Treating mechanical washing wastewater with iron-in-oil characteristics by changing the fate of iron

Machinery wastewater is primarily derived from the water used to wash machinery surfaces and the ground after maintenance. Washing wastewater contains large amounts of surfactants and oils (Liu et al. 2020) and is highly dispersed with a complex chemical composition containing significant amounts of iron. This composition causes challenges when treating environmental discharge (Meng et al. 2012). If wastewater is improperly treated and discharged into the environment, major pollution even can occur, which can endanger human health (Cao et al. 2018). Treatment methods include gravity separation, air flotation, membrane separation, adsorption, and flocculation (Hui et al. 2015; You et al. 2018; Han et al. 2019). Flocculation is an effective method for treating oil-containing wastewater (Zhao et al. 2008; Fard et al. 2016; Deghles & Kurt 2016).

Chitosan, alum, and PAC (polyaluminum chloride) have been used as coagulants to treat residual oil and suspended solids in palm oil wastewater, achieving removal rates of residual oil and suspended solids of >95% (Iskandar et al. 2018). The combination of zinc polysilicate and anionic polyacrylamide (A-PAM) can remove 99% of the oil and leave <5 mg L⁻¹ residual suspended solids when treating heavy oil wastewater (Zeng et al. 2007). For the coagulation and foam separation of PAC and milk casein, 95% of the water-in-oil can be removed (Zhang et al. 2017). Hydrogen peroxide, combined with ultraviolet rays and TiO₂ to oxidize wastewater, yields a 72.5% COD removal rate and a 97.1% turbidity reduction rate (Hodaifa et al. 2019). However, owing to its stable water-in-oil structure, conventional oxidation–flocculation is ineffective for treating emulsified oil wastewater. Therefore, another option is to use Fe(OH)₃ produced by oxidation to promote flocculation and effectively remove oil. However, conventional oxidation–flocculation is ineffective for the treatment of emulsified oil wastewater because of the stable water-in-oil structure.

The addition of NaClO_x and CaO to oil wastewater breaks the surfactant bond between water and oil, forcing the molecules to move to the water or oil phase (Yalcinkaya et al. 2020). As a result, the water-in-oil structure is broken and the oil droplets gather and float to the water surface or settle. Meanwhile, the Fe²⁺ encapsulated in the oil droplets is released. NaClO_x generates oxygen-free radicals and oxidizes Fe²⁺ to Fe³⁺ (Wang et al. 2006). Fe³⁺ reacts with hydroxyl radicals in water to form hydroxides or polyhydroxides. The surface of Fe(OH)₃ is mostly cationic group, which can absorb negatively charged oil droplets and pollutant particles (Cañizares et al. 2007; Chen et al. 2022). These molecules have a large surface area and their affinity for water-in-oil can be leveraged for use as an effective adsorbent to remove emulsified water-in-oil (Crespilho & Rezende 2004). Based on adsorption, bridging and other processes, the size of Fe(OH)₃ increases after absorbing pollutants. As a core, Fe(OH)₃ forms a dot network with the organic polymer chain, which is conducive to the removal of pollutants (Zhang et al. 2015).

Common chemicals such as NaClO_x, CaO, and PAC have been produced on a large scale in the world, with high output and low price. Therefore, using them for industrial wastewater treatment has practical advantages and feasibility (Zheng et al. 2015; Wen et al. 2018; Darvishmotevalli et al. 2019; Ganiyu et al. 2020).

Herein, a novel treatment of emulsified oil wastewater, entitled demulsification–oxidation–flocculation, is proposed. The oil droplets were embedded in paraffin and used for section observations, and the FTIR characteristics of the wastewater and flocs were determined. The distribution of Fe in oil, water, and sediments was tracked and Na⁺, Ca²⁺, and OH⁻ concentrations were determined to clarify their effects on the residual iron and oil contents as well as COD. Based on the removal effect, the optimal ratios of NaClO_x, PAC, and CaCl₂ were determined. Finally, the difference between oxidation after acidification and direct oxidation during the newly developed oil wastewater treatment was investigated. This paper provides an effective new route for the mechanical washing of wastewater using Fe(OH)₃-floc cores produced by oxidation and demulsification to promote the water-in-oil structure decomposition and flocculant precipitation.

