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Article

Recovery of Effective Acid from Waste Generated in the Anodic Oxidation Polishing Process

by
Haiyang Li
1,
Kangping Cui
1,* and
Wenming Wu
2
1
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230000, China
2
CS Link Environmental Technology (Anhui) Co., Ltd., Lu’an 237000, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(9), 1322; https://doi.org/10.3390/w17091322
Submission received: 11 April 2025 / Revised: 25 April 2025 / Accepted: 27 April 2025 / Published: 28 April 2025
(This article belongs to the Section Wastewater Treatment and Reuse)
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Versions Notes

Abstract

:
The high treatment costs associated with wastewater and waste solutions produced by the anodic oxidation polishing section significantly limit industry development. To address this challenge, the present study investigates the characteristics of polishing wastewater and waste solutions, employing extraction and ion exchange combined with diffusion dialysis to recover effective acids. For waste tank solutions, single and dual solvent extraction experiments were conducted to determine the optimal extraction system. Electrostatic potential and interaction region indicator (IRI) analyses were performed to provide theoretical justification. Regarding cleaning wastewater, resin adsorption was applied to selectively remove aluminium ions from waste acid solutions, facilitating effective acid recovery. Static and dynamic adsorption–desorption experiments were initially performed to identify suitable resins. Subsequently, optimised parameters—including adsorption and desorption concentrations, volumes, and flow rates—were systematically established through conditional experiments, and diffusion dialysis was applied to recover acids from the desorbed solutions. The experimental results indicate that tributyl phosphate (TBP) emerged as the optimal single extractant, achieving an effective acid extraction rate of 88.67% under a solvent ratio of 4:1 at a room temperature of 28 °C. A binary solvent system, composed of TBP with 20% sulfonated kerosene, demonstrated superior engineering feasibility due to its reduced viscosity and satisfactory extraction rate of 82.19%. Moreover, adsorption–desorption tests confirmed that the resin-based method effectively recovered acids from cleaning wastewater. Specifically, under optimal operational conditions—downstream adsorption at 0.3–0.5 bed volumes (BV) and 1.0 BV/h, coupled with counter-current desorption at 2 BV and 2.4 BV/h—the acid recovery rate reached ≥95% while removing ≥90% of aluminium ions. Additionally, employing 20% sulfuric acid solution for desorption in diffusion dialysis enabled cyclic desorption. Consequently, this study successfully achieved acid reuse and substantially lowered wastewater treatment costs, representing a promising advancement for anodic oxidation polishing processes.

