Introduction

Biological wastewater treatment system had been identified as a source for the increasing nitrous oxide (N2O) in atmosphere (UNEP 2013.). In denitrification, the mechanism of N2O production and loss is quite clear and uncontested. However, the mechanism of N2O production in nitrification is very complicated. In nitrification, ammonia (NH3) is converted to nitrate (NO3 −) via hydroxylamine (NH2OH) and nitrite (NO2 −). N2O can be produced by chemical breakdown of the unstable nitrosyl radical (NOH) (Poughon et al. 2001; Lemaire et al. 2011). Meanwhile, reduction of nitric oxide (NO) produced from the oxidation of NH2OH can also produce N2O (Stein 2011). Besides, the electrons released from the oxidation of NH2OH were used to sustain ammonium oxidation and satisfy the cell’s reductant needs (Arp and Stein 2003). Generally speaking, NH2OH is an important intermediate formed in oxidation of ammonia/ammonium (NH3/NH4 +) to nitrite (NO2 −) by ammonium-oxidizing bacteria (Vajrala et al. 2013), and it has a direct relationship with the production of N2O in biological wastewater treatment processes (Kim et al. 2010).

Gas chromatography method had been successively used for the determination of NH2OH in seawater (Bulter and Gordon 1986; Gebhardt et al. 2004) and in soils (Liu et al. 2014). Due to short-live and extremely reactive properties of NH2OH (Liu et al. 2014), determination of NH2OH in aqueous solution or in soil with gas chromatography method is not an easy task. Many substances can result in a significant errors in the determination, such as NO2 − (Bremner et al. 1980), transition metal ions and complexes (Brown and Drury 1967; Bengtsson 1973; Bengtsson et al. 2002), O2 (Adhikamsetty et al. 2008), and so on. The composition of the domestic wastewater was more complex than seawater. Orthophosphate (PO4 3−), NO2 −, metal ions and organic matters were common in domestic wastewater. Consequently, determination of NH2OH in biological domestic wastewater treatment processes was more difficult than that in seawater or in soil. Furthermore, as the lack of the quantitative relation between NH2OH and N2O, it caused great difficulties in understanding the mechanism of N2O production in biological wastewater treatment process, especially in nitrification. More recently, with the development of nanotechnology, nanostructure-based electrochemical method achieved great developments in the determination of NH2OH (Sadeghi et al. 2013; Moghaddam et al. 2014; Foroughi et al. 2014; Beitollahi et al. 2014; Mozloum-Ardakani et al. 2015; Rezaei et al. 2015). To the electrochemical method, NH2OH cannot be oxidized at conventional electrode materials, and the electrodes used for the determination of NH2OH must be chemically modified in order to lower the overpotentials and increase the oxidation current response (Moghaddam et al. 2014). However, the monitoring ranges of gas chromatography method and the electrochemical method were at the level of μg/L, and the two methods were not suitable for determining milligram or larger amounts of NH2OH. Besides, the determination procedures of the two methods were also complex.

According to the above points, it was essential to construct a simple and convenient method for the determination of NH2OH in biological wastewater treatment process. The aims of this study were: (1) to verify the feasibility of using the spectrophotometric method to determine NH2OH in biological domestic wastewater treatment process, (2) to examine the effects of PO4 3−, NO2 −, nitrate (NO3 −) and trace elements on the spectrophotometric method and (3) to develop a method for eliminating the interference. The research was carried out from May 2016 to January 2017 in Chang’an University, Xi’an city, Shaanxi province, China.