Washing wastewater was collected and stored at 4 °C before use. Analytically pure calcium hypochlorite, sodium hypochlorite, sodium hydroxide, polyaluminum chloride, petroleum ether, o-phenanthroline, sodium acetate, ammonium acetate, and glacial acetic acid were purchased from Sigma-Aldrich (Missouri, USA). All reagents used in the field experiments were industrial grade. The commercial active oxygen oil-removing agent AOORA-2106 (main component NaClO_x; x = 1, 2, 3, 4; hereinafter referred to as NaClO_x reagent) was provided by Shaanxi Keeping Environmental Sci-Tech Co., Ltd (Xi'an, China).

First, 150 mL of wastewater was poured into a 300 mL beaker along with 400 μL of NaClO_x and 12 mL of 12.8 g L⁻¹ PAC. Subsequently, one of four treatments was applied: (1) 200 μL of 1 mol L⁻¹ CaO; (2) 200 μL of 1 mol L⁻¹ CaCl₂; (3) 200 μL of 1 mol L⁻¹ Ca(OH)₂; or (4) 200 μL of 1 mol L⁻¹ NaOH. The pH values of (3) and (4) were adjusted to 12. The four processed samples were subjected to centrifugation at 150 rpm for 10 s and 30 rpm for 5 min. After standing for 15 min, the supernatant and sediment were collected for further measurements.

The raw and treated wastewater were filtered to obtain flocs, which were then divided into two parts. One part was dried and analyzed using FITR and the other was dyed with Sudan Red III, embedded in paraffin, and sectioned for observation.

First, 150 mL of wastewater was removed and 4, 6, 8, 10, or 12 mL of 12.8 g L⁻¹ PAC were added along with 400 μL of NaClO_x and 200 μL of 1 mol L⁻¹ CaO for Group I; 200, 250, 300, 350, and 400 μL of 1 mol L⁻¹ CaO were added along with 400 μL of NaClO_x and 12 mL of 12.8 g L⁻¹ PAC for Group II; 300, 350, 400, 450, and 500 μL of NaClO_x were added along with 200 μL of 1 mol L⁻¹ CaO and 12 mL of 12.8 g L⁻¹ PAC for Group III. Phenanthroline spectrophotometry (HJ T 345-2007) was used to determine the iron content in the raw water and treated supernatant. The samples were also extracted with petroleum ether for 3 h, and the oil content was measured by absorbance using an ultraviolet spectrophotometer. The COD was determined using the dichromate method (GB11914-89), and each data point was measured in triplicate.

For the acidification + oxidation samples, acid was added to adjust the pH to 6 and 400 μL of NaClO_x was subsequently added after standing for 12 h. Simultaneously, an equal amount of NaClO_x was added to the directly oxidized sample. The supernatant was removed every 4 h and 12 mL of PAC was added. After static sedimentation, the remaining Fe and COD concentrations in the supernatant were determined.

Under surface complexation of Fe, the oil acts as a ligand (L) to chemically combine with Fe: L + H(OH)OFe → L-OFe + H₂O (Moussa et al. 2017). By itself, Fe(OH)₃ is a good flocculant that precipitates with oil by complexation or electrostatic attraction, to remove oil from water. In addition, Ca²⁺ addition reduces the negative charge on the floc surface, disrupting the steady state and initiating coagulation. At this time, the area of the oil droplets accounted for 19.27% of the total view.

Figure 2(c) shows the floc structure generated by the addition of NaClO_x + CaO + PAC. Fe(OH)₃ colloidal particles with the same charge normally repel, maintaining a stable dispersion state, and do not effectively precipitate to become effectively removed. PAC, as a polymer inorganic flocculant, imparts a significant bridging effect on Fe(OH)₃-oil flocs (Chaprão et al. 2018). Compared with the flocculation structure produced by NaClO_x + CaO (Figure 2(b)) and PAC (Supplementary Figure S1), the orange-red oil droplets coalesced more tightly and the flocculated area increased to 19.64% of the total view.

Figure 2(d) shows the supernatant and its contents after NaClO_x + CaO + PAC addition to oil wastewater, wherein the red oil droplets were significantly reduced compared with Figure 2(a), with the oil area accounting for only 0.8% of the total view for a removal rate of 98.92%. Except for a few red oil droplets, blue fluorescent substances were observed. In the context of the literature, it was inferred that the excess Ca²⁺ and CO₂ gas formed CaCO₃ crystals, which emitted blue fluorescence under UV light (Qin et al. 2021). Therefore, a high-density sedimentation tank was suggested to be added at the outlet of the coagulation tank, supplementing the insoluble medium particles. In addition, seed crystals could be added to accelerate the growth of CaCO₃ crystals and allow CaCO₃ gravity sedimentation and adsorption to promote oil removal effects.