1. Introduction

Aluminium and its alloys, as non-ferrous metals, have extensive applications in sectors such as daily chemical products, 3C industries, automotive, construction materials, and aerospace, due to their excellent corrosion resistance and high surface gloss. Typically, aluminium surfaces must undergo anodisation to achieve aesthetic requirements or enhance corrosion resistance [1,2]. The anodic oxidation process includes several steps: degreasing, alkaline etching, ash removal, chemical or electrolytic polishing, anodic oxidation, dyeing, sealing, and drying. This process generates different types of wastewater, including acid–alkaline wastewater, polishing wastewater (with high phosphorus content), dyeing wastewater, and sealing wastewater.
Phosphoric acid and sulphuric acid are widely used in anodic oxidation polishing due to their high viscosity [3,4], which facilitates the formation of a liquid film diffusion layer, essential for achieving optimal polishing effects. Chemical polishing solutions typically involve either a phosphoric–sulphuric acid binary system or a ternary system with phosphoric acid, sulphuric acid, and nitric acid (added as required) [5]. The mass ratio of phosphoric acid to sulphuric acid exceeds 3:1 in chemical polishing [6], whereas electrolytic polishing typically uses a binary system with a phosphoric-to-sulphuric acid mass ratio greater than 2:1. Due to the viscosity of the polishing solutions, large quantities adhere to product surfaces and enter subsequent cleaning tanks. Repeated polishing increases aluminium ion concentration, reducing polishing efficiency. Consequently, periodic solution drainage and acid replenishment are required to maintain polishing performance, generating wastewater and waste liquids with high levels of phosphorus, aluminium, and acidity.
A survey of over thirty anodic oxidation industrial parks and production lines in China indicated that wastewater treatment costs range between CNY 150 and 500 per tonne, with most facilities employing lime neutralisation. Polishing wastewater treatment accounts for over 70% of total treatment expenses. Given the significant effective acid content in polishing wastewater and waste liquids, neutralisation treatments result in considerable resource waste and environmental harm [7,8]. Currently, surface treatment parks generally resist integrating anodic oxidation production lines, highlighting the importance of developing effective recycling and treatment strategies to yield both economic and environmental benefits.
Common treatments for polishing waste include chemical precipitation [9], ion exchange [10,11], membrane separation [12,13], regeneration [14], and solvent extraction [15,16,17,18]. Although there are many methods, each has its own advantages and disadvantages; the specific method comparison is shown in Table 1. Using only one method alone to achieve a good separation effect is very difficult, and the combination of several methods is necessary in order to meet the needs of the production. Generally, waste acid purification is used as the main method for solvent extraction, supplemented by other methods of separation. Different extractants exhibit varying efficacy in extracting aluminium ions from aluminium alloy polishing solutions. Brand et al. utilised alkaline extractants, Amino 308 and Amino 336, achieving limited aluminium extraction efficiency and significant acid loss from chemical polishing waste baths [19]. Acidic extractants, such as naphthenic acid and Cyanex 272, are suitable primarily for extracting aluminium from dilute acidic solutions [20,21]. Conversely, aluminium alloy polishing waste solutions contain high concentrations of phosphoric and sulphuric acids, significantly affecting extraction performance. Tributyl phosphate (TBP), a neutral extractant, demonstrates high efficiency and capacity but tends to emulsify at lower temperatures, complicating phase separation [22,23]. There are not many processes that use a certain solvent alone for extraction, and they are generally extracted by compounding solvents, i.e., extractant + diluent + additives. Commonly used diluents are alkanes, paraffin, and other chemically stable solvents. There are many commonly used additives, such as sulfuric acid [24], hydrochloric acid [25] and other inorganic acids, sodium dodecylbenzene sulfonate [26], tween-20 [27], quaternary ammonium cationic surfactants [28], lignosulfonates [29], polyoxyethylene [30], dehydrated sorbitol fatty acid esters [31] and other surfactants, as well as thiourea, urea [32], ammonium dihydrogen phosphate [33], and sodium dihydrogen phosphate [34], etc. In recent years, there has also been many related research reports on the experiments of complex solvent extraction. Jiang et al. used a complex extraction system of primary amine and hydroxycitric acid to selectively separate germanium from secondary zinc oxide sulphuric acid leachate, and the extraction of germanium reached more than 96% under the optimal operating conditions [35]. Rao et al. recovered copper and gold from e-waste by a two-stage acid leaching process combined with extraction [36]. Chen et al. used TBP in combination with other organic combinations to recover waste acid from waste aluminium polishing solution, and after a series of stripping experiments, 94.05% of phosphoric acid and 91.11% of sulfuric acid were finally recovered from the loaded organic phase [37].
In this study, six extractants—TBP, sulfonated kerosene, n-butanol, methyl isobutyl ketone (MIBK), petroleum ether, and carbon tetrachloride—were comparatively evaluated for their effectiveness in extracting aluminium ions and acids (phosphoric and sulphuric acids) from aluminium alloy chemical polishing waste tank solutions. Additionally, composite extraction systems combining these extractants were tested to identify the optimal extraction system. Regarding polishing wastewater treatment, adsorption–desorption performances of fourteen strongly acidic resins were systematically compared, providing valuable insights for efficient wastewater treatment and recycling strategies in anodic oxidation polishing processes.

