| The production of faulty dyed fabrics, surplus fabrics and wasted fabrics are unavoidable during the manufacture and daily life, reuse of them can not only improve the ratio of comprehensive utilization and relieve the shortage of resources, but also reduce the disposal load and environmental pollution. Excellent stripping performance is the important aspect for the remediation of faulty dyed fabrics and the reuse of waste fabrics. Conventional chemical stripping is easy to damage the fabrics due to the addition of several reagents and the operation conditions of high temperature and strong alkaline, resulting in the complicated and refractory stripping wastewater. Bio-stripping is regarded as one of environmentally friendly and cost-effective alternative. However, the cultivation of effective stripping strain and time consuming restricted its practical application. Using the strong oxidants (such as hydroxyl radicals (·OH)) generated by advanced oxidation processes as stripping agent would be a promising alternative not only due to the weak damage to fabrics but also due to the effective stripping and the reduction of the effluent.In this study, cotton fabrics dyed with Reactive Red X-3B were selected as the fabrics needed to be stripped. The electrochemical stripping was firstly conducted in water under galvanostatic conditions using Ti/TiO2-RuO2-IrO2 or fluorine resin modified β-PbO2 as anode, and stainless steel net as cathode. The parameters affecting the stripping performance, such as the types and dosage of electrolyte, current density and stripping ratio (fabrics:water, w/w), were investigated in terms of stripping percent. Secondly, the parameters affecting the stripping efficiency including dosages of FeSO4 and H2O2, pH and stripping ratio were also investigated and optimized in terms of stripping percent during Fenton stripping of dyed cotton fabrics. The surface morphology and the micro-structure of cotton fabrics before and after stripping were determined by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (F.TIR). The effect of stripping on water was characterized by the chemical oxidation demand (COD) and chromaticity. Finally, the components of stripping water were identified by gas chromatography coupled with mass spectrometry (GC-MS), based on which, the stripping mechanism of electrochemical and Fenton stripping for Reactive Red X-3B dyed cotton fabrics was preliminary infered. The main results were as follows:(1) The electrochemical stripping performance on Ti/TiO2-RuO2-IrO2 using NaCl as electrolyte was superior to that obtained in Na2SO4 due to the indirect oxidation of reactive chlorine generated. The optimized conditions were as follows:2.0 g/L of NaCl,15 mA/cm2 of current density and 1:40 of stripping ratio,98.14% and 97.29% of stripping percent of cotton fabrics dyed with Reactive Red X-3B (dyeing ratio 1.0%) for the inner fabrics near the anode and the outer layer fabrics were obtained, respectively. The COD and chromaticity of stripping effluent were 19 mg/L of and 18 degree, repectively. Substituted hydrocarbons and toluene were detected by GC-MS. SEM results along with FTIR revealed that no hydrolysis and damage were observed on cotton fabrics after electrochemical stripping, but the crystallinity index of cotton fabrics decreased a little. The extent of dye exhaustion of stripped cotton fabrics redyed with Reactive Red X-3B dye in 1.0% of dyeing ratio was 85.73%, and the average value of K/S was 3.76, which was similar to that of dyed cotton fabrics prior to stripping. Thus, the stripping with Ti/TiO2-RuO2-IrO2 anode could not only obtain a good stripping performance and weak damage to the structure of cotton fabrics, but also could reduce the pollution load of stripping water.(2) The stripping percent of cotton fabrics dyed with Reactive Red X-3B with Na2SO4 as electrolyte after 20 min of reaction was inferior to that obtained in NaCl due to the single action of·OH electrogenerated on the β-PbO2 anode and the diffusion limitation of·OH. Above 90% of stripping percent was achieved under the following conditions:0.5 cm of distance between the cotton fabrics and anode,2.0 g/L of NaCl,15 mA/cm2 of current density and 20 min of stripping time. The process was then applied to strip 20 different samples of cotton fabric using the same water bath, the stripping percent of every dyed samples remained above 95%, the COD of stripping effluent was 5 mg/L, even after 20th reuse of the water bath. The components of stripping water reused 6 times were substituted hydrocarbons and toluene. SEM and FTIR analysis showed that the crystallinity index of stripped cotton fabrics decreased after stripping, changes of the micro-structure of cotton fabrics could be ignored. An average of 29.12% of dye exhaustion and 1.78 of K/S value of stripped fabrics redyed with Reactive Red X-3B were reached in the shade strength of 1.0%. Compared with raw cotton fabrics, the dye exhaustion was decreased by 14.57% and the color fastness increased for the similar K/S value after the washing process. Therefore, excellent stripping performance could be obtained on β-PbO2 anode and no obvious damage was observed. Additionally, the stripping water could be reused to strip different dyed cotton fabrics, and the quantity and the load of pollution were reduced.(3) The optimum operation conditions for Fenton stripping of cotton fabrics dyed with Reactive Red X-3B were as follows:pH 3,300 mg/L of FeSO4,900 mg/L of H2O2,60℃ of stripping water. After 70 min of stripping,80.44% of stripping percent could be achieved, the COD of effluent was 87 mg/L. Substituted benzenes and 2,2,4,6,6-pentamethyl-heptane were detected in the effluent. SEM and FTIR validated that no hydrolysis and damnification occurred during Fenton stripping process. The dye exhaustion of Fenton stripped fabric samples was 71.64%, the average value of K/S of the redyed cotton fabrics with 1.0% of dyeing ratio was 1.23. However the washing fastness of the redyed fabrics decreased due to adsorption of ferric ions and its hydroxo complexes during Fenton process.In a summary, electrochemical stripping could effectively remove the dye from dyed cotton fabrics only adding a few of electrolyte at mild conditions, the effect of stripping on the quality of cotton fabric could be ignored. Furthermore, the stripping water could be repeated to strip different samples, and the pollution load of effluent was low. Similarly, Fenton process could also achieved the satisfied stripping performance with the advantages of simple operation and low consumption of reagents. Consequently, electrochemical oxidation and Fenton process offer the efficient, cost effective, environmental friendly methods for decolorizing the dyed cotton fabrics. |
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