differs from place to place; seawater normally contains 2-3mg/L of HS. According to Gaffney et al. (1996), their physical properties like size and their chemical properties like the structure and the number and position of the functional group differ, relying on the origin and the age of the substance (Gaffney et al., 1996).

HS are known to have the ability to change the behaviour of certain pollutants considerably, such as trace metal speciation and toxicity, (Bekbolet & Balcioghu, 1996; Shin et al., 1996), solubilisation and adsorption of hydrophobic contaminants (Chiou et al., 1986; Tanaka et al., 1997) and aqueous photochemistry (Fukushima et al., 2000). HS can act as substrates for bacterial growth, hinder the bacterial degradation of impurities (colours), interact with heavy metals such as Fe, Mn and Pb and thus making them difficult to remove, help to

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 231

that Microcystin toxins can be degraded on immobilized titanium dioxide (TiO2) photocatalyst (Shephard et al., 2002). Matsunaga and Tomoda in 1985 first reported on photocatalytic disinfection.They found that in the presence of high concentrations of microorganisms, disinfection process was effective and more efficient. Similarly, Belapurkar et al., 2006 prepared high surface area TiO2 by hydrothermal method using titanium isopropoxide. They applied for disinfection of water and found to be effective when 1 L of water was photolysed by solar light in a plastic tray containing TiO2 photocatalyst coated on a stainless plate. Their study also proved that the technique can be used for disinfection of ~20 L water daily using solar light. Finally concluded that the photocatalytic technique using solar light, a viable,simple and easy-to-use device for disinfection of drinking water

Lately, the decomposition of humic substances in artificial seawater (highly saline water) and natural seawater were studied by Al-Rasheed and Cardin (Al-Rasheed & Cardin, 2003). Although the decompositions were found to be slower compared with a fresh water media usually employ by other researches, no toxic or hazardous by-products were found

The degradation of some crude oil components (dodecane and toluene) via photocatalysis using seawater media was carried out in 1997 (Minero et al., 1997). No chlorinated compounds were found over the course of irradiation. 100% degradation was recorded after just few hours of illumination. Ziolli and Jardim reported in 2002 that seawater-soluble crude oil fractions can be decomposed under the irradiation of nanoparticles of titania using

The number of new publications regarding photocatalysis for water and wastewater

The photocatalytic oxidation of towards water treatment has caught up most of the attention. Recently, the attention started to shift onto the oxidation of volatile organic or inorganic compounds present in ground water for an efficient treatment. Photocatalytic reduction organic compounds and metal-containing ions and researches on cell destroying and disinfection by irradiated titania has also caught up some attention (Zaleska, 2008).

Subsequently, titania-based photocatalysts has been commercialized in various fields firstly in Japan, followed by the United States and then China. This commercialization of TiO2 based photocatalysts products was started during the mid 90s in Japan. Among commercialisation the purification equipment (e.g. air purifiers, air conditioners, portable water purification system, purification system for pools) and household equipment is more

Though a number of commercial TiO2 is available in market they lag in low sensitivity of photocatalyst towards visible light, which cannot take up the visible spectrum (largest part) in the solar radiation for waste treatment. Hence scientific community is eager to increase the sensitivity of photocatalyst to visible light so that sunlight could be used for excitation

treatment has been increasing significantly since the last decade.

promising achievements in field of water treatment. (Zaleska, 2008).

on liter scale (Belapurkar et al., 2006)

throughout the decomposition process.

artificial light (Ziolli & Jardim, 2002).

**2.7 Current and future scope** 

for a sustainable waste treatment.

**2.6.6 Seawater treatment** 

transport the metals into the environment and also contribute to pipe corrosions (Motheo & Pinhedo, 2000). Besides, HS can also act as a source of methyl groups and hence react with hypochlorite ions which are being used as biocide in water treatment plants to produce disinfectant by-products. Examples of these by-products are trihalomethanes, haloacetic acids, other chlorinated compounds and nitriles. Some of these by-products are suspected to be carcinogenic. Till date, more than 150 products have been recognized as the products of the reaction between HS and chlorine.

The advance oxidation process has been implemented to decrease the organic matter in water including the HS. Its major advantage is that the advance oxidation process does not produce any toxic by-products or residues which required further treatment or disposal. Till date, the degradation of HS using photocatalytic process has not been studied well. The very first study based on this was carried out by Bekbolet in 1996, who studies the effectiveness of photocatalytic treatment on the degradation of model humic substances or humic acid (Bekbolet & Ozkosemen, 1996).

Bekbolet and Ozkosemen studied about the degradation via photocatalytic process using humic acid as a model pollutant. Through the experiment, they found out that after one hour of illumination in the presence of 1.0g of Degussa P25, 40% of the TOC and 75% of the colour (400nm) were removed (Bekbolet & Ozkosemen, 1996).Bekbolet and co. again studied the removal of colour caused by humic acid in the presence of common inorganic ions (e.g. chloride, nitrate, phosphate and sulphate ions) at pH 6.8, and they found some removal (Bekbolet et al., 1998). In other researches where humic acid was used as an additional matrix for the degradation of some organic pollutants, a 80% removal of commercialized humic acid was recorded by using irradiation in the presence of Degussa P25 (Minero et al., 1997). Another similar study showed a reduction around 50% of the concentration of humic acid in just 12 minutes using suspension of Degussa P25 irradiated by a mercury lamp (Eggins et al., 1997).

#### **2.6.4 Removing trace metals**

Trace metals especially mercury (Hg), lead (Pb), chromium (Cr) and many others are considered extremely hazardous to human being. The presence of these metals in water bodies should be removed. Photocatalysis can be used to remove heavy metals like mercury (Hg), chromium (Cr), lead (Pb), cadmium (Cd), arsenic (As), nickel (Ni), and copper (Cu) (Blake, 2001; Olis et al., 1991). Other than that, the photochemical ability of the photocatalysis enables it to recover costly metals from industrial waste discharge such as gold (Au), platinum (Pt) and silver (Ag) (Olis et al., 1991).

#### **2.6.5 Water disinfections**

Photocatalysis can be used in water disinfections because it can kill or destroy various bacteria and viruses. In 1997, a study by Mills and LeHunte reported that *Streptococcus mutans, streptococcus natuss, streptococcus cricetus, escherichia coli*, *scaccharomycescerevisisas*, *lactobacillus acidophilus,* poliovirus 1 were destructed effectively using heterogeneous photocatalysis (Mills & LeHunte, 1997). With algae blooming in fresh water supplies becoming more and more common, the subsequent possibility of cyanobacterialmicrocystin pollution of portable water caused by *Microcystin* toxins. In 2002, Shephard et al. reported that Microcystin toxins can be degraded on immobilized titanium dioxide (TiO2) photocatalyst (Shephard et al., 2002). Matsunaga and Tomoda in 1985 first reported on photocatalytic disinfection.They found that in the presence of high concentrations of microorganisms, disinfection process was effective and more efficient. Similarly, Belapurkar et al., 2006 prepared high surface area TiO2 by hydrothermal method using titanium isopropoxide. They applied for disinfection of water and found to be effective when 1 L of water was photolysed by solar light in a plastic tray containing TiO2 photocatalyst coated on a stainless plate. Their study also proved that the technique can be used for disinfection of ~20 L water daily using solar light. Finally concluded that the photocatalytic technique using solar light, a viable,simple and easy-to-use device for disinfection of drinking water on liter scale (Belapurkar et al., 2006)
