Chloroquine degradation in aqueous solution under electron beam irradiation

CQ phosphate powder (98.5–100.0%), potassium dichromate (99%), silver nitrate (99%), sodium hydroxide (99%), sulfuric acid (95%), and perchloric acid (95%) were purchased from Sigma-Aldrich (Merck, Germany). Distilled water from a Thermo Fischer scientific distillation unit (Merck, Germany) was used in the preparation of all solutions for this study.

The Jasco-V670 UV-VIS (Poland) and Macherey Nagel Nanocolor VIS II (Germany) spectrophotometers were used for the detection of CQ (>1 mg·L^-1) at the maximum absorption wavelength of 343 nm. Nanocolor test kits produced by Macherey Nagel (Germany) were purchased from Aqua Labs (Poland) for photometric determination of total Kjeldahl nitrogen (TKN) (1.0–16 mg·L^-1), total nitrogen (0.5–50 mg·L^-1), nitrate NO3−, NH4+ (0.2–8 mg·L^-1), Cl^- (0.5–50 mg·L^-1), total organic carbon (TOC) (20–300 mg·L^-1), and chemical oxygen demand (COD) (50–300 mg·L^-1). Photometric tests were performed using the Nanocolor VIS II spectrophotometer. The pH measurements were conducted using an Elemetron CX-461 multimeter (Poland). Dissolved oxygen (DO) was measured using a DO meter from Mettler Toledo (Poland).

Radiation processing was done in batch mode. The aqueous solutions containing different CQ concentrations were prepared in distilled water. Irradiation was performed on the ILU6 accelerator at an energy of 1.65 MeV, 2 Hz, and 50 mA. Potassium dichromate (0.0005 M) solution in perchloric acid with silver nitrate and alanine dosimeters were used to assess the applied doses ranging from 0.5 kGy to 7 kGy [43–46]. Low-density polyethylene (LDPE) sleeve bags were filled with 35 mL of CQ solution and irradiated under the accelerator window. The initial pH values of the CQ solution were adjusted with 0.1 M NaOH and 0.1 M H2SO4 to study the effect of pH on the degradation efficiency under electron beam processing.

The stepwise absorption reduction observed for CQ solution at 343 nm and at 330 nm with increasing radiation dose from 0 kGy to 7 kGy is illustrated in Fig. 1. Additionally, the corresponding reduction in CQ concentrations with increasing dose is shown in Fig. 2. The concentration of CQ was reduced from 125 mg·L^-1 to 22 mg·L^-1, which corresponds to an 82% reduction in the initial CQ concentration following electron beam irradiation. At the lowest absorbed dose achieved in this experiment (0.5 kGy), the initial CQ concentration of 125 mg·L^-1 was reduced by ≈50%.

Fig. 1.

Fig. 2.

Free radicals generated during the radiolysis of water drive the reactions leading to the degradation of organic compounds in water. These radicals are generated through a combination of physical, physical-chemical, and chemical processes that occur within 10^-12 s following the interactions of radiation in the water matrix. The species formed and their corresponding G-values are presented in Table 2.

The hydroxyl radical and hydrated electron play important roles in the degradation of CQ molecules via the addition reaction and dissociative electron attachment according to Eqs. (1) and (2), respectively. Reaction rates for the reactions with ^•OH and eaq− at a pH of 8.5 have been previously evaluated and are shown in Table 2 [47]. 1·OH+C18H26 N3Cl→C18H26 N3ClOH 2eaq−+C18H26 N3Cl→C18H26 N3·+Cl−

Initial concentrations of CQ were varied to study the effect of CQ concentration on the degradation efficiency. The removal efficiency decreased with increasing CQ concentration at the same applied radiation dose (Fig. 3a). Generally, concentrations of CQ between 25 mg·L^-1 and 125 mg·L^-1 have degradation efficiencies ≥75% for a maximum radiation dose of 7 kGy. However, for lower concentrations (25 mg·L^-1), more than 99% removal efficiency was achieved.

Fig. 3.

The degradation of CQ can be represented using a pseudo-first-order relation (Eq. (3)) [48]. 3−ln[C][ C0 ]=kD

where C and C₀ are the final and initial concentrations of the target compound at a dose D (kGy) with a rate constant of k. The results of the k values at different concentrations at 0.5 kGy are shown in Fig. 3b. The reaction rate is observed to decrease with increasing concentrations of CQ in the water as shown in Fig. 3c and Table 3.

