In situ treatment of metalworking wastewater by chemical addition-dissolved air flotation coupled with UV, H2O2 & ZnO.

The hybrid treatment techniques have been proven that could be a proper solution to the metalworking fluid (MWF) wastewaters disposal challenge. Hence, this investigation was conducted aim to the assessment of chemical addition-dissolved air floatation (CA-DAF) unit followed with a heterogeneous photocatalytic (PC) process as UV/H2O2/ZnO to treat MWF wastewater produced in one of the central industrial estates in the Middle East. The CA-DAF unit was implemented as trial and errors and had an appropriate efficiency. However, the environmental discharge standards were not achieved only by this unit, so that, a PC process considered for this purpose in a pilot-scale reactor. And also, Chemical oxygen demand (COD), total petroleum hydrocarbons (TPHs) were considered as physicochemical parameters to analyze the applied photochemical reaction throughout the concentrations of ZnO and H2O2, and pH value as variables of the study. The ideal and optimized conditions were observed at pH 10, 600 mg l-1 of ZnO, and 13.11 g l-1 of H2O2 via 99.87% and 97.9% reduction rates in total COD and TPH, respectively. These removal rates were obtained for this integrated strategy under the optimized reaction. By evaluating the synergistic effect, it was found that UV/ZnO could be a predominant reaction in this process. The organic and intermediates analysis appeared 78.46% reduction for all detected organic matters. Besides, PC generation of bis phthalate, mono phthalate, benzene, and benzoic acid to 2,6-bis (1,1-dimethylethyl)-4-mthyl phenol was the reason of the residual phenolic compound concentration in the reaction solution with low removal rate. The kinetic study showed that this reaction could be well fitted with the pseudo-first-order kinetic model by R 2 equal with 0.973 and 0.988 in turn for COD and TPH. The expenditures to treat 1 m3 of the CA-DAF was estimated at 5.335 us$ via cost analysis. Finally, the collected findings indicate that CA-DAF integrated with UV/H2O2/ZnO can be an efficient approach in the MWF wastes disposal or treatment for reuse.

Introduction

One of the critical applied components in the machinery or metal manufacturing industries and machining operations is metalworking fluids (MWFs) as oil-water emulsions that are a type of the engineered materials to optimize metalworking processes such as milling, drilling, boring, grinding, and turning. Cooling and lubricating of metal parts/machinery tools interface in the mentioned processes occur by using of these fluids. Also, they flush away any small metal scraps from tools and work-pieces, as well as diminish and inhibit friction, corrosion, and rusting, that in turn, these advantages develop tool life and the ultimate nature of products [1, 2, 3, 4]. However, MWFs formulations contain chemically complex compounds such as base mineral oil, emulsifiers and surfactants, corrosion inhibitors, foam inhibitors, immense pressure and anti-weld agents, biocides (phenolic and aliphatic derivatives), alkaline reserve compounds, factors affecting friction, and some unknown or toxic materials [5, 6, 7, 8]. All of these compounds cause that spent MWF is identified as the primary origin of oil and hazardous sewages in the metal or machinery activities with strong COD, about 104–15 mg l−1 [9,10]. Besides, the existence of all these toxic and hazardous compounds and microbial agents (fungi and bacteria) makes MWFs wastewater hazardous and non-biodegradable, and are related with health issues include allergies, respiratory diseases, skin diseases, and other diseases especially cancer [5, 9, 11]. Worldwide, the volume of the produced MWFs wastewater is approximately 20, 000, 000 m3 annually, which is achieved by diluting the early used MWFs [7, 12, 13, 14]. Due to hazardous nature and having relative toxicity to aquatic flora and fauna, the disposal alternatives for these oily wastes are costly [15]. Furthermore, the chemically complex compositions of MWFs are private trade information for MWF manufacture, which the disposal of MWFs makes more difficult [5, 12]. However, thanks to the consequences of thermal deterioration, particulate contamination, and biological contamination, it is needed that these emulsion fluids be replaced periodically. Thus, an awful lot of hazardous liquid is generated, which involves purification [16].