Materials and methods

Principles

Under acidic condition, NH2OH can be stabilized as NH3OH+. The spectrophotometric method was based on oxidation of NH2OH to N2O by Fe(III) using ferric ammonium sulfate (NH4Fe(SO4)2, FAS) as oxidation agent (Bengtsson et al. 2002):

$$ {\text{Fe}}\left( {{\text{NH}}_{2} {\text{OH}}} \right)^{3 + } \to {\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{NO}}^{\cdot} + {\text{H}}^{ + } $$
(1)
$$ {\text{Fe}}^{3 + } + {\text{H}}_{2} {\text{NO}}^{\cdot} \to {\text{Fe}}^{2 + } + {\text{HNO}} + {\text{H}}^{ + } $$
(2)
$$ {\text{HNO}} \to 0.5{\text{N}}_{2} {\text{O}} + 0.5{\text{H}}_{2} {\text{O}} $$
(3)

NH2OH was determined through the formation of ferrion, a tris complex of 1, 10-phenanthroline with Fe(II). Ferrion was a red-colored octahedral complex ion and was soluble and stable in aqueous solution in the pH range of 2–9 (Adhikamsetty et al. 2008). Its molar absorption coefficient was 1.11 × 104 mol/cm at 510 nm, with no peak shift due to pH variation (Hughes et al. 1971).

Solution preparation

Stock solution of sodium nitrite (NaNO2): NaNO2 (AR) was dried in an oven at 105 °C for 24 h, and 1.2320 g NaNO2 was dissolved in 1000 mL deionized water. In order to preserve the solution, 2 mL CHCl3 was added to the solution. 1 mL stock solution of NaNO2 contained 250 μg NO2 −-N.

Stock solution of hydroxylamine chloride (NH2OH·HCl): NH2OH·HCl (GR) was dried in an oven at 50 °C for 24 h, and 2.4821 g NH2OH·HCl was dissolved in 1000 mL deionized water. In order to stabilize NH2OH, 1 mL 1 mol/L hydrochloric acid solution was added to the stock solution. The solution was preserved at 4 °C in dark. 1 mL stock solution of NH2OH·HCl contained 500 μg NH2OH-N.

Standard solution of NH2OH·HCl: The standard solution of NH2OH·HCl was prepared by diluting 1 mL stock solution of NH2OH·HCl to 200 mL before each experiment. 1 mL standard solution of NH2OH·HCl contained 2.5 μg NH2OH-N.

Stock solution of sodium phosphate (\( {\text{Na}}_{3} {\text{PO}}_{ 4} \)): \( {\text{Na}}_{3} {\text{PO}}_{ 4} \) (GR) was dried in an oven at 110 °C for 24 h, and 0.2645 g \( {\text{Na}}_{ 3} {\text{PO}}_{ 4} \) was dissolved in 1000 mL deionized water. 1 mL stock solution of \( {\text{Na}}_{ 3} {\text{PO}}_{ 4} \) contained 50 μg PO4 3−-P.

Standard solution of potassium nitrate (KNO3): KNO3 (AR) was dried in an oven at 105 °C for 24 h, and 0.7218 g KNO3 was dissolved in 1000 mL deionized water. In order to preserve the solution, 2 mL CHCl3 were added to the solution. 1 mL stock solution of KNO3 contained 100 μg NO3 −-N.

Standard solution of calcium chloride (CaCl2): CaCl2 (GR) was dried in an oven at 105 °C for 24 h, and 0.1387 g CaCl2 was dissolved in 1000 mL deionized water. 1 mL standard solution of CaCl2 contained 50 μg C 2+a .

Stock solution of trace elements: The stock solution of trace elements contained H3BO3, CuCl2, ZnCl2, \( ( {\text{NH}}_{ 4} ) {\text{Mo}}_{ 7} {\text{O}}_{2} \cdot 4 {\text{H}}_{ 2} {\text{O}} \), MgSO4, CoCl2· 4H2O, AlCl3 and NiCl2. The compositions of the solution are shown in Table 1.

Table 1 Compositions of the stock solution of trace elements
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1 mL concentrated hydrochloric acid was added to the solution for preservation.

Standard solution of trace elements: The standard solution of trace elements was prepared by diluting 1 mL stock solution of trace elements to 200 mL before each experiment.

