Validation of Calculation Method and Carbonate Approach with Real COD Values
The difference between measured COD ( after addition) and original COD ( before addition) is the excess in COD caused by , as a ratio in . Table S1 lists the ratios determined in this work and in the literature for different wastewaters, whereby interference ratios are in the range . These results demonstrate the need for a uniform experimental approach to remove from wastewater samples without changing COD.
To overcome the difficulties in determining COD values in effluents containing
, different methods were investigated. Eq. (
2) was first used to calculate a
of
for all tested wastewaters based on the
added to each sample (
).
, measured in the presence of
, was corrected by
to
and compared with
of the original wastewater (Table
1). Results of the comparison are summarized in Table
2. The difference (ΔCOD) between
and
indicated the predictive quality of the COD calculation based on measured or known
concentrations for different wastewater types.
ΔCOD ranged from
to
, where negative ΔCOD indicated that predicted COD was lower than original (measured) COD. The relative error was between
(for leachate) and 76.6% (for CID) and increased with decreasing COD values. Eq. (
2) may therefore be useful to estimate possible interference with an error of
in COD at an
concentration of
. However, this error is often not acceptable for evaluations of COD removal efficiency. These results and the interferences shown in Table
S1 confirm that the interference ratio is influenced by general wastewater characteristics and not only by
concentration [as stated by Kang et al. (
1999)] in Eq. (
2).
The carbonate approach was then tested for SWW containing
under the conditions proposed by Wu and Englehardt (
2012) [90°C and high
(
)], which resulted in a pH of 11. Samples were taken during the 60-min reaction of SWW at 90°C to determine COD and
concentrations at different reaction times. The standardized values (
) are shown in Fig.
S1. During the first 5 min,
of
was removed but COD also decreased by
. COD was reduced by
in a reaction time of more than 5 min, indicating a competing reaction of COD oxidation caused by oxidative species, as well as
decomposition. Therefore, the role of carbonate/bicarbonate species and their pH effect were studied in further detail.
Decomposition in
To study the effect of
on
decomposition, the decrease in
in two samples of deionized water and 1 M
was observed at ambient temperature. After shaking both samples for 24 h,
concentration decreased from
to 43.2 and
in deionized water and 1 M
solution, respectively. Iodometric measurement confirmed this result, which was directly visible by the different color densities for the deionized water and the
solution, as shown in Fig.
3(a). The calculated
decomposition rate was approximately
in 1 M
solution, while in deionized water
was almost stable for one day.
During shaking, the generation and decomposition of
was expected according to Reaction 3 followed by Pathways B and C, generating the final species:
,
, and
. The generation of oxygen was confirmed by the formation of oxygen bubbles observed in samples containing
. The bubbles started to grow on the wall of sample tubes 20 min after
was added [Fig.
3(b)]. Dissolved oxygen was measured at ambient temperature (∼ 20°C) in both samples, at
for the
solution and
for the deionized water, indicating higher saturation with pure oxygen generated by
in solution.
Optimization for Different Wastewater Types
Table
3 summarizes the progress of
decomposition and pH to determine the appropriate dose of
to completely remove
. The addition of
shifted the pH value of SWW from almost neutral (pH 7.1) to a basic range (
).
concentration decreased at ambient temperature nonlinearly with
concentration over 24 h. In the first 5 h, only
decomposed at an
concentration of
(Sample 1), whereas
and
decomposed when the
concentration was 1 and
, respectively. After 24 h,
of
was still found in Sample 1, whereas
was completely removed in Samples 2 and 3. The minimum dose of
for complete
decomposition of the samples with residual
concentration of up to
was calculated as
(with 1 M
solution).
To clarify the applicability of the proposed method at higher
concentrations, the last run was repeated with an
concentration of
. As shown in Table
3, after 24 h shaking of SWW samples with 1 M
solution, an
residual of
was still found (Sample 4). A 2-mol/L increase in
ensured the complete decomposition of
after 24 h (Sample 5), which met the ratio of
. Calculation of the concentration ratio (
) for samples with complete
decomposition gave a minimum value of
(Sample 2) for
, and
for
. Therefore, we propose a value of
as a minimum ratio to ensure complete
decomposition regardless of
concentration.
