Water purification from pollutants containing various organic substances and their compounds, including medical waste (MW), is one of the main problems facing the world community. According to the World Health Organization, 15% of MW are considered hazardous materials that can be toxic, infectious or radioactive, and harm human health and the environment [1]. Existing methods have serious drawbacks, which makes it necessary to develop new solutions.
Heterogeneous photocatalysis using nanocatalysts is considered as a promising method for cleaning aqueous media from waste from the pharmaceutical industry, but the effectiveness of known photocatalysts is insufficient for use in industrial applications. In previous studies, it was shown that plasma treatment and impregnation of a ZnO-based catalyst with silver nanoparticles leads to an increase in its activity [2].
In this paper, photocatalysts were executed, which were impregnated with silver nanoparticles during the synthesis process. Such an injection mechanism should lead to a change in the morphology of the catalysts and an increase in their activity [2].
ZnO nanoparticles were prepared by dropwise addition of 25 mL of NaOH 0.4 mol/L into 25 mL of ZnSO4 0.2 mol/L at an approximate addition rate of 5 mL/min. After stirring with a magnetic stirrer SOLOR SCUID IKAMAG WHITE (IKA, Germany) at a speed of 150 rpm for 60 min, the solution was kept at 60°C for 2 h. Ag-ZnO composite nanoparticles were prepared by adding 6 mL of ascorbic acid 0.01 mol/L and 13 mL of AgNO3 0.01 mol/L into the solution of NaOH and ZnSO4, while stirring under the same condition as in the first experiment, and again the solution was kept at 70°C for 2 h. The catalysts synthesized by this method should have a size equal to 50-60 nm [3].
The photocatalytic activity of the obtained catalysts was researched in a model decomposition reaction of caffeine simulating pharmacological waste under the action of ultraviolet radiation in aqueous suspensions of synthesized samples. During the experiments for the preparation of the suspension, a 40 mg catalyst was mixed with 20 ml of an aqueous solution of caffeine sodium benzoate with a concentration of 300 mg/l. The mortars are prepared based on distilled water.
The selected concentration of caffeine for the model reaction is in the range of concentrations contained in wastewater. Suspensions of catalysts in aqueous solutions of model substances were exposed to UV radiation. As a source of ultraviolet radiation, a mercury-quartz lamp DRT-240 (power 240 W) was used. During the irradiation, the change in the concentration of caffeine in the solution was monitored using a SOLAR PB 2201 spectrophotometer (SOLAR, Belarus). The irradiation was carried out under constant stirring of the suspension in a magnetic stirrer SOLOR SCUID IKAMAG WHITE (IKA, Germany) at a speed of 200 rpm.
The reaction rate constant was determined by the slope of the graph of the dependence of the logarithm of the concentration of the decomposed substance on time. The relative concentration of the model substance Cr was determined by measuring the optical density at the absorption maximum:
$$C_r = \frac{c(t)}{c_0} \cdot 100\% = \frac{A_t}{A_0} \cdot 100\%, \tag{1}$$where C0 is the initial concentration of the model substance, C(t) is the concentration of model substances after irradiation of the UV radiation at time t, A0 and At the optical density of the solution of the model substance in the absorption maximum (λ=452 nm) before irradiation and at time t after the start of irradiation of the samples, respectively.
To compare the photocatalytic activity of different samples, it was assumed that the photodegradation reaction can be described by a first-order equation, therefore, the kinetic equation has the form:
$$\frac{dc}{dt} = -kC \tag{2}$$Here C – the concentration of the decomposed substance, k- the reaction constant. The solution of equation (2) can be represented as follows:
$$C(t) = C_0 e^{-kt} \tag{3}$$Thus, by constructing a time dependence $\ln \frac{c(t)}{c_0}$, the values of the reaction rate constant can be easily found on the graph.
$$\ln \frac{c(t)}{c_0} = -kt \tag{4}$$The activity of the synthesized samples was compared with the activity of commercially available ZnO (ECOS-1, Russia) with a particle size ranges from 100 nm to 1 μm [2].