Rapid and green detection of manganese in electrolytes by ultraviolet-visible spectrometry to control pollutant discharge.

Controlling the manganese (Mn2+) concentration is important to product quality in the electrolytic manganese industry. Conventional methods for detecting Mn2+ are complex and reagent-consuming, which makes them slow and polluting. It is of great significance to develop a new fast and environmentally friendly method to quantify Mn2+ in electrolyte. The characteristic ultraviolet-visible (UV-vis) absorbance at 401 nm of Mn2+ will vary linearly with the Mn2+ concentration after data correction. Adjusting the pH, calibrating the spectral bandwidth (SBW) and optical path length (OPL), and subtracting the interference from coexisting substances by linear interpolation improve the measuring accuracy. Mn2+ concentration can be determined by direct fast UV-vis absorbance measurement at characteristic peaks without using harmful reagents which facilitates such measurement to be applicated as on-line detection method for electrolytic manganese industry. The method developed in this study will help achieve the goal of improving the detection speed while cutting back on pollutant discharge from concentration analysis process in electrolytic manganese industry.

Introduction

In electrolytic manganese industry, the concentration of manganese (Mn2+), which is the major component in electrolytes, is of great concern. The Mn2+ concentration can be neither too high nor too low during processes. In newly prepared purified leachate, the Mn2+ concentration should be 34.0~38.0 g/L, while that in the electrolyzer cathode should be 15.0~18.0 g/L [1]. Exorbitant Mn2+ concentrations will lead to operating malfunctions, such as excessively alkaline electrolytes, blackened products and manganese ammonium double-salt precipitation, whereas insufficient Mn2+ will result in water electrolysis, plate flaking, and difficult stripping of the manganese plates [1]. An inappropriate manganese concentration leads to a higher rejection rate of electrolytic manganese products, and the risk of waste products being discharged and polluting the environment will consequently be greater. Heavy metals from industrial waste emissions, such as manganese, are well recognized as harmful. Hazards posed by manganese to flora and fauna have been extensively studied [2]. Chronic exposure to manganese can lead to adverse neurological effects in humans [3, 4], including intellectual impairment in children [5]. Overall, it is necessary and important to develop a real-time manganese detection method for timely reporting of manganese concentration during the processing of industrial electrolytic manganese, which facilitates prompt adjustment of operating conditions to prevent malfunctions resulting from an abnormal concentration. When the malfunction rates reduce, the quantity of waste products from the electrolytic manganese industry will also decrease. This will directly save electricity and raw material consumption. More importantly, fewer waste products will indirectly reduce the risk of manganese pollution from industrial waste discharge.

To date, there have been many studies determining Mn2+ concentration using different types of methods. Some researchers measured Mn2+ concentration by colorimetry based on the chromogenic reaction between manganese ion and color-developing agents [6, 7]. The timeliness of this kind of methods is not good due to the relatively long time needed for the completion of chromogenic reaction. Such methods can only measure Mn2+ at low concentrations. However, the concentrations of Mn2+ in leaching solutions and electrolytes of electrolytic manganese industry are generally as high as dozens of grams per liter [1]. If colorimetry is used for Mn2+ concentration determination in industry, laborious dilution will be needed before measurement. Moreover, the high cost of color-developing agents will limit the application of colorimetry in industry. Advanced fluorosensor for trace metal ion detection were synthesized in previous study [8]. But it is not suitable for determination of the high concentrations of Mn2+ in industry. Titrimetric analysis is routinely the foundation of standard methods for measuring Mn2+ concentrations [9, 10]. But the analysis speed of titrimetry is also slow. Besides, toxic reagents such as ammonium sulfide, eriochrome black T, EDTA, N-phenylanthranilic acid, and ammonium iron (II) sulfate are indispensable for Mn2+ concentration measurement by titration. These reagents are highly refractory so that only advanced treatment such as anodic oxidation can degrade them [11]. The waste liquid after titrimetric analysis, which contains harmful Mn2+ and all kinds of other polluting substances, needs to be discharged. This will potentially lead to environmental pollution. There are some studies on developing in situ Mn2+ detection methods with relatively advanced but expensive instruments. Terada et al. performed online Mn2+ detection by total-reflection X-ray fluorescence (TXRF) analysis [12]. However, the sample preparation lasted over 12 h, and tens of minutes are needed for measurement. Wang et al. used a rotating ring disk electrode (RRDE) to measure the Mn2+ concentration [13], but this technique requires a high scan rate (100 mV/s). The resolution of cyclic voltammetry at this scan rate is low, and the operation is complex. The cost of the instrument in existing studies on in-situ Mn2+ concentration measurement is too high to be suitable for industrial application. Furthermore, the speed and performance of such in-situ measuring methods are not satisfactory. Inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) are common methods in scientific studies for determining Mn2+ concentration [14, 15]. In spite of their unsuitably high instrument cost with regard to practical application, they are also only suitable for measuring trace Mn2+ concentration rather than directly measuring the extremely high Mn2+ concentration in the electrolytic manganese industry. An ideal Mn2+ concentration measurement method applied to industry should have characteristics as follows: First, it should be able to directly determine high Mn2+ concentration in electrolytic industry without cumbersome pretreatment. Second, it should be real-time, simple and cheap. Third, no polluting chemical reagents should be discharged from such method. Unfortunately, no existing Mn2+ concentration determination method in practical application or scientific studies can meet the requirements above for industrial application. To the best of our knowledge, there is no research on developing methods specially for accurately measuring the high Mn2+ concentration in electrolytic manganese industry yet.

