**2.6.6 Seawater treatment**

230 Studies on Water Management Issues

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 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 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

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

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

the reaction between HS and chlorine.

(Bekbolet & Ozkosemen, 1996).

(Eggins et al., 1997).

**2.6.4 Removing trace metals** 

**2.6.5 Water disinfections** 

gold (Au), platinum (Pt) and silver (Ag) (Olis et al., 1991).

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 throughout the decomposition process.

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 artificial light (Ziolli & Jardim, 2002).

### **2.7 Current and future scope**

The number of new publications regarding photocatalysis for water and wastewater treatment has been increasing significantly since the last decade.

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

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 for a sustainable waste treatment.

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 233

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In the present scenario, the major difficulty regarding the doped TiO2 photocatalyst is the possible loss of photoactivity due to recycling of photocatalyst and long-term storage. It is believed that the efficiency of the metal-doped TiO2 under visible light wholly depends on the synthesizing and doping method adopted. In some cases, such doped photocatalysts showed zero activity under visible light or considerably lower activity in the ultraviolet spectral range compared to the non-doped TiO2 because of high carrier recombination rates through the metal ion levels. The problem is that the non-metal-doped TiO2 catalyst has very low photoactivity under visible light compared to that under UV light (Zaleska, 2008).TiO2 with visible light absorption can be employed to purify and disinfect the water and make it more suitable for consumption. Beside these limitations the major edge of the photocatalytic oxidation process over other process is because it's an only green and sustainable process towards waste treatment.