As mentioned in Section 3.1, after the structure of the water-in-oil emulsified layer was destroyed, the encapsulated Fe²⁺ was released and oxidized by the active oxygen-free radicals released by NaClO_x to form a Fe(OH)₃-floc core and precipitate. The Fe content of the original wastewater was 3.8 mg L⁻¹, whereas the residual Fe concentration after CaO treatment was 0.185 mg L⁻¹ for a removal rate of 95.13%. After other treatments, the remaining iron was approximately 0.2 mg L⁻¹, with the iron removal rate reaching 94%. Therefore, the Fe removal mechanisms of Ca(OH)₂, NaOH, and CaCl₂ were comparable, but the addition of CaO induced significantly different effects that improved the iron removal efficiency considerably. Although CaO reacted with H₂O to generate Ca(OH)₂, it formed a passivation layer that inhibited further contact between CaO and H₂O, generating less OH⁻ than Ca(OH)₂ at the same molar concentration of Ca²⁺. This was to use NaOH to supplement pH to 12, and discuss whether the influence of Ca(OH)₂ on flocculation was determined by the increase of pH. The result is that at higher pH, the PAC flocculation effect and oil droplet removal effects diminished (Kim et al. 2001; Munirasu et al. 2016); thus, the former flocculation effect was better than the latter. The iron removal efficiency of CaO is significantly higher than that of Ca(OH)₂.

The R–Cl bond gave rise to a moderate absorption peak at 750–700 cm⁻¹ (Deng et al. 2016). After NaClO_x was added, it reacted with alkanes to form R–Cl in the following order: NaClO_x = NaClO_x + CaO > NaClO_x + CaO + PAC. The three produced flocs showed an –OH intermolecular association vibration band at 3,500–3,000 cm⁻¹, and the peak intensity order was NaClO_x + CaO + PAC > NaClO_x + CaO = NaClO_x. This indicates that metal hydroxide Fe(OH)₃ precipitates were formed, which can serve as floc nuclei, facilitate adsorption of surrounding pollutants, and promote floc growth (Atesok et al. 1988). When NaClO_x was added, the iron bound to the oil was released and oxidized to form hydroxide. When PAC was added, the iron hydroxide colloid settled in the flocs. Meanwhile, the halogenated hydrocarbon was also partially converted to alcohol resulting in the NaClO_x + CaO + PAC treatment producing the strongest peak. This also indicated that NaClO_x + CaO + PAC transformed more halogenated hydrocarbons to alcohol, explaining the low halogenated hydrocarbon peak for NaClO_x + CaO + PAC. The flocs showed a strong peak at 1,750–1,600 cm⁻¹ originating from aldehyde groups (Voort et al. 1994) with similar peak intensities for all three flocs. The peak at 1,200–1,000 cm⁻¹ arose from the out-of-plane vibration of cis-disubstituted olefins (Rohman & Man 2010).

As shown in Figure 5(b), the acidic wastewater also exhibited three peaks in the 3,000–2,843 cm⁻¹ range, corresponding to saturated alkane functional groups. However, compared with direct oxidation, the peak signal of the acidification oxidation sample was significantly weaker, indicating an improved oil removal effect. At low pH, the polar groups of the surface-active components cause sufficient electrostatic repulsive interactions to destroy the cohesion of the interfacial membrane (Wong et al. 2015). After acidification and oxidation, the hydrophobic bonds in water were greatly reduced, and the hydrophobic groups were directly oxidized by the acid and converted into hydrophilic aldehyde groups. However, the peak intensity difference in the flocs was marginal, indicating that the change in functional groups after acidification oxidation was the same as that of direct oxidation, and the oil removal mechanism was compatible. Ultimately, acidification destroyed the water-in-oil structure.

Based on the addition of 450 μL of 10% NaClO_x and 0.1 g PAC, the residual iron and oil contents showed an overall upward trend with increasing CaO loading. CaO addition increased the pH and affected the flocculation efficiency of PAC. Lower pH accelerates PAC hydrolysis into positively charged polynuclear hydrolysis products [Al₁₃(OH)₃₂]⁷⁺, which preferentially adsorb contaminant colloids and neutralize the zeta potential (Sun et al. 2017), conducive to coagulation. When the pH was approximately 7, Al(OH)₃ was the primary species involved in the sweeping mechanism. However, at a high pH, hydroxide (OH⁻) increased the negative charge of the contaminants, which inhibited flocculation (Sillanpää et al. 2018). Therefore, charge neutralization or contaminant sweeping can be achieved in the presence of small amounts of CaO (approximately 200 μmol).