2. Materials and Methods

2.1. Materials and Properties of Raw Water

The primary reagents utilised in this study, including hydrochloric acid, sodium hydroxide, concentrated sulfuric acid, tributyl phosphate (TBP), sulfonated kerosene, n-butanol, methyl isobutyl ketone (MIBK), petroleum ether, and carbon tetrachloride, were all of analytical grade (Shanghai National Pharmaceutical Group Chemical Co., Ltd. (Shanghai, China)). Diffusion dialysis equipment and its membrane stacks were purchased from Shandong Tianwei Membrane Technology Co., Ltd. (Weifang, China) Wastewater and waste liquids were collected from the anodic oxidation polishing section of an electroplating facility located in Lu’an City, Anhui Province. The characteristics of the raw water samples are presented in Table 2.

2.2. Acid Recovery from Chemical Polishing Waste Tank Solution

2.2.1. Extraction Capability of Single Solvents

Six extractants—tributyl phosphate (TBP), methyl isobutyl ketone (MIBK), sulfonated kerosene, n-butanol, carbon tetrachloride, and petroleum ether—were selected for comparative extraction experiments. The extraction was conducted at room temperature (28 °C) with organic-to-aqueous phase ratios of 4:1, 3:1, 2:1, and 1:1, respectively. A precise volume of polishing waste solution was transferred into a separatory funnel, followed by the addition of the selected organic phase. After sufficient mixing and phase separation, the concentration of Al³⁺ in the aqueous phase was determined using inductively coupled plasma emission spectroscopy (ICP-ES), following appropriate dilution. Acid–base neutralisation titration was performed using an alkaline burette to measure the initial and post-extraction effective acid concentrations. The volume of waste liquid was 100 mL, the extraction time was about 1 min, and each set of experiments was repeated three times.

2.2.2. Extraction Capability of Binary Solvent Systems

MIBK, n-butanol, sulfonated kerosene, carbon tetrachloride, and petroleum ether were individually used as diluents in binary systems with TBP at varying volumetric dilution ratios (20%, 40%, 60%, and 80%). Extraction experiments were carried out at room temperature (28 °C) with an organic-to-aqueous phase ratio of 4:1. Additionally, the extraction capacities and electrostatic potential distributions for TBP with phosphoric and sulphuric acids were analysed using GAUSSIAN 16W software to provide theoretical support for solvent selection. All structural optimisations and vibrational frequency analyses were performed at the B3LYP-D3(BJ)/def-TZVP level (considering that the purpose of the calculations was mainly to analyse weak interactions such as hydrogen bonding, the DFT-D3 BJ-damping correction was applied to improve the poorer description of dispersion interactions by the conventional exchange-general function, i.e., D3(BJ)); single-point energy calculations of the individual structures were performed at the higher-precision B3LYP/ma-TZVPP level (where ma-TZVPP is the form of def-TZVPP with the addition of a dispersion function to improve the accuracy of calculating the weak interaction energy). Considering that the extraction takes place in the aqueous phase, the implicit solvent model IEFPCM (solvent = water) is used to describe the solvent effects.

2.3. Acid Recovery from Chemical Polishing Cleaning Wastewater

2.3.1. Dynamic Adsorption Capacity

Fourteen types of resins were individually packed into columns for dynamic adsorption tests to remove aluminium ions from chemical polishing cleaning wastewater. The wastewater was introduced from the top of the column at controlled flow rates of 1, 2, 3, 4, and 5 BV/h using a peristaltic pump, with a contact time of 10 min. After the experiment, the aluminium ion concentrations in the effluent were measured, and the removal efficiencies of each resin were calculated. The volume of waste solution was 100 mL and each set of experiments was repeated three times.

2.3.2. Dynamic Desorption Capacity

The fourteen resins used in adsorption experiments were tested for dynamic desorption capacity using 20% sulphuric acid solution. Desorption experiments were performed by introducing sulphuric acid solution from the top of the resin column at controlled flow rates of 1, 2, 3, 4, and 5 BV/h, with a contact time of 10 min. Aluminium ion concentrations were determined in the desorbed solutions, and desorption rates for each resin were calculated. The volume of waste solution was 100 mL and each set of experiments was repeated three times.

2.3.3. Recovery of Desorbed Acid Solution

The 20% sulphuric acid solutions recovered from resin desorption experiments were further treated via diffusion dialysis. Using a peristaltic pump, flow rates of 0.17, 0.34, and 0.68 L/h were tested within the dialysis unit. After fully replacing 3 BV volumes, samples were collected and analysed for orthophosphate concentration, aluminium ion content, and acid concentration, with a reaction time of about 2 h. The effects of varying flow rates on acid recovery efficiency, as well as phosphorus and aluminium removal rates, were systematically evaluated.

