Simultaneous Determination of Mixed Heavy Metal Ions Cr (III) and Co (II) in Wastewater by Cloud Point Extraction Dual Wavelength Spectrophotometry

. Using cloud point extraction technology and the principle of absorbance addition in multi-component system, the mixed metal ions Cr (III) and Co (II) in industrial wastewater were enriched and determined at pH 5.0 and 50℃, using ammonium pyrrolidine dithiocarbamate (APDC) as complexing agent and Triton X-114 as extractant. The results indicate that the contents of Cr (III) and Co (II) can be calculated using a simultaneous equation system. The detection limits for Cr (III) and Co (II) are 0.74 ng/ml and 2.12 ng/ml, respectively. The recovery rates are 95.3% -104.0% and 96.9% -103.5%, respectively. This method has the characteristics of high accuracy, good repeatability, and fast detection speed, and has good practical value.


Introduction
With the rapid development of industrialization in China, the problem of environmental pollution is becoming increasingly serious, and heavy metal ion pollution in water is receiving widespread attention. After heavy metal pollutants enter water bodies, they have a magnifying effect on the food chain and can accumulate in certain organs of the human body, causing chronic poisoning and endangering human health [1] . For example, high concentrations of trivalent chromium can cause acute or chronic damage to the human body [2] , and excessive intake of cobalt can also lead to a series of poisoning symptoms and even death [3] .
At present, the main detection methods for heavy metal ions include electrochemical method [4] , atomic absorption spectroscopy [5] , fluorescence spectrometry [6] , inductively coupled plasma mass spectrometry [7] , biological method, etc. [8][9][10] . Most of these methods have high requirements for detection instruments, or the preprocessing operation is complex and time-consuming, and the analysis cycle is long. Cloud point extraction (CPE) is a new environment-friendly extraction technology emerging in recent years. It uses surfactant aqueous solution instead of volatile organic solvent as the extractant. Based on the solubility and cloud point of surfactant micellar aqueous solution, it can change temperature, pH value, ionic strength and other parameters to make the surfactant produce cloud point phase separation and separate hydrophilic substances from hydrophobic substances. This method can not only promote the enrichment of heavy metals, improve the sensitivity of color reactions, but also avoid the occurrence of side reactions. It has been applied to the determination of various trace metal ions [11] . Therefore, this experiment adopts cloud point extraction technology, using ammonium pyrrolidine dithiocarbamate (APDC) as chelating agent and Triton X-114 as extracting agent. Combined with dual wavelength spectrophotometry, an attempt is made to establish a cloud point extraction spectrophotometry with lower detection limit and higher sensitivity for the rapid and accurate determination of mixed metal ions Cr (III) and Co (II) in industrial wastewater.

Experimental Principles
The cloud point phenomenon of nonionic surfactant TritonX-114 was used to extract, separate and enrich Cr (III) complex and Co (II) complex, and the two metal ions in wastewater samples were determined by dual wavelength spectrophotometry. When the absorption spectra of the X and Y components of the mixture overlap, the concentration of X and Y components can be obtained by measuring λ 1 (X+Y) and λ 2 (X+Y) outside the total absorbance at the maximum absorption wavelengths λ 1 and λ 2 of X and Y, if the absorption law is observed.

Determining the Wavelength of Incident Light λ1 and λ2
Take 1 ml of Cr (III) and Co (II) standard solutions and place them in a 50 ml colorimetric tube. Add 2.5 ml of 0.1% (m/V) APDC solution and 2 ml of 0.5% (V/V) Triton X-114 solution. Adjust the pH value to around 5.0 with 1 mol/l HNO 3 and bring to volume with water. Shake for 1 minute and mix well. Heat the mixed solution in a 50℃ water bath for 10 minutes. If the solution becomes turbid, remove the colorimetric tube and cover it. Shake and let it stand for 5 minutes; Suck the upper colored organic phase solution into a 1 cm colorimetric dish and scan it on a UV-1950 ultraviolet visible spectrophotometer with a wavelength range of 210-640 nm. Measure the maximum absorption wavelengths of chromium (III) and cobalt (II), respectively λ1 and λ2.