Based on the addition of 200 μmol CaO and 0.1 g PAC, increasing NaClO_x loadings were tested, and the remaining iron initially increased and subsequently decreased, while the remaining oil content decreased and then increased. With the demulsification of Ca²⁺ and oxidation of ClO_x⁻, the water-in-oil structure was destroyed. It decomposed into slick and dispersed oil and released Fe²⁺. At smaller loadings of NaClO_x, only the water-in-oil structure could be destroyed and continuously released Fe²⁺, resulting in increased residual iron content. Here, the free water between the oil droplets was removed, which was conducive to oil coalescence and captured by the PAC. Therefore, with increasing NaClO_x concentration, the amount of residual oil in the supernatant decreased. When the amount of NaClO_x exceeded 400 μL, excess active oxygen-free radicals destroyed the water-in-oil structure and oxidized the released Fe²⁺ into Fe³⁺ precipitates. Therefore, the residual iron content in the supernatant decreased with increasing NaClO_x content. However, with excess NaClO_x, the convergent oil was dispersed and broken into smaller oil droplets, which were more difficult to capture and remove by PAC, increasing the remaining oil content. Therefore, the best ratio for direct oxidation was determined to be 400 μL of NaClO_x, 200 μL of 1 mol L⁻¹ CaO, and 12 mL of 12.8 g L⁻¹ PAC.

As shown in Figure 8(b), the removal of COD and Fe by acidification oxidation was almost linearly correlated likely because the oil layer covering Fe was acidified and hydrolyzed, while NaClO_x directly destroyed the water-in-oil structure to fully contact Fe²⁺. Therefore, while the COD was removed by oxidation, Fe²⁺ was oxidized to Fe³⁺ to form a precipitate. The fitted line for direct oxidation is parabolic. In the first 15 h, the angle was >45°, indicating that the reduction in COD was greater than that of Fe. COD removal may be related to the secondary effects of flocculation and precipitation produced by Fe(OH)₃. After 15 h, the fitted line stabilized and the oil was oxidized to form soluble organic matter, complicating its further capture and removal.

Fe²⁺ was encapsulated in water-in-oil due to the presence of surfactants, resulting in unstable water quality of the mechanical washing wastewater. Flocculation is a technical bottleneck that cannot effectively remove oil and iron, necessitating the development of the novel demulsification–oxidation–coagulation technique. Based on the observed floc structure, the orange-red dyed oil droplets of NaClO_x + CaO + PAC interacted with the oil more strongly than NaClO_x + CaO or pure PAC. Ca²⁺ exhibited a better demulsification effect than Na⁺, with the corresponding oil removal rate increasing from 72 to 83%. Because the iron removal efficiency is related to the pH, CaO addition was more advantageous. Oil removal is related to the exothermic heat generated by reagent hydration, so it is more advantageous to add Ca(OH)₂. From the FTIR analysis, the oil wastewater contained many hydrophobic alkanes. In the NaClO_x + CaO + PAC process, the alkanes were converted into hydrophilic alcohols and aldehydes, forming ferrous iron hydroxides that can be used as flocs, thereby increasing the oil removal rate. The best performance was achieved with 400 μL of NaClO_x, 200 μL of 1 mol L⁻¹ CaO, and 12 mL of 12.8 g L⁻¹ PAC. Furthermore, the combination of acidification and oxidation was more efficient than direct oxidation for removing Fe and COD.

In future field applications, mechanically washed wastewater will be initially acidified. Adding CaO to demulsify, combined with NaClO_x oxidation, destroys the water-in-oil structure and rapidly changes the environmental fate of Fe²⁺ to Fe(OH)₃, thereby improving overall oil and iron removal efficiency.

This work was financially supported by the Natural Science Foundation of China (Grant No. 51808044), the Natural Science Foundation of Shaan Xi Province of China (Grant No. 2020JM-262), Shendong Coal Branch Technology Innovation Project of China Shenhua Energy Co., Ltd (Grant No. CEZB210304069), and the Fundamental Research Funds for the Central Universities, CHD (Grant No. 300102281502).

J.Q.: Conceptualization; Methodology; Supervision; Y.H.: Methodology; Writing – Original Draft; B.S.: Formal analysis; Validation; R.W.: Data Curation; Investigation; X.W.: Resources; Project administration; C.Q.: Resources; Project administration; Y.W.: Resources; Project administration.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.