3. Results and Discussion

3.1. Acid Recovery from Chemical Polishing Waste Tank Solutions

3.1.1. Extraction Capability of Single Solvents

The extraction efficiencies of mixed acids, aluminium ions, and the volumes of the extracted phases using different solvents are illustrated in Figure 1. At room temperature (28 °C) and an organic-to-aqueous phase ratio of 4:1, the extraction efficiencies for effective acids from aluminium alloy chemical polishing waste solutions ranked as follows: n-butanol > TBP > MIBK > petroleum ether > sulfonated kerosene > carbon tetrachloride (Figure 1a). The high extraction performance of n-butanol can be attributed to its polar characteristics and hydrophilic hydroxyl groups (-OH), which enhance hydration layer solvation and promote acid extraction. TBP and MIBK contain phosphorus–oxygen (P=O) and carbon–oxygen (C=O) double bonds, respectively, which readily form hydrogen bonds with phosphoric acid molecules, significantly improving acid extraction capabilities. Compared to carbon tetrachloride and sulfonated kerosene, petroleum ether exhibits lower molecular weight and weaker intermolecular van der Waals forces [38,39], thus creating a more pronounced cavity effect, which is advantageous for extraction.
The extraction efficiencies of aluminium ions using different extractants are presented in Figure 1b. The general trend of aluminium ion extraction efficiency at room temperature (28 °C) and an organic-to-aqueous ratio of 4:1 was as follows: n-butanol > TBP > MIBK > petroleum ether > sulfonated kerosene > carbon tetrachloride. Notably, n-butanol exhibited the highest extraction efficiency for aluminium ions (30.15%), likely due to its polar nature and hydrophilic hydroxyl groups (-OH), facilitating aluminium ions from the chemical polishing waste solution to enter the organic phase with water. TBP and MIBK also demonstrated substantial extraction capacities, achieving extraction efficiencies of 29.73% and 25.52%, respectively, under identical conditions. The aluminium ion extraction efficiencies for n-butanol, TBP, and MIBK decreased with decreasing organic-to-aqueous ratios, with TBP showing an almost linear reduction. Carbon tetrachloride, sulfonated kerosene, and petroleum ether displayed minimal extraction capabilities, all with efficiencies below 5%.
The volume change percentages before and after acid extraction are illustrated in Figure 1c. At room temperature (28 °C) and a 4:1 ratio, the residual acid phase volumes for petroleum ether, sulfonated kerosene, and carbon tetrachloride remained above 95%, indicating negligible extraction activity and minimal volume changes. Although n-butanol, TBP, and MIBK demonstrated comparable acid extraction efficiencies, significant differences in extracted phase volumes were observed. Specifically, n-butanol, TBP, and MIBK yielded extracted phase volumes of 29.86%, 44.89%, and 55.43%, respectively. Notably, n-butanol extracted considerable amounts of water, significantly reducing the extracted phase volume. Furthermore, TBP formed hydrates (TBP·nH2O) with water, moderately reducing extracted phase volume compared to MIBK. Consequently, extraction with n-butanol and TBP significantly increased phase viscosity, negatively impacting extraction operability, particularly affecting n-butanol.
In conclusion, due to high viscosity leading to operational challenges in phase separation, transport, and measurement, n-butanol is unsuitable as an extractant for practical applications. Similarly, TBP’s increased viscosity limits its direct usage, necessitating dilution with suitable solvents. Therefore, MIBK, n-butanol, sulfonated kerosene, carbon tetrachloride, and petroleum ether were evaluated as potential diluents for TBP to examine mixed solvent extraction performance.