MnSO4 is the existence form of Mn2+ in industrial electrolytes [1]. Its solution is transparent when its concentration is low. But when MnSO4 concentrations reach dozens of grams per liter, which is its concentration level in electrolytic manganese industry, the color of its solution will be obviously pink (S1 Fig in S1 File). This means that MnSO4 has ultraviolet-visible (UV-vis) light absorption. Many aqueous solutions of transition metal salts are colored. Therefore, these metal ions absorb light in the UV-vis region, and their concentration can be directly determined by UV-vis absorbance with fast speed, simple procedure and no polluting reagents [16-18]. It is reasonable to deduce that the ideal method for measuring Mn2+ concentration in industrial electrolyte can also be developed based on measuring its UV-vis absorbance.

The concentration analysis by UV-vis absorbance is based on Lambert-Beer’s law [17]: where I and I are the intensity of the incident and transmitted light, respectively, T is the transmittance, A is the absorbance, ε is the absorption coefficient (L/(g•cm) or L/(mol•cm)), l is the optical path length (OPL; the length of the medium that the light passes through; common unit is cm), and c is the concentration of the absorbing substance (mol/L or g/L). MnSO4 solution shows color only at high concentrations up to dozens of grams per liter (S1 Fig in S1 File). So, its UV-vis absorbance is very weak. However, a detectable absorbance is essential to accurate quantification. It is necessary to enhance the absorbance intensity of MnSO4 solution. The correlation between concentration and absorbance is shown by Eq (2). As can be seen, there are two ways to increase the absorbance: increasing the OPL (l) or the apparent absorption coefficient (ε).

In this work, Mn2+ concentration in electrolyte was investigated to be directly determined with cost-effective UV-vis absorbance measurement which also discharges no pollutants. The instrumental parameters and the solution conditions for determining the Mn2+ concentration in MnSO4 electrolyte were optimized. Interference from coexisting substances on Mn2+ absorbance was subtracted by mathematical absorbance data processing. To the best of our knowledge, this is the first study quantifying Mn2+ in electrolyte by its UV-vis absorbance.

The idea of introducing UV-vis spectrometry into on-line detection in industry was launched in this study because of its incomparable speed and simpleness. Some substances have weak UV-vis absorption bands. But their UV-vis absorption signal will become clear in industrial process streams with high concentrations. With the growing requirement for precise process control and pollutant reduction, UV-vis detection method development in industry deserves attention. In addition, instrumental parameter adjustment of UV-vis spectrometry is to some extent neglected in chemical analysis. It was analyzed theoretically in detail in this study how adequately choosing the OPL, spectral band width (SBW) will help improving measurement precision.

Materials and methods

Reagents and solutions

Manganese sulfate, ammonium sulfate, ferric sulfate, cobalt sulfate, and nickel sulfate were purchased from Aladdin Industrial Corp. (Shanghai, China). Copper sulfate, zinc sulfate, magnesium sulfate, calcium sulfate, sodium silicate, arsenic acid and sulfuric acid were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the reagents were of analytical grade. The simulated electrolyte was prepared by dissolving analytical-grade manganese sulfate and other reagents in ultrapure water. Sulfuric acid was used to adjust the pH. Different solutions were prepared in 100.0 mL glass volumetric flask.

Instrumentation

The absorption spectra were obtained using a T10CS double-beam spectrophotometer (Persee, China), wherein the OPL can be changed from 1.0 cm to 10.0 cm. The SBW of the instrument is also adjustable. Particle size was measured by a Nano Z Zetasizer (Malvern, UK). Concentrations of coexisting substances were measured by ICP (Agilent 5110, USA). Solution pH was measured by pH meter (Metler Toledo Fiveeasy, Switzerland). Each sample was measured 20 times by the spectrometer and absorbance was averaged as the final outcome.

