Because methyl orange is used as indicator, this value is also known as the m-value. The method is suitable for all kinds of water with a pH value >4.3. This determination can be carried out as a titration. First, the water sample is filtered if it is turbid. Then 3–5 drops of methyl orange solution are added to the water sample (100 mL). Methyl orange has the property to color alkaline and neutral water yellow. If the water becomes acidic, it turns red immediately. The point of change is at pH 4.3. If the solution is yellow, hydrochloric acid at a concentration of 0.1 mol/L is used for the titration. This must be done slowly while the sample is shaken or mixed until the yellow coloration changes to orange (not to red). If the water has a pH below 4.3, a similar procedure with caustic can be used to determine the base capacity to pH 4.3.

3 Transition metal-doped ZnO nanoparticles

Photocatalytic activities of ZnO nanoparticles doped with transition metals were studied for removal of methyl orange under UV light; 12 % of Mn and Co ions were doped onto ZnO nanoparticles and they showed significant removal. Fu et al. (2011) have established that Cu-doped ZnO nanoparticles show more effective photocatalytic ability than ZnO undoped. Ba-Abbad et al. (2013) have analyzed a ZnO matrix doped with Fe synthesized therein by a sol-gel technique. It has been reported that the Fe ions present in lesser quantities had an effect on the photocatalytic activity. Transition metals can be used for doping ZnO to increase the photolytic activity, since the surface defects are increased by doping (Patil et al., 2010; Sood et al., 2014).

It has been shown that the photocatalytic activity of ZnO is increased by doping it with Nd(III) and its effect on the degradation of dyes under solar irradiation has been studied and reported by Shinde et al. (2015). A green approach to solar light-induced removal of methyl red in an aqueous suspension of commercial ZnO was studied and reported by Singh et al. (2013). Xu et al. (2013) reported on the synthesis of flowerlike ZnO-Ag₂O composites by precipitation synthesis and their effect on photocatalytic activity. Zhi-Gang et al. (2012) reported on the preparation and photocatalytic performance of porous ZnO microrods loaded with Ag. In recent years, researchers have prepared a novel nanoscale adsorbent by chemical deposition of nZVI on CNTs, which showed good potential for quick and effective removal of nitrate in water (Azari et al., 2014). A chitin/ZnO composite photocatalyst has been used for the treatment of aquaculture wastewater (Xiajing et al., 2019) and it was reported that the photocatalytic activity was increased to 77.2% of ammonia removal from synthetic wastewater when 5 wt% of CuO was loaded over ZnO using solar radiation for a period of 240 min. Solar energy is capable of excitation of oxides (SnO₂ and ZnO) and dopants are capable of extending the spectrum. The adsorption capacity of a nano-ZnO/chitosan composite membrane (ZnOCTF) was studied by Zhang et al. (2016a,b) using methyl orange as a model dye, and it showed a removal of 90.9%.

Ashebir et al. (2018) reported on the photocatalytic removal of ZnO, Ag-doped ZnO, and Mn-doped ZnO nanoparticles (NPs) and MV as a model has been used to study their photocatalytic decolorization efficiency. The coprecipitation method was used to synthesize Ag-doped and Mn-doped ZnO samples and its removal efficiency was studied against methyl violet and was found that a pH of 9.0, catalyst load of 1 g/L, and initial dye concentration of 4.5 × 10⁴ g/L were optimum parameters. Mn-doped and Ag-doped ZnO showed higher decolorization efficiency when compared to undoped ZnO. The surface defects of ZnO were increased due to the incorporation of Mn and Ag, which in turn increased the removal efficiency of ZnO; because these metals increased the shift of their absorption toward the visible region. Suchada et al. (2019) reported that a graphene quantum dots-ZnO nanocomposite can be used as a photocatalytic material for decontaminating organic dyes and commercial herbicide contaminants because of its cost-effective and environmentally friendly properties.

Sol–Gel Molecular Imprinting

The first synthetic materials with memory for a template were obtained by Dickey in 1949 using a silica gel matrix. Imprinted silica materials were produced by acid precipitation of aqueous solution of sodium silicate in the presence of dyes as templates (e.g., methyl orange). In the following years research on imprinted silicates and metal oxide sol–gel continued and simple amorphous silicates could be imprinted for different dyes, N-heterocycle aromatics, proteins, and for resolution of enantiomers (e.g., camphorsulphonic acid) and stereoisomers (e.g., N-methyl-3-methoxymorphine, nicotine, quinine, quinidine, cinchonine, and cinchonidine). The recognition properties of these materials are dictated by the gel structure and surface chemistry, both of which are influenced by a multitude of factors, such as catalysts, pH, solvents, aging, and nature of precursors. To date, sol–gel derived molecularly imprinted materials have not revealed the same level of success as imprinted polymers for separation, catalysis, and sensor applications. One of the most unique features of imprinted sol–gel materials is their high aqueous compatible nature, allowing incorporation of biologically based guests