3.1.2. Extraction Capability of Binary Solvent Systems

Figure 2a shows the extraction efficiencies of effective acids using binary solvent systems. At room temperature (28 °C) and a 4:1 organic-to-aqueous ratio, the extraction efficiencies for TBP/n-butanol and TBP/sulfonated kerosene systems decreased gradually with increasing dilution ratios, reaching 86.34% and 72.19%, respectively, at a 20% dilution ratio. The TBP/MIBK, TBP/carbon tetrachloride, and TBP/petroleum ether systems initially achieved extraction efficiencies of approximately 55% at a 20% dilution ratio; however, extraction efficiencies significantly declined with increased dilution ratios, following a nearly linear trend.
As the dilution ratio increased, the extraction efficiencies of aluminium ions gradually decreased. The overall trend was as follows: TBP/n-butanol > TBP/MIBK > TBP/sulfonated kerosene > TBP/carbon tetrachloride > TBP/petroleum ether (Figure 2b). At room temperature (28 °C), with an organic-to-aqueous phase ratio of 4:1 and a dilution ratio of 20%, the aluminium ion extraction efficiency of the TBP/n-butanol system reached 50.23%, while the efficiencies of TBP/sulfonated kerosene and TBP/MIBK systems were both approximately 39.14%. The TBP/carbon tetrachloride and TBP/petroleum ether systems exhibited lower extraction efficiencies of around 19.27%.
In summary, each extraction system has distinct advantages and disadvantages. Petroleum ether is cost-effective, but when used as a TBP diluent, the extraction efficiency sharply decreases with increased dilution. The TBP/carbon tetrachloride system shows significant fluctuations in acid extraction efficiency with varying dilution ratios, potentially destabilizing centrifugal extraction due to changes in two-phase distribution properties. The TBP/MIBK system demonstrates good stability; however, MIBK possesses a strong odour, high cost, and low boiling point, resulting in rapid volatilization. Although the TBP/n-butanol system has high extraction efficiency, its high viscosity makes it unsuitable for practical application. Considering the low cost of sulfonated kerosene, the high extraction efficiency of the TBP/sulfonated kerosene system, and its minimal negative impact on aluminium ion extraction, the TBP/sulfonated kerosene system is recommended as the optimal choice.

3.1.3. Electrostatic Potential and IRI Analysis

Electrostatic potential distributions for TBP, sulphuric acid, and phosphoric acid were analysed using GAUSSIAN 16W software (Figure 3). The results indicate that all three alkane chains of TBP exhibited a slight red colour, signifying positive electrostatic potentials. Conversely, the phosphate group appeared distinctly blue, indicating a negative electrostatic potential. Red regions represent electron-deficient areas prone to electrophilic interactions, while blue regions denote electron-rich areas favourable for nucleophilic reactions. For phosphoric and sulphuric acids, electrostatic potential distributions reveal that hydroxyl (-OH) groups displayed pronounced red colouration, indicating positive electrostatic potentials, whereas oxygen atoms (-O) appeared blue, indicating negative potentials. Darker colouration correlates with stronger electrostatic potential, implying that hydrogen bonding is most likely to form between the strongly electron-deficient (dark red) and electron-rich (dark blue) regions when two molecules interact.
Electrostatic interaction analysis between TBP and sulphuric acid for two configurations reveal distinct differences. In configuration 1, the sulphuric acid molecule was positioned between two alkyl chains of TBP; however, no significant overlap in electrostatic potential surfaces was observed. In contrast, configuration 4 shows clear overlapping electrostatic potential surfaces between the two hydrogen atoms of the sulphuric acid molecule and the two oxygen atoms of the phosphate group in TBP, indicating strong electrostatic interactions at this position (Figure 4). Similar to sulphuric acid, both configurations between TBP and phosphoric acid demonstrate pronounced red–blue overlaps of electrostatic potential surfaces, confirming significant electrostatic interactions regardless of whether phosphoric acid was positioned adjacent to the alkyl chains or the phosphate group of TBP (Figure 4).
The weak interaction forces between the molecules were analysed using GAUSSIAN 16W software. Two configurations each of TBP combined with sulfuric acid and phosphoric acid were selected for detailed investigation, presented in two perspectives to facilitate clear observation.
Analysis of the interaction between TBP and sulfuric acid reveals that in configuration 1, the interaction interface predominantly exhibited green regions, indicating weak van der Waals interactions without significant hydrogen bonding. However, in configuration 4, a distinct blue interface emerged between the two hydrogen atoms of sulfuric acid and the two oxygen atoms of the phosphate ester group in TBP, clearly demonstrating the presence of hydrogen bonding and strong attractive interactions (Figure 5).
Similarly, analysis of the interaction between TBP and phosphoric acid (configurations 1 and 5) shows prominent blue interfaces, indicating significant hydrogen bonding between TBP and phosphoric acid molecules (Figure 6). These observations align with previously discussed hydrogen bonding analyses and, thus, are not elaborated further here.
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Figure 4. Electrostatic potential interactions between TBP and sulfuric acid or phosphoric acid.
Figure 4. Electrostatic potential interactions between TBP and sulfuric acid or phosphoric acid.
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Figure 5. The interaction force between TBP and sulfuric acid.
Figure 5. The interaction force between TBP and sulfuric acid.
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Figure 6. The interaction force between TBP and phosphoric acid.
Figure 6. The interaction force between TBP and phosphoric acid.
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3.2. Acid Recovery from Chemical Polishing Cleaning Wastewater