The traditional disposal procedures for MWFs wastes can be hydrothermal oxidation [17], evaporation, thermal splitting [4], landfilling with soil and underground water pollution potential, and incineration that emits a lot of air pollutant such as NOx, SO2, and HCl [7]. Also, other physical, chemical and physicochemical methods for disposal of MWF wastewater can be Microfiltration [18], distillation [19], adsorption [20, 21], coagulation and flocculation [10], flow equalization, gravity separation of free oil, electrocoagulation, clarification/filtration for oil removal, chemical emulsion breaking, chemical stabilization, electrocoagulation, coagulation integrated with dissolved air floatation (DAF) [12, 22], membrane processes [23, 24], that only transfer pollutants from one phase to another without disappearing them [25]. Besides, owing to the existence of surfactants and co-surfactants, the conventional treatment procedures are not just enough for an MWF emulsion [26]. The biological processes in bioreactor based systems for MWF wastes treatment as an individual or coupled with other methods has been evaluated by several researchers [2, 3, 4, 7, 14, 27, 28]. Since the each of biological treatment approaches needs a strong concentration of organic substances along poor low molecular weight and BOD to COD proportion more than 0.4 (a standard rate), the toxicity of these fluids is a downside for applying these processes as a pretreatment or even single treatment [8, 9, 10]. The Ozone-based advanced oxidation processes (AOPs) [29], heterogeneous photo-catalysis by applying semiconductors as TiO2 [30], ZnO as well as homogenous processes such as Fenton reaction have advantage of the fast oxidation of pollutants to harmless end products especially when is motivated by ultraviolet (UV) radiation [9, 16, 25]. Recently, combining processes as hybrid techniques for MWF wastewater treatment has been investigated by many researchers such as incorporating biodegradation and advanced oxidation process [13], hybrid biological, electron beam and zero-valent Nano iron [28], the DAF followed with the photo-Fenton process and other DAF-based hybrid approaches [8, 22, 31]. These integrated approaches revealed the excellent results of final effluent quality in compliance with the environmental standards and are the best options for the complex and hazardous wastewaters.

Among them, the air floatation by separating low-weight particles and oils from wastewater and producing minor sludge volume is more effective and quicker than the precipitation. Furthermore, depends on main operating parameters like separation time and pressure, chemical additive dosage, diameter and size of the gas bubbles, and saturation time, the organic substances removal via this manner could be more than 98% [22, 32, 33, 34].

AOPs definitely are depended on the creation of highly oxidizing hydroxyl radical (OH•) which can oxidize a wide range of the non-biodegradable or difficult degradable compounds. UV/H2O2/ZnO process as a type of heterogeneous photo-catalyst processes degrades organic pollutants in wastewater through oxidants agents (e.g., H2O2 and radicals). The generation pathways of OH• and other radicals could be as Eqs. (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12), (13), and (14) [35, 36, 37, 38].

The privilege of ZnO compared to conventional catalysts such TiO2 is more significant fraction absorption of the UV spectrum, as well as also the pertinent threshold of Zinc oxide is 425 nm [35, 37].

The study's goal was the investigation of TPH and COD removal capacities from pilot-scale UV/H2O2/ZnO reaction as a lab-scale post-treatment reactor in follow of a full-scale CA-DAF system. The individual objectives for the research with regard to TPH and COD as treatment indexes were as: (1) characterizing reagent concentrations (ZnO, H2O2) and the primary pH value effects on both COD along with TPH removal as well as the biodegradability trend, (2) the synergistic impact of ZnO, hydrogen peroxide, and ultraviolet (UV) radiation at the optimum mode, (3) the organic compounds and intermediates analysis; the reduction kinetics of COD, as well as (4) the cost estimation for operating the reaction at the optimum mode.

Experimental

Raw wastewater

The water contaminated by MWF was compiled throughout a machining industry located at the Middle East. The main features of this wastewater were darkness, toxic, and carcinogenic nature depended on the identified matters in follow of this study. It contains the given physicochemical parameters in Table 1 along with standard deviation (SD) of the achieved data.

Table 1

Characteristics of MWF wastewater [8].

Parameter	TOC	TPH	COD	pH	Oil & grease	BOD5	BOD5/COD	Turbidity	Suspended solids
Unit	mg/l	mg/l	mg/l	-	mg/l	mg/l	-	NTU	mg/l
Rate	5800 ± 50	3200 ± 20	35000 ± 50	9	15500 ± 50	280 ± 10	0.008	710	5700 ± 20

Chemicals and reagents

The species of OH radicals were supplied by Hydrogen peroxide (H2O2; 30%, Catalog Number 1085979025) as well as Zinc oxide (ZnO; 99%, CAS Number 1314-13-2). The pH change of wastewater medium into the pilot also was carried out via adding sulfuric acid (H2SO4; 98%, CAS Number 7664-93-9) and sodium hydroxide (CAS Number 1310-73-2). N-pentane 99% (CAS Number 109-66-0) was used in the sample preparation for GC-MS test. COD over measuring issue influenced by H2O2 in the taken samples resolved from MnO2 addition(CAS Number 1313-13-9) to samples [8, 39, 40].