Solution of ferric ammonium sulfate (FAS): 1.9287 g NH4Fe(SO4)2· 12H2O (AR) was dissolved in 1000 mL 0.1 mol/L hydrochloric acid solution.

Solution of 1, 10-phenanthroline (1, 10-Phe): 1.8021 g 1,10-Phe (AR) was dissolved in 1000 mL 1 mol/L acetic acid solution.

Solution of sodium acetate and acetate (SAA): 96 mL 1.0 mol/L sodium acetate solution and 114 mL 1.0 mol/L acetic acid solution were mixed. The mixture was preserved at 4 °C in dark.

Procedure

a. Preparation of standard curve: The standard curve of NH2OH-N was prepared in the range of 0.00–1.00 mg NH2OH-N/L by diluting 0.00, 0.20, 0.50, 1.00, 2.00, 4.00 and 10.00 mL of the standard solution of NH2OH·HCl to 25 mL.

b. Color development: 1.00 mL SAA, 1.00 mL FAS and 1.00 mL 1, 10 Phe were added to the 25 mL samples of NH2OH in turn. Before the addition of each reagent, the samples were thoroughly mixed.

c. Photometric measurement: 30 min after adding color reagents to the sample, the absorbance of the sample was read at 510 nm (TU1810PC, PERSEE· Beijing, China), and the absorbance of the deionized water was set at zero. The light path was 1 cm.

During determination of NH2OH-N in other samples, the samples were treated with the same manner as the standard curve.

Experiments

Determination of the time for color development

For determining the time for color development of the spectrophotometric method, the color development time of samples containing different concentrations of NH2OH-N was evaluated. The concentrations of NH2OH-N in the samples were set at 0.00, 0.02 and 0.80 mg/L, respectively.

Impact of NO2 − on the spectrophotometric method

The concentrations of NH2OH-N in samples were set at 0.02, 0.05, 0.10, 0.20 and 0.40 mg/L. In order to examine the impact of NO2 − on the spectrophotometric method, NO2 − was added to the samples. In the experiment of each concentration of NH2OH-N, the concentrations of NO2 −-N in the samples were 2.00, 5.00, 10.00 and 15.00 mg/L, respectively. Each group of the experiment was conducted in triplication.

Impact of PO4 3− on the spectrophotometric method

The concentrations of NH2OH-N in samples were set at 0.02, 0.05, 0.10, 0.20 and 0.40 mg/L. In order to examine the impact of PO4 3− on the spectrophotometric method, PO4 3− was added to the samples. In the experiment of each concentration of NH2OH-N, the concentrations of PO4 3−-P in the samples were 1.00, 2.00, 3.00 and 4.00 mg/L, respectively. Each group of the experiment was conducted in triplication.

Elimination of the interference of PO4 3− on the spectrophotometric method

In order to eliminate the interference of PO4 3− on the method, extra experiments were conducted. The concentrations of NH2OH-N in samples were set at 0.05 and 0.10 mg/L. In the experiment of each concentration of NH2OH-N, the concentrations of PO4 3−-P in the samples were 1.00, 1.50 2.00, 2.50, 3.00, 3.50, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00 and 10.00 mg/L, respectively. Each group of experiments was conducted in triplication.

Verification

In order to verify the spectrophotometric method and the correction method proposed in this study, samples containing different concentrations of NH2OH-N, PO4 3−-P and NO2 −-N were determined by the spectrophotometric method. Meanwhile, the application of the correction method for improving the accuracy of the spectrophotometric method was evaluated. The compositions of the verification samples are shown in Table 2.

Table 2 Compositions of samples for verification of the spectrophotometric method
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Impact of NO3 − on the spectrophotometric method

The concentrations of NH2OH-N in samples were set at 0.05, 0.10, 0.20 and 0.40 mg/L. In order to examine the impacts of NO3 − on the spectrophotometric method, NO3 − was added to the samples. In the experiment of each concentration of NH2OH-N, the concentrations of NO3 −-N in the samples were 6.00, 10.00, 16.00 and 24.00 mg/L, respectively. Each group of the experiment was conducted in triplication.

