Abstract

Oily wastewater with acidic and highly emulsified properties is prevalent in numerous industrial sectors, including petrochemicals, metal processing, and food fermentation. Accurate determination of oil content in such complex water matrices is crucial for process control, environmental protection, and compliance with discharge regulations. However, current national standard methods often face challenges when dealing with highly emulsified and strongly acidic water, such as low extraction efficiency, persistent emulsification layers, and significant result deviations. Based on frontline testing practices, this paper deeply analyzes the application bottlenecks of current standard methods. It systematically proposes a series of improvement strategies and experimental validation protocols, covering sample pretreatment, demulsification techniques, extraction optimization, and instrument calibration. The aim is to provide a more operable and reliable technical pathway for the accurate determination of oil content in highly emulsified acidic water within the industry.

I. Current Status and Challenges

Currently, the determination of oil content in water is primarily based on national standards such as HJ 637-2018 Water quality - Determination of petroleum and animal and vegetable oils - Infrared spectrophotometry. Its core principle involves extracting oil substances from water samples using extractants like carbon tetrachloride or tetrachloroethylene. After dehydration and drying, the absorbance at specific wavebands is measured using an infrared oil analyzer.

However, when water samples are acidic (especially at low pH values) and highly emulsified, the limitations of standard methods become particularly prominent:

1. Sharp Decline in Extraction Efficiency: Acidic conditions can alter the existing form of oil substances and affect the affinity between the extractant and oil droplets. A highly emulsified state means that oil droplets have an extremely small particle size (usually at the micron or even nanometer level), stably encapsulated by surfactants or solid particles, forming a strong oil/water interface film. This significantly hinders the effective contact and dissolution of oil by the extractant.

2. Difficulty in Breaking Emulsification Layers: Standard methods commonly employ demulsification techniques such as adding demulsifiers (e.g., aluminum sulfate), acidification (HCl), or centrifugation. However, for highly stable emulsified systems, these conventional methods often have limited effectiveness. This results in a blurred three-phase interface (oil-water-solvent) after extraction, with a persistent intermediate emulsification layer, causing not only oil loss but also difficulties in quantitative transfer.

3. Poor Result Accuracy and Precision: Due to incomplete extraction and entrainment losses in the emulsification layer, the final measured oil concentration is typically lower than the actual value. Additionally, the relative deviation between parallel samples increases, reducing the credibility of the data.

4. Solvent and Personnel Safety Risks: To break persistent emulsification, operators may tend to add excessive amounts of strong acids or perform repeated extractions. This not only increases the risk of exposure to hazardous solvents and chemical burns but also generates more hazardous waste.

II. Method Optimization and Improvement Strategies

To address the aforementioned challenges, Sinokle proposes a systematic optimization scheme. On the premise of not violating the core principles of national standard methods, this scheme enhances and refines the pretreatment process.

1. Targeted Sample Pretreatment and Enhanced Demulsification

Synergistic Demulsification by Gradient Acidification and Salting-Out: For unknown or highly emulsified samples, a preliminary small-scale test is recommended. In a separating funnel, add an appropriate amount of NaCl first (salting-out effect to destroy the electric double layer), then slowly add HCl or H₂SO₄ dropwise until pH ≤ 2, and gently shake to observe simultaneously. Gradient pH adjustment can be attempted to find the optimal demulsification point. For certain emulsified systems, combining physical methods such as low temperature (e.g., refrigeration at 4℃ for several hours) or heating (e.g., water bath at 40-50℃, noting volatility) can further promote oil-water separation.

Screening and Application of High-Performance Demulsifiers: Explore efficient and low-interference demulsifiers suitable for the target water sample system. For example, for wastewater from specific industries (such as metalworking fluid wastewater), cationic or non-ionic polymer demulsifiers can be tested. Key step: A spiked recovery experiment must be conducted to verify that the selected demulsifier itself does not introduce infrared interference, and does not adsorb or encapsulate oil, leading to negative deviations.

Ultrasonic-Assisted Demulsification and Extraction: Under controlled temperature and time conditions, perform short-term, low-frequency ultrasonic treatment on the samples after acidification and salting-out. The cavitation effect of ultrasound can effectively break emulsified droplets and interface films, significantly improving the subsequent extraction efficiency. It is necessary to optimize ultrasonic power and action time to avoid excessive heating or the formation of finer secondary emulsions.

2. Refined Operation of the Extraction Process

Multiple Small-Volume Extraction Strategy: Abandon single large-volume extraction and adopt 2-3 consecutive extractions of the same water sample with a small volume (e.g., 10-15 mL) of fresh extractant each time. Combine all extracts. This method can improve the total extraction efficiency, especially for systems with unfavorable distribution coefficients.

Optimization of Extraction Oscillation Mode and Time: Replace fixed-time mechanical oscillation with intermittent, observation-based oscillation of "vigorous oscillation (30 seconds) - standing for layer observation - supplementary oscillation". The total time can be flexibly adjusted according to the reduction of the emulsification layer to ensure sufficient contact.

Targeted Treatment of Emulsification Layers: If an emulsification layer still appears, one or a combination of the following measures can be taken:

- Filtration and separation through glass wool or anhydrous sodium sulfate column (adsorption loss shall be verified in advance).

- Carefully aspirate the emulsification layer with a microsyringe or capillary tube, add a small amount of absolute ethanol or a more polar solvent (such as dichloromethane, instrument compatibility shall be verified) separately for secondary demulsification and extraction, and combine with the main extract.

- Transfer the entire emulsification layer to a centrifuge tube, centrifuge at high speed, and take the lower clear liquid (extractant layer).

3. Instrument Calibration and Quality Assurance

Matrix Matching of Standard Substances: When preparing the standard curve, use blank water samples with a matrix similar to that of the sample to be tested (adjusted to the same acidity and added with the same demulsifier) for spiking and extraction as much as possible to offset the matrix effect during the pretreatment process.

Whole-Process Spiked Recovery Rate Monitoring: For each batch or each type of difficult sample, a whole-process spiked recovery experiment must be performed. The spiking points shall cover the expected concentration range, and the recovery rate results (suggested target range: 80-120%) shall be used as the key basis for judging the effectiveness of the pretreatment method in this time, and for correcting the results when necessary.

Method Blank and Cross-Contamination Control: Highly emulsified samples are prone to residue, so all contacted vessels must be thoroughly cleaned (soaking with special detergent and solvent is recommended). The frequency of method blanks should be increased to ensure that the background value is stable and low.

III. Experimental Verification and Application Suggestions

Sinokle conducted comparative tests on an acidic metalworking wastewater (pH=3.5, severe emulsification) using the above-mentioned optimized methods. Compared with strictly following the original standard procedure, after adopting the demulsification process of "salting-out + gradient acidification + ultrasonic assistance" combined with three times of small-volume extraction, the spiked recovery rate of the target oil components increased from the original ~65% to ~95%, and the relative standard deviation improved from more than 15% to within 5%.

Application Suggestions:

1. Establish Standard Operating Procedures (SOPs): Laboratories should determine the optimal type and dosage of demulsifiers, acidification degree, and number of extractions through experiments for the common types of highly emulsified acidic water in their own laboratories, and solidify them into internal SOPs.

2. Personnel Training and Experience Accumulation: Strengthen operators' ability to judge and handle emulsification phenomena, encourage recording the demulsification "formulas" and effects of different water samples, and form a knowledge base.

3. Cooperate with Instrument Suppliers: Explore the application possibility of online or offline pretreatment equipment (such as fully automatic demulsification and extraction instruments) with higher anti-interference performance and lower detection limits