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. 2022 Apr 26;23(9):4752.
doi: 10.3390/ijms23094752.

Treating High COD Dyeing Wastewater via a Regenerative Sorption-Oxidation Process Using a Nano-Pored Activated Carbon

Shih-Fu Ou  1 , Dun-Sheng Yang  2 , Jia-Wei Liao  3 , Shyi-Tien Chen  4
Affiliations

Treating High COD Dyeing Wastewater via a Regenerative Sorption-Oxidation Process Using a Nano-Pored Activated Carbon

Shih-Fu Ou et al. Int J Mol Sci. .

Abstract

Nowadays, the structural complexity of dyes used in the textile industry and the widely adopted water-saving strategy in the dyeing processes often fail plants' biological wastewater treatment units due to chemical oxygen demand (COD) overload. To alleviate this problems, this study investigated a regenerable adsorption-oxidation process to treat dyeing wastewater with COD around 10,000 mg/dm3 using a highly nano-pored activated carbon (AC) as a COD adsorbent, followed by its regeneration using hydrogen peroxide as an oxidizing reagent. In addition to studying AC's COD adsorption and oxidation performance, its operational treatment conditions in terms of temperature and pH were assessed. The results firstly demonstrated that about 50-60% of the COD was consistently adsorbed during the repeated adsorption operation before reaching AC's maximum adsorption capacity (qmax) of 0.165 g-COD/g-AC. The optimal pH and temperature during adsorption were 4.7 and 25 °C, respectively. Secondly, AC regeneration was accomplished by using an initial peroxide concentration of 2.5% (by wt %) and EDTA-Fe of 2.12 mmole/dm3. The reuse of the regenerated ACs was doable. Surprisingly, after the first AC regeneration, the COD adsorption capacity of the regenerated AC even increased by ~7% with respect to the virgin AC. Thirdly, the results of a five-consecutive adsorption-regeneration operation showed that a total of 0.3625 g COD was removed by the 5 g AC used, which was equivalent to an adsorption capacity (q) of 0.0725 (= 0.3625/5) g-COD/g-AC during each adsorption stage. Based on the obtained results, a regenerable COD adsorption-oxidation process using a nano-pored AC to treat the high-textile-COD wastewater looks promising. Thus, a conceptual treatment unit was proposed, and its potential benefits and limitations were addressed.

Keywords: a regenerable adsorption–oxidation process; activated carbon (AC); adsorption capacity; high COD wastewater; hydrogen peroxide.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Adsorption characteristics of the used AC. (A) Plots of the percent COD removed versus the AC amount used in adsorbing the dyeing wastewater COD. (B) Plot of the Ce/qe ratio versus Ce. The maximum adsorption capacity (qmax) at 24 h in (A) was equal to 0.165 (calculated by 1/6.054) g-COD/g-AC.
Figure 2
Figure 2
Plots of the residual COD over the sampling time for one spike (A) and multiple spikes (B) of the dyeing wastewater. For one spike run, the adsorption was nearly completed in one day. For the multiple spikes run, the percent removal of COD dropped to less than 50% on the 4th spike of the wastewater.
Figure 3
Figure 3
Effects of pH at ambient temperature (A) and temperature at pH = 4.7 (B) during AC adsorption runs. A lower pH value showed a greater percent COD removal, but the temperature had little effect.
Figure 4
Figure 4
Plots of the changes in the AC adsorption capacity after its first regeneration at various peroxide concentrations and a fixed initial iron-complex content of 2.12 mmole/dm3. The results showed that the use of the initial peroxide concentration at 2.5% was sufficient to regenerate the saturated ACs.
Figure 5
Figure 5
Plots of the percent COD removed after its regeneration at various initial Fe-complex dosages and a peroxide concentration of 2.5%. The use of an initial Fe complex at 2.12 mM had the highest percent COD removal value. A higher Fe complex could consume the oxidative capacity of the peroxide and resulted in the drop of the percent COD removal.
Figure 6
Figure 6
Plots of the COD residues (by weight) in the regenerant and the amounts of the remaining COD in the AC-treated wastewater under various regenerative pH conditions at ambient temperature. The variation of the pH showed little effect on the AC regeneration.
Figure 7
Figure 7
Plots of the COD residues (by weight) in the regenerant and the amounts of remaining COD in the AC-treated wastewater under various regenerating temperatures and a fixed pH value of 4.7. The lower the regeneration temperature, the less COD remained after adsorption.
Figure 8
Figure 8
Plots of the percent COD removed versus the number of times AC was regenerated. The results showed that, at the targeted 50% adsorption of the COD, the reuses of the generated ACs seemed doable.
Figure 9
Figure 9
Plots of the cumulative COD adsorbed over the consecutive daily operations of AC adsorption and regeneration. The results showed a linear increase in terms of the cumulative COD adsorption during each adsorption–oxidation cycle.
Figure 10
Figure 10
A proposed layout of the two-step treatment unit in treating a high-COD dyeing effluent directly from a 200 L dyeing bath. The designed unit can handle the high-COD wastewater drained from the 200 L dyeing bath and starts its adsorption–oxidation cycle to remove the COD in the wastewater.
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