Impacts of other matters on the spectrophotometric method

In order to examine the impacts of C 2+a and trace elements on the spectrophotometric method, the CaCl2 solution and the trace elements solution were added to the samples. The concentrations of NH2OH-N in samples were set at 0.05, 0.10, 0.20 and 0.40 mg/L. The compositions of the samples are shown in Table 3. Each group of the experiment was conducted in triplication.

Table 3 Compositions of samples containing C 2+a and trace elements
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Results and discussion

The determination time and the standard curve

When NH2OH-N concentrations in the samples were 0.00, 0.02 and 0.80 mg/L, the absorbance of the samples almost not varied when the time for color development was longer than 20 min (Fig. 1), especially in the range of 20–60 min. In the chromogenic process, NH2OH was oxidized to N2O by Fe(III), and Fe(III) was reduced to Fe(II). Then, ferrion formed by coupling 1, 10-phenanthroline with Fe(II). The reaction of NH2OH and Fe(III) at 30 °C was completed in 3 min (Dias et al. 1979). This process was a chemical process and did not need a very long reaction time. So, the determination time of the spectrophotometric method was set at 30 min after all reagents were added.

Fig. 1
figure 1

Time for color development of samples containing different concentrations of NH2OH-N

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When the concentration of NH2OH-N was in the range of 0.02–1.00 mg/L, there was a linear relation between the concentrations of NH2OH-N and the absorbance (y = 1.5078x-0.0132, R 2 = 0.9991) without the presence of interfering substances (Fig. 2).

Fig. 2
figure 2

Standard curve of the spectrophotometric method for determination of NH2OH-N

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The monitoring ranges of the gas chromatography method (Kock and Bange 2013) and the electrochemical method ((Sadeghi et al. 2013; Moghaddam et al. 2014; Foroughi et al. 2014; Beitollahi et al. 2014; Mozloum-Ardakani et al. 2015; Rezaei et al. 2015)) were at the level of μg/L. Compared with the gas chromatography method and the electrochemical method, the monitoring range of the spectrophotometric method was at the level of mg/L. The spectrophotometric method was more suitable for determining milligram or larger amounts of NH2OH.

Impact of NO2 − on the method

The determined NH2OH-N concentrations of the samples containing NO2 − are shown in Table 4.

Table 4 Determined NH2OH-N concentrations of the samples containing different NO2 −-N concentrations
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When the concentrations of NH2OH-N in the samples were 0.02 mg/L, the recoveries of NH2OH-N were only in the range of 58.30–68.33% while NO2 −-N presented, and the spectrophotometric method cannot give satisfied determination results of NH2OH-N. When the concentrations of NH2OH-N in the samples were higher than 0.05 mg/L, the spectrophotometric method can give relatively accurate results of NH2OH-N concentrations when NO2 −-N concentration was lower than 15.00 mg/L. The recoveries of NH2OH-N were in the range of 90.49–113.58%. Compared with the method which directly determined N2O converted from NH2OH-N (Kock and Bange 2013), the presence of NO2 −-N only had a minor interference with the spectrophotometric method when the concentration of NH2OH-N in the samples was higher than 0.02 mg/L.

Under acid condition, Fe(II) can react with NO2 − as follows (Heil et al. 2016):

$$ {\text{Fe}}^{2 + } + {\text{NO}}_{2}^{ - } + 2{\text{H}}^{ + } \to {\text{Fe}}^{3 + } + {\text{NO}} + {\text{H}}_{2} {\text{O}} $$
(4)

However, from the color development process, Fe(II) coupled with 1, 10-phenanthroline to form ferrion instead of reacting with NO2 −. The presence of NO2 − did not affect the formation of ferrion.

