Synergetic degradation of atenolol by hydrodynamic cavitation coupled with sodium persulfate as zero-waste discharge process: Effect of coexisting anions

Highlights

• Synergetic degradation of atenolol by HC-PS as zero-waste discharge process was investigated.

• The contribution of SO₄⁻ radical was higher than that of OH radical.

• The presence of SO₄²⁻ ions has the highest promoting effect on the performance of HC-PS process.

Abstract

This research investigates atenolol (ATL) removal from the aqueous solution by a combination of hydrodynamic cavitation (HC) and sodium persulfate (HC-PS) as zero-waste discharge process. The effect of pH, reaction time, PS dose, initial ATL concentration, coexisting anions and natural organic matters (NOMs) has been studied. The highest ATL removal by HC and HC-PS processes was obtained at pH 3 and 6, respectively. The presence of coexisting anions viz., Cl⁻, NO₃⁻, I⁻, SO₄²⁻, and PO₄³⁻ promoted the degradation of ATL, while the efficiency of ATL degradation was inhibited by the presence HCO₃⁻ and CO₃²⁻ anions and NOMs. The highest synergetic and strongest inhibition effect of ATL degradation by the HC-PS process was observed in the presence of SO₄²⁻ ions (synergetic index value, 4.61) and CO₃²⁻ ions (synergetic index value, 0.44). Radical trapping experiments showed that the relative contributions of hydroxyl and sulfate radical to degrade ATL were 41.7% and 58.3%, respectively. In case of HC-PS process, degradation initiated with the protonation of ATL in the presence of free active radicals was followed by the ether bond cleavage, oxidation of primary and secondary amine moieties, and finally mineralization. The HC-PS process is proved to be a potential treatment method to destroy pharmaceutical pollutants from water and wastewater sources.

Introduction

New challenges of treatment methods for polluted water are being actively investigated to tackle heavy water pollution by the discharge of pharmaceuticals and their residues in ground, surface, drinking waters and waste streams. These toxic chemicals enter into the environment via inappropriate disposal of expired or unused medications (55–80% of pharmaceuticals in urine and feces) [1], [2]. Their presence of even at trace concentrations can remain for a long time in the environment due to their low rates of biodegradation, which if present, can disrupt the physiological behavior of the humans and animals [3]. Beta (β)-blockers such as atenolol (ATL) belong to a significant class of pharmaceuticals that are primarily used in the treatment of cardiovascular disorders, and presently these are extensively detected in the effluents of wastewater treatment plant (WWTP) as well as surface waters in the concentration range of ng/L to μg/L [4], [5]. The presence of ATL even in minute quantity may cause harmful environmental effects and human hazards. Even after the chlorination of wastewater, phytotoxicity of ATL remains active in water [6], [7] and the biodegradation of ATL requires long treatment time [8].

Conventional treatment processes of municipal wastewater may not decompose all the pharmaceuticals [9], [10] present in wastewater and hence, adsorption and nanofiltration methods have been explored to remove such toxic pharmaceuticals. Alternatively, it is necessary to develop the highly efficient, low-cost and environmental-friendly treatment technologies to remove ATL from the wastewater streams before they are being discharged into natural waters. Recently, many non-biological treatment technologies have been adopted including advanced oxidation processes (AOPs) based on hydroxyl (OH) radical formation (2.80 V oxidation potential), which can be efficient to remove the non-biodegradable pollutants [11], [12]. The Fenton and photo-Fenton processes, photocatalytic oxidation, UV-based processes, ozonation and cavitation (acoustic and hydrodynamic) are some of the alternative approaches used in the literature [13], [14]. Acoustic and hydrodynamic cavitation methods are used in wastewater treatment; but acoustic cavitation is not an efficient method on a full scale due to high power consumption and high operating costs. Recently, hydrodynamic cavitation has been adopted on a large scale due to its low energy consumption, cost-effectiveness, and simple structure [15], [16]. Additionally, due to high degradation, there is no problem of by-products resulting from the degradation of the desired pollutant [17]. Application of activated carbon is insufficient process in removal of ATL due to the hydrophilic nature of ATL [18].

Activation methods for peroxodisulfate such as alkaline, UV irradiation, heat, and use of transition metals may lead to the formation of sulfate (SO₄⁻) radicals [19]. Compared to OH^• radical, SO₄⁻ radicals have higher reaction stoichiometric efficiency, longer half-life, and the process is independent of pH [20]. Activation of PS using transition metal can be performed without the need for external energy, and therefore the method is cheaper and is less complex than the other methods. However, the use of metal catalysts is limited due to the disadvantages of secondary pollution and added environmental toxicity [21]. In addition, direct irradiation of visible and UV light irradiation can be inefficient to produce SO₄⁻ radicals from the sulfate source [20].

Alternatively, in a hydrodynamic cavitation (HC) approach, the bubble (cavity) is formed by the variation of pressure in a fluid flow due to local velocity fluctuations caused by passing the liquid stream through the geometric constructions such as venturi tubes, narrow valve, orifice plates, etc., [22], [23], [24]. When these cavities reach their maximum size, they can explode due to compressive effect of the bulk liquid over an extremely small interval of time (microseconds). Furthermore, when cavities are carried to a higher-pressure region, they implode violently, thus creating a very high pressure and temperature. This collapse releases an enormous energy to provide hot spots by raising the temperature up to 10³–10⁴ K [25], [26] and pressure to 100 MPa [27]. Under this condition, OH and H radicals can be formed in water, which is similar to ultrasonic cavitation [28].

Under this situation, molecules of water hemolytic decompose inside the cavities, leading to the generation of OH radicals and other reactive species [29]. The combination of AOPs with other suitable methods or additives can then lead to improved overall performance of the hybrid process. Choi et al., [30] studied the combination of persulfate (PS) with HC and iron ions for the decomposition of pentachlorophenol. The results of their study showed that the mechanism of PCP treatment was a redox reaction, and the contributions of OH⁻and SO₄⁻ were 81.1 and 18.9%, respectively. The authors concluded that PS is not only an oxidizing agent, but also can produce SO₄⁻ which is more oxidizing through an activation process by Fe(II). Lebik-Elhadi et al., [31] applied heat and ultrasound (US) to activate PS in the removal of thiamethoxam to demonstrate that in the presence of US, the conversion time of PS to SO₄⁻ decreases with increasing concentration, while the PS in the range of 20–100 mg/L would result in a complete conversion after 45 min of the reaction, thus a further increase to 500 mg/L could reduce the time to 20 min.

Realizing the importance of these findings, it was felt necessary to combine HC process with sulfate (SO₄^-) radical to decompose ATL from the aqueous media. In order to achieve this goal, we attempted to (i) optimize hydrodynamic cavitation and sodium persulfate (HC-PS) process in ATL removal, (ii) examine the effect of coexisting anions on the performance of HC-PS and (iii) determine the dominant role of the radical to understand ATL degradation pathway.