Results and discussion

Determining the characteristic absorption peaks

To determine the characteristic absorption peak with which to quantify Mn2+, we scanned a pure aqueous MnSO4 solution at wavelengths from 300 to 800 nm. A Mn2+ concentration in solution as high as 80.0 g/L was tested to ensure clear visualization of the absorption peak of Mn2+. The SBW of the spectrometer was 2.0 nm, which is the default instrument setting. Cuvettes with OPL of 1.0 and 10.0 cm were used. A 2.0 mol/L H2SO4 solution was prepared and measured as a reference. The SO42- concentration in the reference was much higher than that in the MnSO4 solution in order to find out whether the absorption spectrum of MnSO4 is influenced by SO42-. If SO42- also absorbs in the wavelength region studied, the spectrum will change in shape and intensity and the absorbance will be negative when SO42- concentration in the reference is much higher than that in the MnSO4 solution. The spectrum of MnSO4 has absorption peaks at 336 nm, 357 nm, 401 nm, 433 nm, and 530 nm, whether measured in the typical 1.0 cm cuvette or in a spectrum-enhancing 10.0 cm cuvette (Fig 1). These peaks were potentially correlated with the Mn2+ concentration. The spectrum of MnSO4 measured with ultrapure water as the reference was the same as that measured using H2SO4 as the reference. As was shown, SO42- did not affect the Mn2+ spectrum. Ultrapure water was used as the reference in the following experiments.

Fig 1

Spectra of a MnSO4 solution measured with different cuvettes and references.

Absorption coefficient measurement and effect of pH

Solutions with different concentrations of MnSO4 were prepared in 100.0 mL glass volumetric flasks, and their absorbance peaks were measured in a 1.0 cm cuvette. Fig 2 shows that the absorbance at 401 nm was not linearly related to the concentration, disobeying Lambert-Beer’s law. As can be seen, when Mn2+ concentration increases from 50.0 g/L to 100.0 g/L, the absorbance at 401nm was almost the same. The tested MnSO4 solution was stored for one day. The next day, the originally transparent pink solution was brown with flocs and sediment in it (S1 Fig in S1 File).

Fig 2

Spectra of MnSO4 solutions of different concentrations without pH adjustment.

MnSO4 is unstable under neutral to alkaline conditions [19, 20]. It will reciprocally react with OH- to generate Mn(OH)2 and further react with oxygen to generate insoluble oxides. S2 Fig in S1 File shows the spectra of MnSO4 solutions with the same concentration but different pH values. As the pH increased, the absorbance obviously increased, especially at shorter wavelengths. This is similar to the experimental results in other articles studying the influence of scattering on spectra for turbidity measurement [21]. Turbidity from generation and oxidation of Mn(OH)2 caused scattering of light in MnSO4 solution, which contributes to part of absorbance. As absorbance was not totally from Mn2+ absorption, it was not linearly related to Mn2+ concentration.

The particle size distribution in two solutions of MnSO4 with pH = 1.8 and pH = 7.4 was measured, and the results in S3 Fig in S1 File show that the majority of the particles’ size in the sample with pH = 7.4 were larger than those in the other solution. Therefore, scattering will be severer at higher pH which is in accordance with results in S2 Fig in S1 File.

In actual production, reductants such as SeO2 and SO2 are added to the electrolyte to prevent the generation of such oxidized solids [1]. However, to minimize influence from Mn(OH)2 generation and oxidation, the pH should be adjusted to acidic before measurement. New solutions of MnSO4 at different concentrations were prepared (10.0, 30.0, 60.0, 80.0 and 100.0 g/L) in 100.0 mL glass volumetric flasks by dissolving MnSO4 powder in ultrapure water and 5.0 mL concentrated sulfuric acid (18.4 mol/L H2SO4) was added to each flask before dilution to volume to ensure acidic pH. Fig 3 shows the absorption spectra of different solutions. There was no obvious increase in absorbance at shorter wavelengths. Therefore, the influence from turbidity was significantly reduced. It could be seen that the MnSO4 solution became stable under acidic conditions. The values of the peak absorbance of these solutions (1.0 cm cuvette), shown in S4 Fig in S1 File, were linearly related (R2>0.98) to the solution concentration after pH adjustment. Fitting formulas including absorption coefficient of different peaks are also shown in S4 Fig in S1 File. The absorption peak at 401 nm was highest and was selected as the characteristic peak for the following experiments.

Fig 3

Spectra of MnSO4 solutions with different concentrations and low pH.

The precise pH that ensures solution stability should be determined to minimize the amount of sulfuric acid added for cost considerations. Four solutions with the same concentration but different pH values were prepared and placed in an indoor environment for 30 min before measuring their spectra (S5 Fig in S1 File). An increase in absorbance at shorter wavelengths was still obvious when pH = 3.84, as shown in S5 Fig in S1 File. However, when the pH was under approximately 2.0, the spectra were stable. Therefore, the optimum pH was 2.0 and solutions were adjusted to pH≈2.0 in the following experiment. H2SO4 was used as the pH controlling reagent and no new ions were introduced during pH adjustment into the MnSO4 solution which can also be treated as electrolyte. If method developed in this work is used in industry for determination of Mn2+ concentration in electrolyte, samples after measurement can be returned directly to the production line so that there will not be waste fluid discharge.