The proposed method was then applied to the four wastewaters (Table
1) containing
at two concentrations (44.5 and
). For each wastewater, four samples were prepared by mixing wastewater with
solutions of 1 (Samples A and B) and
(Samples C and D) (
volume per volume). The
ratio was almost met in Samples B and D (
and
, respectively). Samples A and C were run as blanks without
. After 24-h shaking, no
was detected in Samples B and D for
wastewater types, as shown in Table
4, demonstrating that
was effectively decomposed by
, the comparison of blank and treated samples indicated minimal changes considering the standard deviation between original COD and COD after
pretreatment.
Under the experimental conditions, there appeared to be more than one factor influencing the decomposition of
into its final products (
, and
). The first factor was the high
concentration versus the low
concentration (
). The high concentration of
activated the decomposition reaction (Reaction 3) forming
(
Bakhmutova-Albert et al. 2010), which was subsequently decomposed through the preferred pathways (B and C). The second factor was the basic pH value of all samples (Table
4). A basic pH of 8.62–8.82 increased the abundance of
species according to lime-carbonic acid equilibrium (
Zeebe and Wolf-Gladrow 2005); however, it also supported the self-decomposition of
according to Reaction 4 (
Kolyagin and Kornienko 2003).
The extrapolation of COD results in Table
4 was supported by the formation of oxygen bubbles observed in Samples B and D (active
decomposition by
), as shown in Fig.
3(b). This observation confirmed the dominance of
reactions in Pathways B and C until
decomposition was complete.
A comparison of Sample A with Sample B and Sample C with Sample D for each wastewater showed very close COD values, with a deviation of
(9.5% relative to
) to
(4.8% relative to
). Additionally, Samples A and C (Table
4) confirmed the original COD values for all wastewater samples (Table
1) with absolute deviations between 0 and
. These results indicated that the addition of
did not interfere with the COD measurements (see Runs A and C) and that the decomposition of
was complete (see Runs B and D), while the original sample values were verified after
decomposition (Table
4).
Additionally, the calculated standard deviation (SD) of the COD analysis (Table
4) relating to the accuracy of the spectrophotometer (1%) and sample preparation was approximately 1%–11% relative to the original COD. Therefore, the deviations derived by the proposed approach were in the same range as those derived by the standard method. Indeed, Samples B and D, with
solution, showed a small decrease in COD (Table
4) compared with the COD of the original sample (
in Table
1). This small decrease may indicate that the competing reaction pathway (A in Fig.
1) made only a small contribution to COD oxidation, as proposed by Yang et al. (
2019), because
reacts with electron-rich compounds, such as organic sulfides (
Richardson et al. 2000), amines (
Balagam and Richardson 2008), and alkenes (
Yao and Richardson 2000), which may exist in untreated wastewater. In untreated leachate, this fraction seemed to be higher than in the other wastewaters examined, resulting in the highest SD, which could be neglected, however, due to the much higher COD value of the leachate.
In treated wastewaters, especially those treated by AOPs including the addition or generation of
, organic electron-rich compounds is not expected due to their oxidation by AOPs. Therefore, the competition reaction (A in Fig.
1) is expected to be less relevant for treated wastewater and proposed pretreatments with
. This approach would be more useful for correct COD measurements, which are essential to determine the required limit of COD effluent or to evaluate the COD removal effeciency by AOPs. In summary, the proposed approach at ambient temperature removed
without any significant change in the COD of original samples for the four wastewaters.