CA-DAF unit

The MWF wastewater purification was carried by applying a CA-DAF unit located at a chemical purification unit operated at batch mode as well as periodical discharge. The chemical additives dosage applied for 7 m3 of MWF stream is given in Table 2.

Table 2

The chemicals applied for CA-DAF process per 7 m3 of MWF swage undergoing purification [8].

Al2 (OH) nCl 6-n (coagulant)(kg)	Micronized CaCO3(kg)	NaOH(kg)	Poly-aluminum chloride(flocculant) (kg)	H2O2(kg)
82	14	17.5	0.05	2.65

UV/H2O2/ZnO process

PC pilot set-up

The UV/H2O2/ZnO process was carried out in a lab-scale unit utilized with batch-mode, as displayed in Figure 1. This section was comprised of a 1-L volume photochemical reactor-a packed plexiglass container with a low-pressure mercury-vapor UV lamp (20W) with a quartz glass sleeve along with a steel shield. It was jointed to a 5-L size Plexiglas recirculation tank enclosed by a water jacket container to adjust the target temperature (26 ± 2°C). The recirculation tank was armed with a mechanical mixer (485 rpm) to mix the tank volume of 3.5 L. Accordingly, the pilot volume of swage undergoing purification regarding the recirculation pump and fittings content (0.8 L) calculated as 5.3 L. The recirculation flow rate (Qr) between the photochemical reactor and the recirculation tank was adjusted at 10 l min−1 using a recirculation pump. A collection of valves was employed aims to the flow rate harness and sampling via the pilot as well as avoiding the current comes back through the pump to the tank [8].

Figure 1

The pilot-scale UV/H2O2/ZnO process followed by the full-scale CA-DAF unit.

UV/H2O2/ZnO reaction

The DAF effluent (5.3 L) was injected into the mixing tank and then recalculated (between the mixing tank and the photochemical reactor) by a centrifugal pump into the system. The operating mode for pre-test run was regarded as (1) the recirculation pump was turned on (at a Qr that was ago indicated in wastewater volume), and the early pH of the solution was adjusted at 9, (2) the UV lamp along with the mechanical mixer turned on, as well as the reaction medium of 0.4 g l−1 (200 ml) ZnO was added, (3) the timing (100 min total) of this run was begun along with the starting of slow adding H2O2 (at a rate of 0.85 ml/min for 60 ml total), (4) at the end of reaction, double samples (each 30 ml) were taken for the sampling time (5) for the next runs, one-at-a-time method was applied to obtain the optimum conditions of treatment to the optimum variables (pH, ZnO, H2O2). All of the experiments were done with the equal concentration of influent.

Analytical methods

To analyze TOC, TP, BOD5, and oil in the samples, the methods were adopted from the standard method protocols [41]. For the COD measurement, the initial as well as treated sewage samples were spectrophotometrically measured by using Hack Lange cuvette test (Hack Lang, Dusseldropf) and tested with a Hach Lange DR 5000 Model spectrophotometer (Hach Lange, Dusseldropf) [8]. The pH control and ORP monitoring were done by WTW pH 330i pH & Redox messgerät im Koffer. The total dissolved solids (TDS) and the electrical conductivity (EC) were achieved by WTW Cond 330i Conductivity Meter. The total organic carbon (TOC) was analyzed by a Shimadzu TOC-5000A TOC analyzer [42]. In terms of the oil analysis, samples extracted by Freon solvent (1,1,2-trichlorotrifluoroethane; 99%, CAS Number 76-13-1) and standards (from 0 ppm to 100 ppm) for quantification were ultimately analyzed by Shimadzu IR-470 device [8, 43]. The TPH measurement of samples from sampling and holding to gas chromatography (GC) trace was done in accordance with TNRCC-1005 with some modifications [8, 44]. In order to this, the prepared samples were eventually injected to A7890/C5975 GC–MS system (Agilent, USA) armed with a CP-Sil 5 CB capillary column (30 m long, 0.25 mm ID, and 0.25 mm film). The TPH standardization was done with unleaded gasoline, and the correlation for the attained calibration curves was 0.99 as Eq. (15) [8].

TPH content of the taken samples was estimated from Eq. (16) [8, 44].

Cost estimation

The power usage was computed applying Eq. (17) evolved by Bolton and Stefan [45]:Where, EEO: reformed Electrical Energy per experiment run that is the energy in kWh required to reach 90% devastation of pollutants in 103 L of polluted stream, P: lamp power in kW, t: irradiation time in min, V: volume of the treated wastewater in liters, Cinf: influent COD concentration in mg l−1, and Ceff: effluent COD concentration in mg l−1 [8].