Theoretical analysis of optimum instrumental bandwidth

Samples with a concentration of 80.0 g/L in a 1.0 cm cuvette were scanned at different SBW. Fig 4 shows that when the spectral bandwidth varied from 0.1 nm to 3.6 nm at increments of 0.5 nm, the absorbance first increased and then decreased. The relationship between the measured and real absorbance is expressed by Eq (3): where Ams is the measured absorbance, Arl is the real absorbance and Aerr is the measurement error. Aerr comes from instrument or measurement. So, it is fixed under stable measuring condition. Higher Arl minimizes the impact of Aerr by lowering the relative error. As the absorption coefficient of MnSO4 in solution is only 6.557*10−4 L/(g•cm) at 401 nm (SBW = 2nm, S4(c) Fig in S1 File), it is important to increase the absorbance to avoid deviation from the true value. Relationship between absorbance and SBW has been reported in previous studies [22, 23] revealing that total absorbance varies with SBW.

Fig 4

Spectra of an 80.0 g/L MnSO4 solution measured at different SBWs.

0.1~3.6 nm, 0.5 nm increments. Inset is absorbance at approximately 401 nm at different SBWs (0.6~1.3 nm, 0.1 nm increments).

It is supposed that there may exist an optimum SBW producing the highest measured absorbance which will make measuring result more accurate. Fig 4 inset reveals that when the SBW changes from 0.6 to 1.6 nm at a step width of 0.1 nm, the absorbance follows the same trend as Fig 4. It first increases and then decreases before reaching a maximum when SBW is 1.0 nm. It is determined by the test results above that 1.0 nm shows to be the optimum SBW value which produces the highest absorbance. Fig 6 shows that, at an SBW of 1.0 nm, the absorbance and concentration are still linearly correlated. Therefore, the SBW was adjusted to 1.0 nm in the following experiment.

Fig 6

Spectra of MnSO4 solutions with different concentrations and the revised absorbance at 401nm.

Inset is the revised absorbance (by subtracting Asub from linear interpolation) versus the concentration.

Effect of the OPL

Eq (2) indicates that the absorbance is also affected by the OPL (l). Suitable path length selection is essential to the measurement of absorption spectra [24]. Measurement sensitivity is influenced by variations in the OPL. By transforming Eq (2), the relationship between the incident and transmitted light can be expressed as follows:

The transmitted light intensity variation per unit analyte concentration is defined as the sensitivity |SEN|, whose expression is:

The instrument noise of a spectrophotometer is mainly thermal noise from the detector, which affects both the incident and transmitted light [24]. Because the incident light is stronger than the transmitted light, noise from incident light can be ignored. Defining the noise from transmitted light as σIt and the deviation in concentration as σc, the concentration deviation caused by noise is obtained as:

A high |SEN| can lower the concentration deviation due to noise. At the same time, |SEN| is dependent on OPL as revealed by Eq (5). So, OPL may be a factor influencing measurement accuracy. S1 Table in S1 File provides a summary of the absorbance measured at the same concentration (40.0 g/L) but different OPLs (1.0, 2.0, 3.0, 5.0, 10.0 cm). Each sample was measured 20 times and the standard deviation for each sample was approximately the same, indicating that the absolute value of the deviation in absorbance from the true value was nearly the same for each sample. However, when the path length was longer, the absorbance measured at the same concentration was higher. Therefore, the relative deviation in absorbance and concentration was lowest for samples measured at the longest OPL. 10.0 cm should be the optimum OPL in the available path length series and is chosen as OPL used in the following experiment.

The Mn2+ concentration in qualified leaching solutions and electrolytes varied within a narrow range (newly prepared leachate, 36.0~40.0 g/L; electrolyte in electrolyzer, 15.0~18.0 g/L) [1]. It is hard and important to report the precise real-time Mn2+ concentration. A method for measuring the amount of Mn2+ in electrolyte should be sensitive enough to detect the slight change in concentration. MnSO4 solutions with concentrations of 16.0, 17.0, 18.0, 38.0, 39.0, and 40.0 g/L were measured in a 10.0 cm cuvette. S6 Fig in S1 File presents the spectra of these solutions. There were distinguishable differences in the absorbance values of the different spectra. The concentrations calculated with these absorbance values were nearly equal to the true concentrations.