It was observed that the recoveries of NH2OH-N were decreased with the increasing NO2 −-N concentration, especially when the concentration of NH2OH-N was higher than 0.02 mg/L. This phenomenon indicated other reactions related to NH2OH and NO2 − occurred in the systems resulting in the decrease in the recoveries of NH2OH-N. As mentioned before, NH2OH is an extremely reactive matter. Although, it can be stabilized under acid condition, NH2OH can react with NO2 − to form hyponitrous acid (H2N2O2) which can decompose to N2O and water rapidly (Bothner-By and Friedman 1952):

$$ {\text{NH}}_{2} {\text{OH}} + {\text{HNO}}_{2} \to {\text{H}}_{2} {\text{N}}_{2} {\text{O}}_{2} + {\text{H}}_{2} {\text{O}} $$
(5)
$$ {\text{H}}_{2} {\text{N}}_{2} {\text{O}}_{2} \to {\text{N}}_{2} {\text{O}} + {\text{H}}_{2} {\text{O}} $$
(6)

Based on the results obtained in our study, the above reactions maybe happen slowly. However, they were not the dominant process; otherwise, the recoveries of NH2OH-N of all samples would be much lower than the obtained values. Nevertheless, the accuracy of the spectrophotometric method was obviously affected when the concentration of NH2OH-N was at a very low level (e.g., 0.02 mg/L). It was recommended that when using the spectrophotometric method to determine the concentration of NH2OH-N in samples containing NO2 −, the determined concentration of NH2OH-N higher than 0.05 mg/L was considered to be effective.

Impact of PO4 3− on the spectrophotometric method

The determined NH2OH-N concentrations of the samples containing PO4 3− are shown in Table 5.

Table 5 Determined NH2OH-N concentrations of the samples containing different PO4 3−-P concentrations
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It was observed that the recoveries of NH2OH-N were decreased with the increasing of PO4 3−-P concentrations. When the concentrations of NH2OH-N in the samples were higher than 0.20 mg/L, PO4 3−-P almost had no impacts on NH2OH-N determination by the spectrophotometric method. When the concentration of NH2OH-N in the samples was 0.10 mg/L, the existence of PO4 3−-P had minor impacts on NH2OH-N determination by the spectrophotometric method. The recoveries of NH2OH-N were in the range of 86.35–102.90%. However, when the concentrations of NH2OH-N in the samples were lower than 0.05 mg/L, there were big errors between the actual concentrations and the determined concentrations of NH2OH-N. The recoveries of NH2OH-N were only in the range of 25.71–91.50%. The errors caused by PO4 3−-P were increased with the increasing PO4 3−-P concentrations. Overall, when the concentrations of NH2OH-N in the samples were lower than 0.10 mg/L, the existence of PO4 3− interfered with the determination of the spectrophotometric method.

In the spectrophotometric method, NH2OH was oxidized to N2O by Fe(III) in FAS; meanwhile, Fe(III) was reduced to Fe(II). When samples contained PO4 3−, Fe(III) and Fe(II) reacted with PO4 3− and formed precipitate (Galal-Gorchev and Stumm 1963; Ghassemi and Rcht 1971). In the spectrophotometric method, the formation of the precipitate was prevented by the low pH value (~4.5). However, due to the reaction between Fe(III)/Fe(II) and PO4 3−, when the concentration of NH2OH-N in the samples was lower than 0.10 mg/L, the accuracy of the spectrophotometric method was affected. 0.10 mg/L of NH2OH-N in samples seemed to be the threshold concentration for the spectrophotometric method.

Numerical method for eliminating the interference of PO4 3−

In order to eliminate the impact of PO4 3− on the spectrophotometric method, the interferences of different PO4 3−-P concentrations on the method were examined when the concentrations of NH2OH-N were 0.05 and 0.10 mg/L, respectively.