Results and discussions

The CA-DAF effluent nature after the operation at the condition named in above is given as Table 3. By understanding the nature of effluent in this stage of purification, the research was conducted towards UV/H2O2/ZnO experiments operated as a lab-scale batch-mode.

Table 3

The physicochemical treatment parameters metered in CA-DAF and UV/H2O2/ZnO effluents, and reduction yield to. Here, a: by running CA-DAF unit, b: by running the optimum UV/H2O2/ZnO process, and c: total removal yield.

Parameter	Unit	Value		Removal efficiency %
		A	B	A	B	c
pH	-	7.8	10	-	-	-
COD	mg/l	506	45.54	98.5	91	99.87
BOD5	mg/l	110	18	61	83.63	93.57
BOD5/COD	mg/l	0.217	0.395	-	-	-
TOC	mg/l	143	39.58	97.5	72.32	99.3
TPH	mg/l	958	67.06	70	93	97.9
Oil	mg/l	35	6.1	99.77	82.57	>99.9
Turbidity	NTU	29.5	4.73	95.8	83.96	99.33
T-P	mg/l	0.09	<0.01	-	>88.8	-
TDS	mg/l	2065	3450	-	-	-
ORP	mV	27	286	-	-	-

MWF wastewater analysis

The MWF used in the mentioned industry was semi-synthetic one, so that, it's the FTIR spectrum is given in Figure 2. Semi-synthetic MWFs are a kind of water-based MWFs which generally include 2 to 50 percent of mineral oil and also substances applied in synthetic MWFs [46]. The main compounds and functional groups regarding the FTIR spectrum and the raw sample's chromatogram utilized by GC-MS were already detected and by concerning on Figure 2 are as Phenols and Alcohols, Alkanes, Alkynes, Alkenes, Aromatic compounds, cyclic amines, aliphatic amines, and Alkyl Halides. Therefore, the low degradability of the MWF wastewaters could be the result of the presence of these compounds.

Figure 2

The FTIR spectrum peaks of the used MWF: Phenols and Alcohols (wavenumber of 3401 cm−1), Alkanes (wavenumber of 2926 cm−1, 2857 cm−1, 1457 cm−1 and 1374), Alkynes (wavenumber of 2085 cm−1), Alkenes (wavenumber of 1640 cm−1), Aromatic compounds (wavenumber of 1559 cm−1 and 1403 cm−1), Cyclic amines (wavenumber of 1339 cm−1), Aliphatic amines (wavenumber of 1073 cm−1 and 1027 cm−1), Alkyl halides (wavenumber of 571.799 cm−1).

Impact of the initial pH and reagents

The pH value is one of the most effective parameters on the PC processes. Accordingly, the impact of pH on the DAF effluent treatment was checked out in various pH values from 2.5 to 11 throughout the seven times charge of the pilot-scale unit along with the other fixed condition, and the related results are shown in Figure 3. It has been obtained that the COD and TPH removal efficiency rose and their residual concentrations dropped gradually by arising pH so that the best performance of the applied process was attained in the neutral to alkaline conditions. Such that, the maximum removal rates of 64% and 69% were achieved at pH 10 for the COD and TPH that was in similar with some studies carried out by Wang, Chen, and Akyol [47, 48, 49]. Due to the role of various factors, the explanation of pH impact on the removal yield of the applied PC process is a complicated work. As shown in Figure 3, the lowest removal rates of 28% as well as 30% respectively for COD and TPH is observed in acidic pH to neutral (especially pH 2.5) where the photo-corrosion oxidizes metal oxides (in this case ZnO) to ionic form (Zn2+) as leaching throughout Eq. (1). Hence, the leaching is the cause of the inactivated catalyst at the acidic pH, and subsequently, the dropping of the removal efficiency in follow of Krishnakumar's study. The leaching phenomenon is lowest at the alkaline pH [50]. The enhancement of Zn2+ concentration in the reaction medium is well proved from XRF of the formed sludge in the solution with the high weight percentage of ZnO, as shown in Figure 4. Furthermore, a drop in the removal of COD and BOD in acidic conditions can be due to the reduction of PC degradation (PCD) simultaneously with very high adsorption at low pH. This circumstance can occur when the catalyst surface is coated with petroleum hydrocarbons-where adsorption of UV radiation also is decreased on the catalyst. The generation of anions at higher pH via Eqs. (9) and (13) facilitates photo-generation of considered as main oxidizing species responsible for the PCD of the many organic and toxic substances [51]. However, in very alkaline conditions (here pH 9 to up) owing to the release of the H2O2 from the reaction solution and reducing of OH from some paths such as Eqs. 4, 5, 9, and 14 as the result of that, the COD and TPH removal efficiency in the reaction solution is reduced. Besides, as the zero-point charge (zpc) for ZnO is 9–10, and Zinc oxide surface is positively charged below pH 9 and above this value, the surface is negatively charged by the adsorbed anions, while the presence of an awful lot of the ions on the catalyst surface as well as in the reaction solution helps the construction of OH [35, 50]. On the other interpretation, Wang described the amphoteric effect of ZnO as it can be dissolved in the beyond of neutral pH towards both alkaline and acidic conditions. Therefore, in this work, at the lower pH values (4 and 2.5) due to the dissolution of ZnO, the active catalyst is decreased severely, so that results in the PD deterioration [47]. Also, concerning the presence of phthalate in MWF wastewater that will be discussed in the following section, Chung proved pH 10 even can be the optimum for TiO2 based PD [52].