Impact of pH value and Temperature
In the process of cloud point extraction, pH value plays an important role in the formation of complex and its chemical stability. This study investigated the effect of pH range 3.0~9.0 on the detection results. As shown in Figure 1, at a pH value of 5.0, the test results of the two ionic complexes reached the maximum, and pH 5.0 was chosen as the optimal reaction condition. To maximize the enrichment factor of cloud point extraction, the water bath temperature must reach the cloud point temperature required by the surfactant, and this temperature must be maintained as the equilibrium temperature for cloud point extraction. Maintaining this temperature for a period is the equilibrium time of cloud point extraction, and the water bath temperature is an important factor affecting the degree of chelation of chelates. This experiment selected a water-bath temperature range of 25 to 75℃ for investigation, and the results are shown in Figure 2. The experiment found that the optimal temperature for water bath ultrasound assisted heating was around 50 ℃, and the extraction reaction was completed well within about 10 minutes.

Experimental results and detection limit
Prepare cobalt series standard solution and chromium series standard solution respectively according to Step 2.3.2. Take 2 ml, 4 ml, 6 ml, 8 ml, and 10 ml of chromium standard solution and cobalt standard solution respectively. Place them in a 50 ml colorimetric tube. Add 2.5 ml of 0.1% (m/V) APDC solution and 2 ml of 0.5% (V/V) Triton X-114 solution. Measure under optimal reaction conditions. The absorption of Cr (III) and Co (II) series standard solutions prepared were measured at λ 1 and λ 2 respectively. As shown in Figure  3&4. According to the four standard curve equations drawn in Fig.3 and Fig.4, combined with the following simultaneous equations (1) and (2) Take 10 ml of the wastewater to be tested and treat it according to step 2.3.2. Measure the absorbance of the sample to be tested at λ1 and λ2 respectively, and conduct a recovery experiment with added standards. Calculate the concentration of Cr (III) and Co (II) in the wastewater using the simultaneous equations (1) and (2). The specific results are shown in Table 1. The method used in this article determines Cr (III) and Co (II) in wastewater, with recovery rates of 95.3% -104.0% and 96.9% -103.5%, respectively. The relative standard deviation (RSD) (Cr (III), n=6) is 0.97% -1.6%, and the RSD (Co (II), n=6) is 1.2% -2.8%.

Conclusions
Spectrophotometry is one of the most commonly used analytical methods, with advantages such as strong universality, wide application range, and easy operation. Even in today's highly popular instrumental analysis, its precision and accuracy still do not affect the fact that photometry has become a very attractive analytical method. CPE is a hydrophobic chelate formed by chelating agents and heavy metal ions in the presence of surfactants, exhibiting prominent spectral characteristics, and surfactants can induce an increase in absorption intensity. Cloud point extraction not only promotes the pre enrichment of heavy metals, thereby improving the sensitivity of color reaction, but also avoids the occurrence of side reactions.
This article combines cloud point extraction and dual wavelength spectrophotometry to determine Cr (III) and Co (II) in wastewater. Experiments have shown that under conditions of pH 5 and temperature 50℃, 2.5 ml 0.1% (m/V) APDC as chelating agent and 2 ml 0.5% (V/V) Triton X-114 as extractant can simultaneously enrich and determine mixed metal ions Cr (III) and Co (II) in industrial wastewater. This method uses fewer extractants for cloud point extraction, has a high enrichment factor, and reduces the consumption of experimental reagents. The linear regression correlation coefficient R 2 of the two element standard solutions is greater than 0.9990, and the linear regression curves are ideal. The detection limits are 0.074ng/ml and 2.12ng/ml respectively, which meet the standard testing requirements. The recovery rates of two different concentrations of samples were 95.3% -104.0% and 96.9% -103.5%, respectively, indicating that the method has high sensitivity and accuracy. It can effectively achieve rapid and efficient monitoring of experimental wastewater, and can meet the rapid and simultaneous detection of trace Cr (III) and Co (II) in wastewater. Therefore, this method has reliable practical value.