When the concentrations of NH2OH-N were 0.05 and 0.10 mg/L, the determined NH2OH-N concentrations were decreased with the increasing PO4 3−-P concentrations (Fig. 3). When the concentration of PO4 3−-P in the samples was 10.00 mg/L, the recoveries of NH2OH-N were only 42.37 and 63.80%, while the concentrations of NH2OH-N were 0.05 and 0.10 mg/L, respectively. PO4 3− seriously interfered with the spectrophotometric method (Table 6).

Fig. 3
figure 3

Determined NH2OH-N concentrations in samples containing different PO4 3−-P concentrations

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Table 6 Determined NH2OH-N concentrations of samples containing different PO4 3−-P concentrations
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In order to eliminate the interference of PO4 3−, a numerical method was proposed. Steps of the numerical method were as follows:

Step 1 was the normalization. The recoveries of NH2OH-N in the samples containing different concentrations of PO4 3−-P were divided with the corresponding concentrations of PO4 3−-P.

Step 2 was the fitting. The concentrations of PO4 3−-P and the normalized recoveries of NH2OH-N were set as the X and Y axes, respectively, for plotting; then, the power function was used to fit the data (Fig. 1 in Supplementary Materials). The mean values of the scaling factor and the exponent of the power function under six experiments were 0.9166 ± 0.0514 (n = 6) and −1.1715 ± 0.0448 (n = 6). Consequently, the correction function for the following step was determined as:

$$ y = 0.9166x^{ - 1.1715} $$
(7)

where x represented the PO4 3−-P concentration in samples, mg/L, and y represented the normalized recovery of NH2OH-N under different PO4 3−-P concentrations.

Step 3 was the correction. The correction procedure included three sub-steps: (1) concentrations of PO4 3−-P and NH2OH-N in samples were determined. The PO4 3−-P concentration was determined according to the standard method (APHA 1998). Based on the determined PO4 3−-P concentration, the normalized recovery of NH2OH-N can be ascertained via Eq. (7); (2) the normalized recovery was revised by multiplying the determined PO4 3−-P concentration; (3) the determined NH2OH-N concentration was divided by the normalized recovery in sub-step (2). As a result, the NH2OH-N concentration in sample can be determined eventually.

Verification of the correction method

In order to verify the availability of the correction method proposed in this study, two conditions were verified. The first condition was samples contained PO4 3−-P except for NH2OH-N, and the second condition was samples contained PO4 3−-P, NO2 −-N and NH2OH-N. The determined NH2OH-N concentrations in samples before and after correction are shown in Table 7.

Table 7 Determined NH2OH-N concentrations in samples before and after the correction
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When samples only contained PO4 3−-P except for NH2OH-N, the numerical correction for the results obtained by the spectrophotometric method was very effective. The accuracy of the spectrophotometric method was improved obviously after the numerical correction. When the NH2OH-N concentration in the sample was 0.06 mg/L and the PO4 3−-P concentrations varied in the range of 1.40–3.80 mg/L, the determined NH2OH-N concentrations were in the range of 0.046–0.055 mg/L, and the recoveries were in the range of 76.67–91.67%. After correction by the numerical method, the determined NH2OH-N concentrations were in the range of 0.062–0.064 mg/L, and the recoveries were in the range of 103.18–105.95%. The similar results were achieved when the concentration of NH2OH-N in the samples was 0.08 mg/L. However, when PO4 3− and NO2 − simultaneously existed in the samples containing NH2OH-N, the situation was more complicated.

When the NH2OH-N concentration was 0.10 mg/L, and the PO4 3−-P concentrations were 3.00 and 4.00 mg/L, respectively, the recoveries of NH2OH-N obtained by the spectrophotometric method were higher than 83.00% when the concentration of NO2 −-N in the sample was lower than 10.00 mg/L. After correction by the numerical method, the recoveries were improved to an extremely high level, and unfortunately, the errors of the spectrophotometric method were also improved. Under these situations, the spectrophotometric method can be used to determine the concentration of NH2OH-N directly.