Eliminating influence from coexisting substances

Leaching solutions and electrolytes contain substances other than Mn2+ due to additive addition and incomplete impurity removal [1]. We sampled real qualified electrolyte from one electrolytic manganese plant in China. Concentrations of all the coexisting substances were measured by ICP or acquired from the operation parameters. The data are summarized in S2 Table in S1 File. The spectrum of a mixture containing 40.0 g/L Mn2+ and all the other coexisting substances (concentrations in accordance with data in S2 Table in S1 File) is shown in S7 Fig in S1 File. The absorbance at 401 nm of this mixture was higher than that of a pure MnSO4 solution containing 40.0 g/L Mn2+, revealing that coexisting substances will interfere with the Mn2+ concentration measurement by increasing the absorbance.

Fig 5A presents the spectrum of a mixture solution containing all the coexisting substances except Mn2+ at the same concentration shown in S2 Table in S1 File. The absorbance at 401 nm was nonzero. Therefore, the absorbance due to coexisting substances should be subtracted. Fig 5B was obtained by the second derivation of the spectrum in Fig 5A. The derivative value around approximately 401 nm was almost zero, which means that the absorbance around approximately 401 nm caused by coexisting substances follows a function whose order is no more than 1. Therefore, the absorbance around 401 nm from coexisting substances follow a linear functional relationship with wavelength. Thus, this absorbance value can be calculated by linear interpolation using the absorbance values at two wavelengths of absorption valleys on the left and right sides of 401 nm. In the pure MnSO4 spectra, the two absorption valleys nearest to 401 nm were at approximately 386 nm and 411 nm. Therefore, A line is drawn between the two valley points. The value on this line at the wavelength of 401 nm is to be subtracted from the measured absorbance at 401 nm on the spectra of the electrolyte containing all kinds of coexisting substances. The absorbance value to be subtracted is defined as: where Asub is the absorbance to be subtracted from total absorbance at 401 nm, A386 is the absorbance on the spectra of the electrolyte at 386 nm, and A411 is the absorbance on the spectra of the electrolyte at 411 nm. 386, 401 and 411 represents the corresponding wavelengths. The fraction formula in Eq (7) represents the ratio of distances between 401 nm, 411 nm and 386 nm which is also the ratio of (Asub-A386) to (A411-A386). Both coexisting substances and Mn2+ contribute to absorbances in Eq (7) which makes Asub include absorbance from both coexisting substances and Mn2+ spectrum. Therefore, the subtraction may change relationship between Mn2+ absorbance and concentration. To test whether subtracting Asub from absorbance will influence the Mn2+ concentration measurement, Asub were calculated with spectra of pure MnSO4 solutions at different concentrations and were subtracted from the original absorbances at 401 nm to obtain the revised absorbances, which still had a good linear relationship with the Mn2+ concentration (Fig 6). For example, A386 and A411 of 40.0 g/L pure Mn2+ solution is 0.0615 and 0.085 respectively as is shown in Fig 6. Therefore, Asub for this solution is 0.07560 according to Eq (7). As the measured absorbance at 401 nm of the solution is 0.29925, revise absorbance at 401 nm is 0.22365 by subtracting Asub from measured absorbance. Revised absorbance of different solutions is all calculated in this way. The correlation coefficient (R2>0.999, inset graph of Fig 6) between the revised absorbance and concentration was even higher than that between the unrevised absorbance and concentration in S4(c) Fig in S1 File. Therefore, subtracting Asub will not change the linear relationship between Mn2+ concentration and absorbance. As can be seen in Fig 6, there is good linear relationship between the revised absorbance and concentration at a concentration range from 10.0 g/L to 80.0 g/L. This concentration range covers the range of Mn2+ concentration in the electrolytic manganese production which is mentioned in the introduction section (newly prepared leachate, 36.0~40.0 g/L; electrolyte in electrolyzer, 15.0~18.0 g/L). So, the concentration range of Mn2+ to be analyzed in the practical production is well within the linear range of the measurement method in this study. The limit of detection (LOD) and the limit of quantification (LOQ) based on absorbance of this method were calculated as 3 times and 10 times the absorbance noise of the spectrometer respectively. The absorbance noise was provided by the manufacturer as 0.0005. So, the LOD and LQD are 0.0015 and 0.005 respectively. Converted to concentration using linear equation in Fig 6, the LOD and LQD of this method are 1.1 g/L and 1.7 g/L respectively.

Fig 5

Spectra of mixture solution containing all substances at their respective concentrations in industry.

(a) original spectrum and (b) second derivative spectrum.

The method above was applied to calculate the revised absorbance at 401nm of simulated electrolyte samples containing different concentrations of Mn2+ and all the coexisting substances at their respective concentrations in industry (S2 Table in S1 File) first. Then, the Mn2+ concentrations were back calculated with the revised absorbance and the fitted curve in Fig 6 inset. Concentrations calculated with the unrevised absorbances and fitted curves in S4 Fig in S1 File are compared with the concentrations calculated with the revised values by method in this section (S3 Table in S1 File). It is revealed in S3 Table in S1 File that revision leads to accurate calculated concentrations with relative error less than 1.7% while the concentrations calculated with unrevised absorbances exhibit relative errors over 23.0%. Linear interpolation is an effective method to eliminate interferences from coexisting substances and acquire accurate concentrations when other parameters, such as the pH, SBW and OPL, have been optimized.