Figure 3

The impact of pH on the reduction rate of the COD and TPH, as well as the residuals; ([ZnO] dose = 200 mg/l, the primary COD concentration = 506 mg l−1, primary TPH concentration = 958 mg l−1, solution volume = 5.5 l, reaction time = 100 min, temperature = 26 ± 2 °C, H2O2 = 45 ml (9.027 g), weight ratio = 17.94, weight ratio = 9.476.

Figure 4

The XRF of the trace sludge produced in the natural to alkaline condition and overcoming Zn2+ than other ions.

By fixing pH in 10, the effect of the ZnO-photocatalyst loading on this process was investigated in 5 concentrations ranges as 100, 200, 400, 600 and 800 g l−1 to optimizing of the catalyst dosage and the results are given in Figure 5. Among them, 600 g l−1 of ZnO with 83% and 85% removals respectively to COD and TPH appeared the highest removal rates. In the concentrations lower than 600 mg l−1, removal rates are a function of the added catalyst amount and are increased spontaneously by raising the catalyst amount and consequently developing the OH• generation rate. This trend can be due to an increment in the number of active places on the photocatalyst surface which spontaneously raises the amount of OH• and the superoxide radicals into the reaction solution. In catalyst quantities higher than 600 mg l−1, the reduction rate will be reduced slightly. Therefore, it can be interpreted that the UV penetration is fell off by increasing the reaction solution turbidity created by catalyst particles that consequently limits the efficiency of this process at the surplus ZnO amounts. This evident interpretation coincides with Chakrabarti's study and others [49, 51].

Figure 5

Influence of catalyst loading on the removal rate and residuals of COD and TPH. Reaction conditions: pH 10, the primary COD and TPH concentrations in turn 506 mg/l and 958 mg l−1, 5.5 l solution volume, 100 min reaction time, 26 ± 2°C temperature, 45 ml (9.027 g) H2O2, the weight ratio = 17.94, weight ratio = 9.476.

By fixing pH and the ZnO loading, the study was directed toward evaluate hydrogen peroxide effect on COD and TPH reduction rates that the obtained results are shown in Figure 6. For this aim, five concentrations scopes of 5.04, 9.027, 13.11, 17.147, and 20.17 g l−1 H2O2 as the volumes 25, 45, 65, 85, and 95 ml were considered in this step of the survey. The best reduction efficiency was acquired at 13.11 g l−1 or 65 ml with 91% and 93% for COD and TPH, respectively. Without any adding H2O2 and only with 600 mg l−1 of ZnO and UV irradiation in during 100 min of the reaction were not attained removal rates better than 46% and 47% for COD and TPH. Only by adding 25 ml of the oxidant, the removal efficiency for these parameters was reached to 70% and 72%, while beyond of the optimum amount as 65 ml or 13.11 g l−1 in concentration decreased the process efficiency with the increase of oxidant concentration. This procedure can be due to the destructive function of H2O2 on OH• as a scavenger throughout Eqs. (5) and (6), where was mentioned by Peternel [37]. After the latest step in the optimization of this process, the residual amounts of some physicochemical treatment parameters such as COD, BOD, TPH, TOC, etc. were attained as Table 3, and each one has been compared with the initial value. Apart from this, the sizeable SDs was not found from the data.

Figure 6

The H2O2 concentration influence on the reduction rates of COD and TPH and their residuals. The experiment conditions were as pH 10, 600 mg l−1 ZnO, the retention time of 100 min, the primary COD and TPH concentrations in turn 506 mg l−1 and 958 mg/l, 5.5 l solution volume, 26 ± 2°C temperatures.