3.2.1. Dynamic Adsorption Capacity

The selection of the 14 resins in our study was based on commercially available products that are well-established in the industry. The dynamic adsorption capacities of fourteen strongly acidic resins at different flow rates are presented in Figure 7. At a flow rate of 1 BV/h, NKC-9 resin exhibited the highest aluminium ion removal efficiency (94.56%), followed by LSI010 resin (83.24%). Gel resin 001 × 16 and macroporous resins D001 and D072 showed comparatively weaker dynamic adsorption capacities.
As flow rate increased, the aluminium ion removal efficiencies of all tested resins notably decreased. At lower flow rates, aluminium ions primarily occupied available adsorption sites on the resin surface, resulting in longer hydraulic retention time, lower mass transfer resistance, and higher adsorption efficiency. With increased flow rates, fewer surface adsorption sites were available, forcing aluminium ions to penetrate deeper into the resin’s internal pore structures. Consequently, mass transfer resistance gradually increased, causing a reduction in adsorption efficiency.
As shown in Figure 7, except for HD-8 resin, aluminium ion removal efficiencies at flow rates of 1–2 BV/h were significantly higher compared to higher flow rates.

3.2.2. Dynamic Adsorption and Desorption Capacity

Fourteen resins were loaded into resin columns for dynamic desorption tests. The desorption was performed by downward feeding of 20% sulfuric acid at controlled flow rates of 1, 2, 3, 4, and 5 BV/h using a peristaltic pump, with a contact time of 10 min. Upon test completion, the aluminium ion concentration in the solution was measured, and the desorption efficiency for each resin was calculated (Figure 8).
As shown in Figure 8, two clear trends emerge. First, resins with higher degrees of cross-linking, such as 001 × 16, exhibit higher desorption efficiencies. Second, macroporous resins generally show greater desorption efficiencies, as exemplified by resins D001 and D072. The differences in desorption performance among the strongly acidic resins mainly result from variations in pore blockage. Macroporous resins typically have stable and well-developed pore structures that do not require swelling in water [40]; thus, they experience minimal pore shrinkage during ion-exchange processes. Conversely, gel resins undergo significant volumetric shrinkage (>20%) when transitioning from hydrogen-type to aluminium-type, leading to micropore closure, pore blockage, and decreased resin performance due to reduced active site accessibility.
Among gel resins, those with higher cross-linking degrees, such as resin 001 × 16 (cross-linking degree of 16%), showed reduced shrinkage and expansion during transformation, resulting in fewer pore blockages and higher desorption efficiencies. Notably, resin 001 × 16 achieved a high aluminium desorption rate of 97.14%.
In summary, gel resins (e.g., 001 × 16) and macroporous resins (e.g., D001 and D072) demonstrated strong desorption capacities; however, as indicated in Section 3.2.1, their adsorption capacities were comparatively weaker. In contrast, NKC-9 and LSI010 resins exhibited excellent adsorption capacities along with moderate desorption efficiencies. Moreover, an optimal flow rate of 2 BV/h during adsorption ensured both a high aluminium ion adsorption efficiency and rapid treatment of water samples.