Conclusion

Simple UV-vis spectrometry was used to determine the Mn2+ concentration in this study, and the developed method met the requirements for both speed and accuracy without a pollution risk. The method is easy to realize the on-line mode when the sampling procedure is automated due to the fast speed of UV-vis measurement, which further facilitates the application effectiveness. Considerable expenses, such as reagent and labor costs, are incurred for Mn2+ detection in industry, which also generates pollution because current standard methods measuring Mn2+ are based on titration which consumes many polluting reagents while the detection speed is low [25-27]. Application of the method developed in this work will bring about economic and environmental benefits, especially in China which is the largest electrolytic manganese producer in the world.

In this work, influence from parameters such as pH, SBW and OPL on precise Mn2+ concentration determination was described in detail. It is stressed in this study that appropriate instrumental parameter adjustment will improve analytical performance which also should be paid enough attention to during all kinds of UV-vis measurement.

Metals such as Cu, Ni, and Co are electrolytically produced in industry, and their sulfates, which are the major constituents in their respective electrolytes, also absorb light in the UV-vis spectral region in aqueous solution [16]. Therefore, the developed method for Mn2+ quantification, including the pH, SBW, OPL optimization and the interference removal via mathematical treatment of the absorbance, may be universally applicable. Electrolytic metal industries other than electrolytic manganese can refer to this work to develop green real-time detection methods and cut back on the contaminants discharged into environment.

(DOCX)

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24 Jan 2022

9 Feb 2022

Response to Reviewers

Dear editor and reviewers,

Thank you for the editor’s email informing us of manuscript revision and the reviewers’ comments on our manuscript entitled "Rapid and green detection of manganese in electrolytes by UV-vis spectrometry to control pollutant discharge" (Manuscript Number: PONE-D-21-38889). These comments are of great value and help for improving our manuscript.

We read up on the comments and made corresponding response carefully. The color of newly added contents in the revised manuscript are in red. Deleted contents still exist but are marked in red and with red strikethrough. All the numbers of pages, and lines in this response to reviewers are in accordance with revised manuscript with track changes.

Our point-by-point responses to the reviewers’ comments are as follows:

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Response: We have reviewed our reference list to ensure that it is complete and correct. We added more articles in the reference of the revised manuscript according to the reviewer's comment. This is mentioned in the Response to Reviewers.

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Response: Work underlying this manuscript such as the design of this study, the organization and analyses of data, the paper writing and the propriety and organization of language have been taken seriously. We have done our best to make sure that this manuscript is technically sound and the data support the conclusions.

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: N/A

Response: We were careful and serious with the organization and analyses of data and the propriety and organization of language to make sure that the statistical analysis has been performed appropriately and rigorously.

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3. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: Yes

Reviewer #2: No

Response: We have checked this manuscript carefully in the first submission to make sure that all raw data underlying our findings can be found in the figures and tables of the manuscript and its Supporting Information. During the revision this time, we checked this manuscript and its Supporting Information again carefully and tried our best to find out if we missed any raw data in the manuscript and Supporting Information. Maybe S3 table of Supporting Information missed absorbance at 386 nm and 411 nm which are raw data for calculation of the revised absorbance at 401 nm in this table. We added these data into S3 table of Supporting Information. We have made it clear in the Data Availability Statement in the manuscript PDF file that all relevant data are within the manuscript and its Supporting Information files. We ensure that we have made all data underlying the findings in our manuscript fully available.

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5. Review Comments to the Author

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Reviewer #1: In electrolytic manganese industry, controlling manganese (Mn2+) concentration is very important to product quality. However, the traditional method of detecting manganese (Mn2+) concentration is complicated and time-consuming. Xue et al. reports a new method for rapid and environmentally friendly determination of Mn2+ in electrolyte and explores the best determination conditions. The following comments need to be addressed before the work can be further considered for publication.

1. The abbreviations that first appear in this article should be defined.

Response: Thank you for your helpful comment. We checked the manuscript again carefully to add the full name of every abbreviation to the position of its first appearance in the manuscript.

2. The resolution of illustrations in this paper is too low, so it is recommended to improve. There are shadows in the picture.

Response: Thank you for your helpful comment. We improved the resolution of all illustrations in this manuscript and uploaded them together with the revised manuscript again. Our manuscript is converted to PDF for you to review. The resolution of the illustrations indeed decreases if directly read in the PDF manuscript. There is download link for every uploaded illustration in the page where this illustration exists in the PDF manuscript. You can download the illustration to read it in a high-resolution form.