The synergist effect of ZnO, H2O2, and UV

The optimum condition for the UV/H2O2/ZnO process was evaluated for the synergistic influence of which component on the COD and TPH removal, separately. The COD and TPH reduction rates just with UV/H2O2 and 13.11 g l−1 or 65 ml H2O2 was attained as 31% and 33%, in order. In follow, the H2O2/ZnO reaction had 33.8% and 35% efficiencies for these parameters. Finally, maximum reduction efficiencies were observed in the UV/ZnO reaction via 58% and 61% reduction for COD and TPH in the solution. These documents disclosed that UV/ZnO combination is a decisive reaction in the UV/H2O2/ZnO process. This result could well be interpreted from equations of (1) and (2) and the reaction of with organics generated as the result of the photo-inducing of the catalyst.

Organic substances and intermediates analysis

In a BOD5/COD (B/C) proportion more than 0.4, the refractory wastewaters could be biodegradable easily [8, 53]. The raw sewage was immensely non-biodegradable with a B/C proportion of 781–833 × 10−5, which improved to 217 × 10−3 via the CA-DAF unit. Since then, eventually, this ratio reached a favorable value of 395 × 10−3 after the optimized UV/H2O2/ZnO. To analyze the biodegradability improvement obtained dramatically after the applied integrated CA-DAF and UV/H2O2/ZnO, the effluent samples were injected to GC-MS. Therefore, the primary organics were detected and illustrated through the chromatogram of Figure 7, and then listed as Table 4. These outputs include strain-chain paraffin (45%), alcohol (5%), aromatics (25%), and other hazardous organics (25%) compared with GC-MS data amassed in the NIST library as well as via the same ones. These compounds, as GC-MS analysis outputs formerly were identified by the raw MWF wastewater FTIR analysis in Figure 2, generally. By the detailed GC-MS report, it was deducted that low biodegradability of CA-DAF effluent and even untreated wastewater can be due to the dominant aromatics consisted of alpha-terpinene (0.14%), 2,6-bis (1,1-dimethylethyl)-4-ethyl phenol (0.32%), bis (2-ethylhexyl) phthalate (0.31%), and mono (2-ethylhexyl) phthalate (99.12%) in accordance with Amin's study and Wei's research [8, 53], which are non-biodegradable and toxic, and could affect the biological units of wastewater treatment. All of the organics detected in Figure 7 and listed in Table 4 were averagely removed 78.46% as mineralization rate in the final effluent from the optimized UV/H2O2/ZnO process. This reduction yield is evident in Figure 8 from the area of each compound's peak. Among these matters, 2,6-bis (1,1-dimethylethyl)-4-mthyl phenol, bis (2-ethylhexyl) phthalate, and mono (2-ethylhexyl) phthalate with removal efficiencies of 67%, 68%, and 75% remained in final effluent in the trace amounts. The reason of low removal rate for phenol can be the PC conversion of phthalate, benzene, benzoic acid, and other aromatics to phenol as the intermediate into solution during the reaction, so that, this issue is well mentioned by Chung, Huang and other researchers [52, 54, 55, 56, 57]. The photo-catalytic conversion of bis (2-ethylhexyl) phthalate and mono (2-ethylhexyl) phthalate to 2,6-bis (1,1-dimethylethyl)-4-mthyl phenol is well illustrated in Figure 9. Also, Eqs. (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12), (13), and (14) can be understood from the reaction medium by Figure 9. The proposed degradation and mineralization pathways of phthalate and phenol are shown in Figure 9. All of the detected compounds in CA-DAF system effluent were comparable with MWF formulations early discovered by Dihora et al. [58].

Figure 7

The GC–MS chromatogram of CA–DAF effluent (e) with the major peak belonged to Phthalate compound, figures (a)–(d) respectively are the abundance of m/z for Phenolic matter detected by GC-MS analysis (a); the matter detected by libraries of NIST (b) 1, 2 benzenedicarboxylic acid, mono (2-ethylhexyl) ester; WILEY (c) 1, 2-benzenedicarboxylic acid, Diisooctyl ester; and PEST (d) 1, 2-benzenedicarboxylic acid, bis (2-ethylhexyl) ester. All of the compounds detected in the CA-DAF effluent are listed in Table 4 [8]. The right-hand column is related to the abundance of various m/z value (mass to charge ratio) in ionization and its similarity for (a).

Table 4

The organic matters detected in the CA-DAF unit effluent and the optimized UV/H2O2/ZnO system effluent by GC-MS analysis [8].