3.2.3. Recovery of Desorption Solution

The results of diffusion dialysis at different flow rates are presented in Figure 9. The surface area of the membrane is 0.34 m2 and the contact time is about 2 h. As the flow rate of waste acid increased, the acid recovery rate decreased, whereas the removal rates of aluminium and phosphorus increased. In other words, the waste acid flow rate was inversely proportional to the acid recovery rate but directly proportional to the removal efficiencies of aluminium and phosphorus. It is also important to note that variations in the waste acid flow rate significantly influenced acid recovery and phosphorus removal rates, but had a relatively minor effect on aluminium removal efficiency.

3.3. Limitations of the Study

During the diffusion dialysis process, a portion of the acid (approximately 10% of the desorbed liquid acid content) remains in the residual liquid and cannot be recovered or reused. This results in resource loss and increases the overall cost of wastewater treatment due to additional water treatment requirements. Future work will focus on improving the efficiency of acid recovery from the residual liquid to reduce this waste and enhance the economic feasibility of the process.
The use of organic solvents, such as TBP, in the extraction process raises concerns regarding solvent loss due to evaporation during operation. This solvent loss not only reduces the overall efficiency of the process but also presents health risks to the operators working with these solvents. Exposure to volatile organic solvents can lead to significant health hazards, including respiratory issues and skin irritation. To mitigate these risks, it is essential to employ proper ventilation, personal protective equipment, and solvent recovery systems. Further research will explore the development of safer and more efficient solvent systems to address both environmental and health concerns.
These limitations provide opportunities for further research to improve the scalability and sustainability of the proposed method. Addressing these issues will be crucial for the industrial adoption of this process in anodic oxidation polishing wastewater treatment.

4. Conclusions

The extraction method was employed for treating the waste slurry, comparing the extraction efficiencies of six extractants—tributyl phosphate (TBP), n-butanol, methyl isobutyl ketone (MIBK), petroleum ether, carbon tetrachloride, and sulfonated kerosene—on aluminium alloy polishing slurry, to identify the most suitable extractant. Under single-solvent conditions at room temperature (28 °C) and an organic-to-aqueous ratio (Vo/Va) of 4:1, the effective acid extraction rates from chemical polishing solutions ranked as follows: n-butanol > TBP > MIBK > petroleum ether > sulfonated kerosene > carbon tetrachloride. Due to the excessively high viscosity of n-butanol, TBP was recommended as the preferred single-solvent extractant, achieving an extraction efficiency above 70%. To address TBP’s viscosity challenge, a diluent was introduced. At 28 °C, with a Vo/Va ratio of 4:1 and a 20% dilution ratio, the TBP/sulfonated kerosene system achieved an effective acid extraction efficiency of 82.19%. Moreover, this combination showed minimal interference from aluminium ions, and sulfonated kerosene’s low cost further supports its practical use as an extractant for aluminium alloy slurry. Additionally, electrostatic interactions and hydrogen bonding between TBP and phosphoric and sulfuric acids provided a theoretical basis supporting the optimal selection of the TBP/sulfonated kerosene extraction system.
For cleaning water and wastewater treatment, a resin-based approach was utilised. Comparative analysis of adsorption and desorption performance for aluminium ions among fourteen strongly acidic resins, alongside a cost evaluation for practical engineering applications, indicated that the aluminium alloy chemical polishing solution recovery system based on LSI010 resin achieved up to 90% acid recovery. Furthermore, compared to other resins, the LSI010 resin ensured high adsorption and desorption efficiency while substantially reducing material costs, offering valuable insights for effective treatment and recycling strategies for anodic oxidation polishing wastewater.

Author Contributions

Data curation, H.L.; project administration, K.C.; software, H.L.; supervision, W.W.; validation, K.C.; writing—original draft, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported from the National Key R&D Programme of China (2019YFC0408500).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Author Wenming Wu was employed by the company Zhongxin Link Environmental Technology (Anhui) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflicts of interest.