3. The best reading range of spectrophotometer is 0.2-0.8. The absorbance values of some illustrations in this paper are not in this range. Does it have any influence?

Response: Thank you for your helpful comment. The conclusion that the best reading range of spectrophotometers is 0.2-0.8 is classical but a little out-of-date due to the fast improvement of UV-vis spectrophotometers nowadays. The best reading range of a spectrophotometer is decided by its quality and performance. The spectrophotometer used in this study is a T10CS double-beam spectrophotometer (Persee, China). It is relatively expensive and has very excellent performance among all kinds of spectrophotometers from different brands. The detailed performance parameters of T10CS can be found in the manufacturer's official website.

The equation with which the best reading range is calculated can be expressed as follows:

E_r=dT/TlnT*100%

Where Er is the relative error in UV-vis measurement. T is the true transmittance. dT is the absolute measurement error of transmittance. The detailed derivation procedure of this equation can be seen in the journal article in brackets (Analytical Chemistry 1955, 27, 5, 716–725). dT is strongly associated with the quality and performance of UV-vis spectrophotometers. Poorer spectrophotometers have larger dT values. Better spectrophotometers have smaller dT values but the price will be higher. The corresponding values of Er, dT, T and absorbance are listed in the table as follows:

T % 95 94 93 92 91 90 80 70 65

Absorbance 0.022 0.027 0.032 0.036 0.041 0.046 0.097 0.155 0.187

Er %(dT=1.0%) 20.522 17.193 14.817 13.036 11.652 10.546 5.602 4.005 3.571

Er %(dT=0.5%) 10.261 8.597 7.408 6.518 5.826 5.273 2.801 2.003 1.786

Er %(dT=0.3%) 6.157 5.158 4.445 3.911 3.496 3.164 1.681 1.202 1.071

Er %(dT=0.15%) 3.078 2.579 2.223 1.955 1.748 1.582 0.840 0.601 0.536

T % 60 50 40 36.8 30 20 15 10 5

Absorbance 0.222 0.301 0.398 0.434 0.523 0.699 0.824 1.000 1.300

Er %(dT=1.0%) 3.263 2.885 2.728 2.718 2.769 3.107 3.514 4.343 6.676

Er %(dT=0.5%) 1.631 1.443 1.364 1.359 1.384 1.553 1.757 2.171 3.338

Er %(dT=0.3%) 0.979 0.866 0.819 0.815 0.831 0.932 1.054 1.303 2.003

Er %(dT=0.15%) 0.489 0.433 0.409 0.408 0.415 0.466 0.527 0.651 1.001

As can be seen, Er at the same T values changes with dT. If the ideal Er is set as less than around 3.5%, it can be revealed from the table above that the best T should vary in a range of around 65% to 15% at the poor dT of 1.0%. Converted from transmittance to absorbance, this range equals to about 0.2~0.8. Calculation above is the origin of the conclusion mentioned by you that the best reading range of spectrophotometers is 0.2-0.8. But the quality and performance of UV-vis spectrophotometers are improving fast nowadays. For the T10CS spectrophotometer used in this study, the nominal dT from its manual is 0.3% which is much better. The technicist from the manufacturer told us that the actual dT of this spectrophotometer can be less than 0.15%. Under such excellent dT values, if the ideal Er is still set as less than around 3.5%, the corresponding best absorbance reading range of T10CS spectrophotometer should be 0.04~>1.3 (dT=0.3%) and <0.02~>1.3 (dT=0.15%) respectively. So, the absorbance values in this paper are all within the best absorbance reading range of UV-vis spectrophotometer due to the high quality and performance of T10CS spectrophotometer.

4. The linear range and detection limit of this method?

Response: Thank you for your helpful comment. The linear range and detection limit of this method were described and calculated in the revised manuscript with track changes (Pages 19-20; lines 404-416).

5. According to Fig S7, coexisting substances have certain influence on the detection of Mn2+. Can other methods be used to reduce the influence?

Response: Thank you for your valuable comment. There is a section named "Eliminating influence from coexisting substances" in this manuscript which specially discussed the method to reduce the influence from coexisting substances on the actual UV-vis absorbance of Mn2+. Linear interpolation was found to be a good method to determine the interfering absorbance value which is to be subtracted from the measured absorbance of the electrolyte containing all kinds of coexisting substances. After subtraction, there was still linear relationship between the revised absorbance and concentration with which the Mn2+ concentration in electrolyte could be accurately calculated and the interference from coexisting substances on concentration determination was removed (see Supporting Information S3 Table). Detailed contents of this section can be seen in pages 17-20; lines 351-433 in the revised manuscript with track changes.