	Retention Time (min)	Boiling point (°C)	Formula	Chemicals	Similarity(%)	DAF effluent	Area (105) UV/H2O2/ZnO effluent	Removal (%)
1	5.739	174	C10H16	Alpha-Terpinene	86	0.78345	ND	100
2	10.286	272	C16H26O	Phenol, 2,6-Bis (1, 1 – dimethylethyl)-4-mthyl	98	1.759	0.58	67
3	10.754	243	C11H24O	1 –Undecanol	35	0.05663	0.00623	89
4	11.281	254	C16H34	Hexadecane	53	0.50204	0.1	80
5	11.838	302	C17H36	Heptadecane	43	0.23485	0.01644	83
6	12.480		C21H14FeN2O3	iron, tricarbonyl (N-(phenyl-2-pyridyl methylene) benzenamineN,N)	59	0.66645	0.1866	72
7	13.535	317	C18H38	Octadecane	93	0.21103	0.06753	68
8	14.59	330	C19H40	Nonadecane	73	0.52329	0.16222	69
9	15.689	343	C20H42	Eicosane	87	0.58189	0.1513	74
10	18.102	369	C22H46	Docosane	83	0.62866	0.0691	89
11	19.402	391	C24H50	Tetracosane	50	0.26959	0.0835	69
12	20.734	432	C27H56	Octacosane	72	0.69644	0.08357	88
13	22.172	400	C24H38O4	1,2-Benzenedicarboxylic acid, bis(2-ethylhexyl) ester	91	1.72665	0.55258	68
14	22.714	409	C16H22O4	1, 2-benzenedicarboxylic acid, mono(2-ethylhexyl) ester	91	549.18425	137.296	75
15	24.774	474	C33H68	Tritriacontane	45	0.64618	0.09	86

ND: non-detected.

Figure 8

The GC–MS chromatogram of UV/H2O2/ZnO process effluent under optimized conditions (e) with the peak belonged to the residual Phenolic matter. Figures (a)–(d) respectively: (a) the abundance of Phenolic compound detected by GC-MS analysis; and the percentage of their similarity with the matters detected by libraries of NIST (b) Phenol, 2,6-Bis (1, 1 –dimethylethyl)-4-mthyl, WILEY (c) Phenol, 2,6-Bis (1, 1 –dimethylethyl)-4-mthyl; and PEST (d) 4-Methyl-2,6-Bis (1,1-Dimethylethyl) Phenol. The right-hand column is related to the abundance of various m/z value (mass to charge ratio) in ionization and its similarity for (a).

Figure 9

The PC conversion of phthalate, benzene, and benzoic acid to phenol in UV/H2O2/ZnO reaction and probable mineralization pathways.

Kinetic study and cost analysis

By considering the heterogeneous content of the used wastewater and also the complexity intermediates produced during UV/H2O2/ZnO reaction that evolve inoperative a precise kinetic study of dissipate extraordinary reactions appearing within chemical oxidation, the PD reaction kinetics of COD as well as TPH can be interpreted by a modified Langmuir-Hinshelwood model in accordance with the presented works [59]. The relationship between COD and TPH concentration is illustrated via the kinetic model as Eq. (18), where, k is the inherent rate coefficient (M min−1), t is the time of irradiation (min), and K is the adsorption equilibrium constant of COD and TPH on the photo-catalyst surface (M−1). When the adsorption is relatively low as well as the concentration of organics is low, the Eq. (18) could be modified to pseudo-first-order kinetics with an obvious constant (k) as Eq. (19). Following, it was resulted that the simple and complex organic matters can be vanished within the one hundred minute of the PC degradation in the dissipate pH quantities could be expressed as pseudo-first-order kinetics in COD and TPH concentrations, and dependable information in Figure 10. The achieved kinetics the reaction were well fitted with R as 0.973 and 0.988 for COD (Figure 10a) and TPH (Figure 10b), respectively. The used kinetic model indicated that the reaction rate would be restricted just through OH• in the reaction medium as a function of the initial pH quantity [8, 60]. In regard to the Eq. (19), the rate constant for the reaction is given in Figure 10 for the COD and TPH in each pH value. Therefore, it is assumed that the COD and TPH decomposition follows pseudo-first-order kinetics throughout Eq. (19). A plot of –Ln (C/C0) versus reaction time (t) yields a straight line, and the slop is obvious rate constant (k). Here, C and C0 are initial COD and TPH concentrations of the MWF wastewater or DAF effluent and final COD and TPH concentrations after t min of reaction [53, 60, 61].

Figure 10

Pseodue first-order plot versus time for COD (a) and TPH (b) removal for each pH, and the obtained pseodue first-order rate constant (k). The fixed experimental conditions: 0.5 g l−1 ZnO, 50 ml H2O2 (1.6323 mol or 55.476 g), 100 min retention time (t), 26 ± 2 °C temperature, 506 mg l−1 initial COD, 958 mg l−1 initial TPH, weight ratio = 19.933, weight ratio = 10.528.