Catalytic Decomposition Acceleration by Increased Temperature
The 24-h pretreatment described is sometimes unacceptable due to delays such as for process evaluation. A typical method to accelerate chemical reactions is increasing the temperature. A simple approach for the described method of decomposition is to use the same COD standard test equipment for heating and decomposition. The thermal heat block system commonly includes preset temperatures (e.g., 70°C, 90°C, 100°C, 148°C) for the reaction of organic compounds in the standard COD test. Here, for the mixture of SWW and as well as for subsequent heating and reactions at a fixed standard temperature, empty cuvettes appropriate for the thermal heat block system were used.
The
mixture, with the proposed ratio of
(
) was heated at 70°C, 100°C, and 148°C to accelerate the decomposition of
. Each experiment was stopped when the
was no longer present. Fig.
4 shows that decomposition was complete after 5 min at 148°C. In contrast to the results at ambient temperature (
),
COD removal occurred, indicating oxidation of the original COD.
At 100°C and 5 min reaction time, 6 mmol
remained in the samples: a 12% increase in COD compared with the original tested wastewater. Further heating at 100°C for 30 min caused an additional decrease in COD, to the same concentration as at 148°C. The results for 100°C and 148°C indicated oxidation of a constant fraction of COD in the SWW by
following the oxidation pathway (A in Fig.
1). Parallel to these experiments, blank samples of
without
were heated at 70°C, 100°C, and 148°C for 60, 30, and 20 min, respectively. No COD removal was detected. A decrease in COD at
is expected to be catalytic oxidation by the
mixture.
At 70°C, decreased from 90 to 6 and after 20 and 30 min, respectively. The measured COD after this pretreatment was approximately equal to before treatment. Therefore, 30-min pretreatment at 70°C seems to accelerate decomposition without COD removal. The optimal temperature for a wide range of water and wastewater types is recommended for further study.
To further elucidate the effect of pH on
decomposition, experiments were repeated four times at 70°C and side by side for
and
as
decomposition catalysts. This resulted in pHs of
and
, respectively. Fig.
5 summarizes the results with mean values and SD (details of each experiment are provided in Table
S2). The carbonate catalyst decomposed
approximately 4–5 times faster than the bicarbonate catalyst. Additionally, the COD of the original SWW sample decreased by
with the carbonate catalyst (from 1,195 to
) after 1 h heating, whereas COD removal was four times lower (
) during
decomposition by the bicarbonate catalyst. The main difference between the carbonate and the bicarbonate was pH.
To confirm the importance of pH, additional experiments were carried out using sodium bicarbonate, in which pH was increased from
to
by NaOH addition, followed by
addition and a 30-min reaction at 70°C. According to lime-carbonic acid equilibrium (
Zeebe and Wolf-Gladrow 2005), this action is supposed to force most
to change into
prior to the activation reaction of
. Fig.
S2 shows that
decomposition increased in the first 20 min while COD degraded to below the original concentration after 30 min, similar to COD degradation with
(Fig.
5). The difference between the effect of carbonate and that of bicarbonate on COD removal is most probably related to the quantity and quality of oxidative species formed at different pH values. Other than
and
as oxidative species, singlet oxygen (
) was probably generated through the recombination of
formed during
decomposition, as observed by Bokare and Choi (
2015) for alkaline solutions. Moreover, the copresence of
and
could lead to the slow generation of hydroxyl radicals via the Haber-Weis mechanism (
Bokare and Choi 2015). Using NBT (1.5 mM) to detect
caused the occurrence of deep-blue formazan pigment precipitates after just 1 min in the carbonate sample, while the bicarbonate was still light yellow (Fig.
S3). The dark color of the carbonate sample indicated the generation of
radicals produced via the catalytical decomposition of
(by carbonate). Continuous shaking for 24 h caused the reaction between
and NBT in both samples to become much darker, with more precipitates in the carbonate sample than in the bicarbonate sample, which was light blue with few precipitates in [Figs.
S3(b and c)]. This confirmed that
enabled much less
species generation than
, which was responsible for the original COD that remained unchanged. This suggests that
is beneficial for
decomposition to avoid interference in spectrophotometric COD tests. A quantitative determination of those oxidative species will be a topic for future research.