The solvent pH influences the percentage of the different radicals generated during the radiolysis of the water. Under alkaline conditions, ^•OH readily reacts with OH^- to generate O^•- thereby reducing the concentration of ^•OH and hence the degradation efficiency. 4·OH+OH−→O•−+H2O

In acidic media, the hydrated electrons eaq− react with H⁺ to produce ^•H. 5eaq−+H+→•H

Therefore, under electron beam irradiation, the pH enhances or inhibits the production of reactive oxidizing or reducing species and therefore affects the degradation process. In this study, the degradation of CQ was observed to increase with increasing pH. The removal efficiency decreased slightly with increasing pH at irradiation doses between 0.5 kGy and 2 kGy. The acidic pH of 2 to neutral pH of 7 had relatively higher removal efficiencies for CQ (≈84%). In neutral to an alkaline pH of 10, the removal efficiency was at ≈80% at irradiation doses of 7 kGy (Fig. 4). However, slight increments in the removal efficiency were observed with increasing pH from neutral to pH = 10 at an irradiation dose of 3 kGy. According to Eq. (2), reactions with the hydrated electron have a higher reaction rate compared to hydroxyl radical reactions. More hydrated electrons are produced in alkaline conditions, which lead to higher degradation of CQ. However, in strongly alkaline conditions (pH = 12), the removal efficiency was inhibited. This could be attributed to the reactions of hydroxyl radicals with hydroxide (Eq. (4)). Conversely, at lower pH, hydrated electrons are consumed, which would result in lower removal efficiency for CQ. Additionally, hydroxyl radicals are known to have a higher reduction potential (2.72 V) at acidic pH compared to alkaline conditions (1.89 V); therefore, their role in CQ degradation would be affected at different pH [49].

Fig. 4.

The changes in the initial pH during the irradiation process are presented in Fig. 5. Other than affecting the production of water radiolysis products, the pH value can affect the reaction pathways by changing the structure and characteristics of contaminants such as their solubility. Certain compounds retain their molecular form when the pH is less than its pK_a. However, when the pH >pK_a, the compound loses a proton and becomes negatively charged. The pKa of CQ is 8.76, and it is a weak acid with a tendency to donate electrons. Therefore, at pH <pKa (between pH 2 and 7), CQ is in the non--ionic form (protonated). At pH >7, the divalent form of CQ (CQH22+) is dominant alongside lesser concentrations of the monovalent and neutral CQ [50]. Compounds such as diclofenac are insoluble at low pH values (around 3), which is a favorable condition for electron beam irradiation. Therefore, the optimization of pH may influence the solubility and degradation of some pollutants [51]. Neutral or acidic media are preferable to an alkaline solution for the degradation of HCQ, a CQ derivative under gamma irradiation [52, 53]. In the present study, acidic to neutral pH favors the degradation of CQ under an electron beam. Acidic pH has been reported to favor electro-Fenton oxidation, sulfate radical AOPs, and the Fe(VI) oxidation of CQ (Table 1). The pH of wastewater is a vital component before and after treatment for the eventual discharge of wastewater.

Fig. 5.

The results for the degradation byproducts are based on the degradation of 125 mg·L^-1 of CQ solution under electron beam irradiation. The CQ solution was irradiated between 0.5 kGy and 7 kGy. The presence and degradation of organic compounds influence parameters used in wastewater such as pH, TOC, COD, chloride ion, and nitrogen. The formation or reduction in these physicochemical parameters was used to evaluate the effectiveness of CQ treatment under electron beam irradiation.

Change in the solution pH

During the radiolysis of the CQ solutions, the pH was observed to reduce from 6.5 before irradiation to 3.2 at 7 kGy (Fig. 6). A similar decrease in pH has been observed in the degradation of ciprofloxacin where pH decreased from 7.00 to 4.22 at 5.0 kGy, consistent with the formation of lower molecular weight organic acids and dissolved ions resulting from EB treatment [51, 54].

Fig. 6.

In the degradation of CQ under electro-Fenton oxidation, the change in pH was attributed to the formation of carboxylic acids such as oxamic and oxalic acids [20]. Additionally, pH changes in the degradation of diuron under radiation were attributed to the formation of aldehydes and ketones [55].