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Water 17 01322 g001
Figure 1. The influence of different extractants on the extraction rate ((a) Effective acid extraction rate (b) Aluminum ion extraction rate (c) Residual phase volume).
Figure 1. The influence of different extractants on the extraction rate ((a) Effective acid extraction rate (b) Aluminum ion extraction rate (c) Residual phase volume).
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Figure 2. The changes in extraction rate after dilution with different proportions of the second solvent ((a) effective acid extraction rate (b) aluminum ion extraction rate).
Figure 2. The changes in extraction rate after dilution with different proportions of the second solvent ((a) effective acid extraction rate (b) aluminum ion extraction rate).
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Figure 3. (a) Electrostatic potential distribution of TBP (front view on the left, bottom view on the right); (b) electrostatic potential distribution of phosphoric acid (front view on the left, top view on the right); (c) electrostatic potential distribution of sulphuric acid (front view on the left, top view on the right).
Figure 3. (a) Electrostatic potential distribution of TBP (front view on the left, bottom view on the right); (b) electrostatic potential distribution of phosphoric acid (front view on the left, top view on the right); (c) electrostatic potential distribution of sulphuric acid (front view on the left, top view on the right).
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Figure 7. Dynamic adsorption curves of fourteen strong acidic resins.
Figure 7. Dynamic adsorption curves of fourteen strong acidic resins.
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Figure 8. Dynamic desorption curves of fourteen strongly acidic resins.
Figure 8. Dynamic desorption curves of fourteen strongly acidic resins.
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Figure 9. Variation diagram of diffusion dialysis results at different flow rates.
Figure 9. Variation diagram of diffusion dialysis results at different flow rates.
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Table 1. Comparison of existing methods.
Table 1. Comparison of existing methods.
Processing MethodsKey AchievementsDrawbacks
Chemical precipitationMature and reliable process, shorter process, wide range of applicationsLow processing efficiency, high sludge volume, needs to be used with other processes
Ion exchangeHigh selectivity, good treatment results, high automation potentialHigh pre-treatment requirements, limited applicability, operating cost issues
Membrane separationAcid reuse potential, high pollutant removal, low chemical consumptionSerious membrane contamination, poor adaptability to high-concentration waste liquids, concentrated liquid treatment problems
RegenerationEfficient resource recovery, reduced hazardous waste disposal, and significant economic benefitsStringent pre-treatment requirements, high technical complexity, recovery purity limitations
Solvent extractionHigh recycling efficiency, no secondary pollution to the environment, adaptableHigh technical requirements, harsh requirements for the extractant, high operating costs, less industrialised domestic applications
Table 2. Properties of raw water.
Table 2. Properties of raw water.
Chemical Polishing of Waste Tank Liquid
Density1.6037 g/mL
Total acid62.96 wt.%
Aluminium ion31,281 mg/L
Mass ratio of phosphoric acid to sulfuric acid in mixed acid4:1
Chemical Polishing Cleaning Wastewater
Density1.2097 g/mL
Total acid28.41 wt.%
Aluminium ion10,514 mg/L
Mass ratio of phosphoric acid to sulfuric acid in mixed acid4:1
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Li, H.; Cui, K.; Wu, W. Recovery of Effective Acid from Waste Generated in the Anodic Oxidation Polishing Process. Water 2025, 17, 1322. https://doi.org/10.3390/w17091322

AMA Style

Li H, Cui K, Wu W. Recovery of Effective Acid from Waste Generated in the Anodic Oxidation Polishing Process. Water. 2025; 17(9):1322. https://doi.org/10.3390/w17091322

Chicago/Turabian Style

Li, Haiyang, Kangping Cui, and Wenming Wu. 2025. "Recovery of Effective Acid from Waste Generated in the Anodic Oxidation Polishing Process" Water 17, no. 9: 1322. https://doi.org/10.3390/w17091322

APA Style

Li, H., Cui, K., & Wu, W. (2025). Recovery of Effective Acid from Waste Generated in the Anodic Oxidation Polishing Process. Water, 17(9), 1322. https://doi.org/10.3390/w17091322

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