6. There are some formatting errors in the references, please check them carefully.

Response: Thank you for your valuable comment. We checked the reference carefully again to ensure that all formatting errors have been corrected.

7. More recent work is suggested to be referred: Electrochem. Commun. 2021, 123, 106912; Anal. Chem. 2017, 89, 2191-2195.

Response: Thanks for your helpful comment. The two journal articles mentioned in this comment were all added to the reference of the revised manuscript (reference NO. 8 and 11).

Thank you very much for your comments and advices.

Reviewer #2: 1) The continuity in the text is missing in the starting paragraph and appears as fragmented sentences.

Response: Thank you for your valuable comment. We rewrote the introduction section according to your comment. The language and contents were reorganized to make the introduction section more fluent, concise and logical (Pages 2-10; lines 39-198 in the revised manuscript with track changes).

2) line 61- current methods [8,9] - the term "current methods" may be modified for readability. The unique results observed in the cited reference should be properly highlighted.

Response: Thank you for your valuable comment. The whole sentences have been revised according to your comment (Page 5; lines 101-109 in the revised manuscript with track changes). The two cited references all used standard titrimetric method to determine Mn2+ concentration. This is the reason we cite them. Their unique results are not relevant to analytical methods development so that they are not necessary to be highlighted.

3) line 75- expensive instrument like.... so on [12,13]- avoid short phrases such as so on, rather briefly present different instruments and discuss their advantages and disadvantages.

Response: Thank you for your valuable comment. The whole sentences have been revised according to your comment (Page 6; lines 118-123 in the revised manuscript with track changes).

4) line 75- few researchers .. the author must cite with proper references and present the supporting data.

Response: Thank you for your helpful comment. Actually, after a careful search, we found no research paper on developing methods specially for accurately measuring the high Mn2+ concentration in electrolytic manganese industry. There is no reference for us to cite. We used the word "few" in this sentence of the unrevised manuscript to express negative meanings and for sake of prudent expression. This sentence was revised to avoid misunderstanding (Page 6; lines 130-132 in the revised manuscript with track changes).

5) UV-visible absorption is highly sensitive detection method and it is prone to possible interferences from other components since many substances absorb broad regions of the spectrum. Author must run liquid chromatography of the sample to rule out any interferences.

Response: Thank you for your valuable comment. The research object of this study is electrolyte in electrolytic manganese industry. It is actually an inorganic salt solution. Liquid chromatography is used to separate organic substances. So, it will not be useful for ruling out any interference in this study. Deep purification is necessary for the preparation of manganese electrolyte. So, manganese electrolyte is actually a relatively simple mixed solution containing mainly MnSO4 and some inorganic additives and trace inorganic impurities. The concentration and composition of additives and impurities in manganese electrolyte are definite and well-known which are listed in S2 Table of Supporting Information. The additives and impurities indeed interfered with the accurate UV-vis absorbance measurement of Mn2+. So, there is a section named "Eliminating influence from coexisting substances" in this manuscript which specially discussed the method to reduce the influence from coexisting substances on the actual UV-vis absorbance of Mn2+ (Pages 17-20; lines 351-433 in the revised manuscript with track changes). Linear interpolation was found to be a good method to determine the interfering absorbance values from coexisting substances which is to be subtracted from the measured absorbance of the electrolyte in this study. After subtraction, there was still linear relationship between the revised absorbance and concentration with which the Mn2+ concentration in electrolyte could be accurately calculated and the interference from coexisting substances on concentration determination was removed (see Supporting Information S3 Table).

6) Author claimed that the UV-visible approach for quantitative analysis adopted in current studies to be accurate without pollution risk, but H2SO4 used in this study to reduce turbidity of sample is highly corrosive! how would author explain this?

Response: Thanks for your helpful comment. We have taken the environmental pollution risk of discharging any waste liquid after measurement into consideration. In this study, only H2SO4 were added into the electrolyte sample to be analyzed which mainly contained MnSO4. No other ions were introduced into the electrolyte sample to be analyzed during Mn2+ concentration measurement with UV-vis approach in this study. So, waste liquid produced after being analyzed by the method in this study can be directly returned to the production line of electrolytic manganese industry. Hence there will be no discharge of waste liquid and no pollution risk for the Mn2+ concentration measurement method in this study. There is no need to worry about the corrosive H2SO4 being discharged. The contents above describing the pollution-free nature of the method in this study have already been briefly included in the unrevised and revised manuscript (Page 14; lines 287-291 in the revised manuscript with track changes).

________________________________________

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14 Feb 2022

Rapid and green detection of manganese in electrolytes by ultraviolet-visible spectrometry to control pollutant discharge

PONE-D-21-38889R1

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18 Feb 2022

PONE-D-21-38889R1

Rapid and green detection of manganese in electrolytes by ultraviolet-visible spectrometry to control pollutant discharge

Dear Dr. Li:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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