Cl^- generation

During the radiolysis of organic compounds with good leaving groups such as halogenated compounds, dissociative electron attachment as shown in Eq. (2) is proposed to be the most preferred reaction process of the aqueous electron [56, 57]. The Clions present in CQ were continuously released with increasing radiation dose (Fig. 7). Almost complete Cl-detachment was observed at 7 kGy indicative of the dechlorination of the CQ compound.

Fig. 7.

Most polychlorinated cyclic compounds are stable, lipophilic, and tend to bioaccumulate in the environment with slow degradation. Chlorinated compounds i.e., pesticides, biphenyls, dibenzodioxins, and dibenzofurans are genotoxic (mutagenic and carcinogenic) and therefore pose a risk to living organisms [58–60]. Therefore, the dechlorination of CQ under electron beam treatment would be considered to reduce its toxicity. However, in the present study, no toxicity studies were undertaken to ascertain the reduction in toxicity.

Nitrogen

The generation of NO3− and NO2− in the aqueous phase in the presence of ionizing radiation is normal when dealing with nitrogen-containing compounds [49]. The oxidative degradation of nitrogen-containing compounds releases nitrate, nitrite, and ammonium ions [61, 62]. Nitrogen often enters water and wastewater via agricultural, domestic, and manufacturing wastes. The ammonium, organic nitrogen, nitrate, and nitrite concentrations are the most relevant for the determination of nitrogen in water and wastewater. The total organic bound nitrogen (CQ[N]) was reduced by 50% as well as the TKN (Fig. 8). The eventual formation of inorganic nitrogen (NO3−) is also observed to increase with increasing doses. Additionally, the formation of the NH4+ ions with increasing dose was observed alluding to the nitrification process.

Fig. 8.

Organic nitrogen (TKN) is normally converted into inorganic nitrogen (NO3− and NO2−) during the wastewater-treatment processes. At the end of the water-treatment process, predominantly inorganic nitrogen is present. Nitrogen in freshly polluted water is originally present in the form of organic nitrogen and ammonia (TKN + NH₃). Completely nitrified wastewater will have little or no organic nitrogen. However, secondary reactions of mineralized nitrogen species (nitrate and nitrite ions) with the parent or intermediate compound generate nitrogenous disinfection byproducts that are also a concern [61].

Changes in the oxygen demand

Dissolved oxygen (DO) reflects the amount of oxygen dissolved in water and is available to living aquatic organisms. The decay of organic matter consumes the DO and the presence of excess organic material, biochemical oxygen demand (BOD), or COD in lakes and rivers causes eutrophication (oxygen deficiency). A large amount of DO is consumed by aerobic microorganisms to decompose the organic matter. The DO is affected by the water temperature, salt concentration, atmospheric pressure, and other conditions, and the saturation level decreases with increases in water temperature. During the radiolysis processing of wastewater, the DO will react with radicals formed. This may significantly reduce O₂ concentrations. However, from the experiment, the dissolved O₂ varied from 5.5 mg·L^-1 before irradiation to 4.7 mg·L^-1 at 7 kGy as shown in Fig. 9.

Fig. 9.

High DO has been found to influence the mineralization of azo dyes under electron beam irradiation by improving the cleavage of the aromatic rings and promoting the reduction of TOC [63]. The DO was more important for the destruction of substituted aromatic rings in the dye molecule than the chromophore and needed to be maintained at optimal concentrations during electron beam radiolysis [64]. However, DO scavenges eaq− and H^•, therefore, reducing the equilibrium concentration of these species and their reactions with target pollutants [65].

The oxygen demand determines the amount of organic pollution in water, waste loadings of treatment plants, and in evaluating the efficiency of treatment processes. BOD, COD, and TOC are used to measure the oxygen demand. There was a slight decrease in the COD and TOC with increasing doses and the decreasing concentration of CQ (Fig. 10). These observations in TOC and COD degradation indicate the formation of other organic byproducts that are not easily degraded in the present conditions [66]. In other studies, even with the apparent insignificant reduction in COD and TOC, the resulting degradation products exhibited a higher biological oxygen demand and therefore were more susceptible to biological degradation.

Fig. 10.