When the NH2OH-N and PO4 3−-P concentrations were 0.05 mg/L and 3.00 mg/L, respectively, the recoveries of NH2OH-N obtained by the spectrophotometric method were in the range of 64.00–80.00% when the NO2 −-N concentrations varied in the range of 2.00–15.00 mg/L. After correction by the numerical method, the recoveries of NH2OH-N were improved to 84.30–105.37%. When the NH2OH-N and PO4 3−-P concentrations were 0.05 mg/L and 4.00 mg/L, respectively, the recoveries of NH2OH-N obtained by the spectrophotometric method were in the range of 56.00–70.00% when the NO2 −-N concentrations varied in the range of 2.00–15.00 mg/L. After correction by the numerical method, the recoveries of NH2OH-N were improved to 77.49–96.87%. Under these conditions, the recoveries of NH2OH-N obtained by the spectrophotometric method were improved obviously by the numerical method proposed in this study.

Compared with the gas chromatography method (Gebhardt et al. 2004; Kock and Bange 2013) and other spectrophotometric method for NH2OH determination (Kolasa and Wardencki 1974; Dias et al. 1979), the method proposed in this study was suitable for determining milligram amounts of NH2OH-N in aqueous solution. There was a linear relation between the concentrations of NH2OH-N and the absorbance, while the concentration of NH2OH-N was in the range of 0.02–1.00 mg/L. When NO2 − and PO4 3− presented in samples, the linear range varied to 0.05–1.00 mg/L. The side reactions of dissolved NO2 − can result in significant errors in the NH2OH determination by the gas chromatography method (Kock and Bange 2013). However, when NO2 −-N concentration was lower than 15 mg/L, the spectrophotometric method can obtain relatively accurate results. The presence of NO2 − did not cause significant errors for the NH2OH determination. The presence of PO4 3− can result in a significant error in the NH2OH determination when the NH2OH-N concentration was lower than 0.10 mg/L. The numerical method proposed in this study can improve the accuracy of the spectrophotometric method significantly.

However, each method had its applicable scope, and the spectrophotometric method was no exception. If the concentration of NH2OH-N in samples was extremely low, and the concentrations of NO2 −-N and PO4 3−-P in samples were high, the spectrophotometric method was probably not a good choice for the determination of NH2OH-N. It was noteworthy that if the concentration of NH2OH-N in samples was high enough (e.g., 0.4 mg/L), NO2 − and PO4 3− did not interfere with the spectrophotometric method.

Impacts of other matters on the spectrophotometric method

The impacts of NO3 −, C 2+a and trace elements are shown in Table 1 and Table 2 in Supplementary Materials.

When the NO3 −-N concentration varied in the range of 6.00–24.00 mg/L, the recovery of NH2OH-N under four conditions was in the range of 104.40–111.85%. When the C 2+a concentration varied in the range of 2.00–8.00 mg/L and volume of the standard solution of trace elements added to the samples varied in the range of 1.00–5.00 mL, the recoveries of NH2OH-N under the four conditions were in the range of 98.31–112.55%. NO3 −, C 2+a and trace elements did not interfere with the determination of NH2OH-N by the spectrophotometric method.

Conclusion

The spectrophotometric method taking ferric ammonium sulfate and 1, 10-phenanthroline as the oxidant and the chromogenic agent, respectively, can be used to determine the NH2OH-N concentration in biological wastewater treatment processes quickly and conveniently.

  1. 1.

    When the concentrations of NH2OH-N in the samples were higher than 0.05 mg/L, the spectrophotometric method can give relatively accurate results when NO2 −-N concentration was lower than 15.00 mg/L.

  2. 2.

    NO3 −, C 2+a and trace elements did not interfere with the method.

  3. 3.

    The impact of PO4 3− on the method was complicated. The interference of PO4 3− on the method can be eliminated by the numerical method proposed in this study.

The determination method proposed was helpful to understand the quantitative relationship between NH2OH and N2O emission and reveal the function of NH2OH in N2O production in biological wastewater treatment processes.