By considering rate constants attained in the applied kinetic model that indicated in Figure 10, the pseudo-first-order rate constants are deteriorated beyond of pH 7 to 10. Additionally, the reaction times more than 100 min due to low decomposition hardly would be practical in a sight of the economic aspect.

Here, a cost analysis was carried out for the optimized UV/H2O2/ZnO to calculate the quantity of the required chemicals, and electrical energy via commercial price and Eq. (17), respectively. Thus, EEO for UV lamp was obtained as 6kWh m−3. The basic price of the chemicals utilized into the pilot as well as the applied power was as Table 5. Also, the UV/H2O2/ZnO expenditures for 1 m3 of CA-DAF effluent is shown in Table 5. Overall, the total cost for this heterogeneous PC treatment according to Table 5 will be achieved as 5.335 $ per a cubic meter of CA-DAF effluent that is very economical as the second part of this hybrid treatment procedure.

Table 5

The implementation costs for the investigated UV/H2O2/ZnO in the optimum mode.

Process constitution	Unit cost ($)	Consumed in 5.3 L (pilot volume)	Costs ($ m−3)
UV (kWh−1)	0.05	6	0.35
ZnO (99%) (kg)	2.35	0.0033	1.41
H2O2 (30%) (kg)	1.5/kg	0.01311	3.575

Conclusions

In this research, in follow of the evaluation of the integrated MWF wastewater treatment approaches were studied combining a CA-DAF system with a heterogeneous UV/H2O2/ZnO process around one of the industries located in the Middle East. The goals of this investigation were obtained as:

The optimum conditions achieved as pH 10, 600 mg l−1 ZnO, 13.11 g l−1 H2O2. The role of ZnO reaction can influence the pH values lower and higher than the optimum range (7–10). Since the ZPC for ZnO is 9, the higher and lower pH values can harness OH• in the reaction medium. Moreover, H2O2 can be released from solution at the very alkaline condition. H2O2 is effective in amounts lower than 13.11 g l−1 to effective generation in OH•, while in higher values can be a scavenger for OH•. The catalyst amount beyond the optimum value could be the cause of turbidity and limits UV radiation penetration consequently.

By evaluating the synergistic effect, it has resulted that the influence of process components could be as UV/ZnO > H2O2/ZnO > UV/H2O2. Hence, the UV/ZnO is determinative in this heterogeneous PC reaction as many of researchers already were pointed out in the previous studies.

By intermediates analysis, it was disclosed that a phenolic structure could be the product of phthalate degradation in the reaction medium. Additionally, it is recommended that Phenol, 2,6-Bis (1,1–dimethylethyl)-4-mthyl can be monitored as a 1,2-Benzenedicarboxylic acid, bis(2-ethylhexyl) ester, and 1, 2-benzenedicarboxylic acid, mono(2-ethylhexyl) ester photo-catalytic can be destruction by-product in MWF wastewater treatment. The producing of phenol by benzene and phthalate was mentioned via some studies previously.

This photo-catalytic reaction was compatible with pseudo-first-order kinetics. Since the reactants and intermediates number is high in the reaction medium, this observation could be due to the complexity of MWF wastewater constituents and the achieved kinetic plot [62].

The total implementation costs for one cubic meter of CA-DAF effluent treatment by using this process was estimated as 5.335 US $ with the consumed energy equal with 1090.9 kWh, which can be economical remediation.

The hazard of the MWF wastes for human health has been proved by many researchers [63]. Therefore, the environmental disposal of these hazardous wastes involves safe treatment as well as safe reuse. Hence, among the various hybrid remediation ways, the applied and integrated processes may be the economical and eco-friendly alternative along with very high efficiency. However, it is suggested that to complete this investigation, some experiment as UV/H2O2/ZnO effluent bioassay or enzyme assay for characterizing of final current toxicity at the optimized conditions could be performed. Also, the effect of anions and cations on the heterogeneous reaction and the produced intermediates, the aeration influence on the process efficiency and by-products, the salinity impact on both CA-DAF and UV/H2O2/ZnO units and kinetics before a full-scale implementation, and finally analyzing fixed-bed catalyst-based technologies to improve the catalyst consumption could be another future outlooks around this survey.

Declarations

Author contribution statement

Mohammad Mehdi Golbini Mofrad: Performed the experiments; Wrote the paper.

Hamidreza Pourzamani & Mohammadreza Alipour: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Iman Parseh & Mohammad Mehdi Amin: Analyzed and interpreted the data.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.