| In dyeing and finishing industry, the reactive dyes are widely usedto dye and print the cellulose fabrics for their reasonable price, ease ofapplication, good color values and excellent fastness properties.However, the reactive dyes suffer disadvantage of incompleteexhaustion up to20%～40%during dyeing. The vagrant dye consistedof unreacted and mainly hydrolyzed dyes can sharply decrease thefastness of fabrics. Generally, this vagrant dyes are wiped off through awash-off process with a great deal of dyeing wastewater producing.According to incomplete statistics, one-third of the effluent (COD)derived from the total dyeing technics are related to the washingprocedures since the vagrant dyes and auxiliaries existing. Moreover, agreat of energy and water consumption, as well as the treatment of theresulting effluent, typically accounts for the major part of the totaldyeing cost (about50%). The polluted water badly affects thephotosynthesis of hydrophyte and can also cause severe damage tohuman beings because of their mutagenic effect and carcinogenicity.There are two ways to solve the dyeing pollution, one is theend-of-pipe control, recovering the wastewater through various disposalmeasurements before the discharge; the other is to decrease thewastewater outflow from sources by means of clean productiontechnics.Dyes are chromogenic organic compounds, so the routinebiological and chemical treatments cannot reach the aim of disposingdyeing wastewater. At present, adsorption is one of the best choices forindustrial dyes wastewater advanced disposal. But the adsorptiontreatment has the disadvantages of low adsorption capacity, high-cost,second solid rejectamenta producing and difficulty to recycle for the weak desorption character.According to the problem mentioned above, Chitosan-gelatincomposite microspheres were prepared by the inverse suspensioncrosslinking reaction， using cationic chitosan (CS) and amphotericgelatin as the raw materials and taking glutaraldehyde as thecross-linking agent. The chitosan/gelatin microspheres(CGMSs)prepared have the excellent adsorption and desorption quality andsuitable for disposing the anionic dyeing wastewater especially reactivedyes. In addition, CGMSs display the merits of saving water, sparingtime, decreasing COD and reclaiming dyes during the wash-offtechnology.When CGMSs were prepared, the factors such as dosage ofchitosan, gelatin and emulsion, selection of emulsion and crosslinkingagents, the ratio of water-oil, agitation speed, emulsification time andtemperature were investigated. The products were characterized bySEM and laser particle size analyzer. The CGMSs obtained wereindicated spherical, irregular surface morphology, and good fluidnesswith23.53μm of volume mean diameter and6.0of isoeletric point. Theoptimal conditions were as follows: chitosan was0.02g/L, gelatin was0.08g/L, Span-80was0.04g/L, the ratio of water–oil was1/6, thetemperature was60℃emulsifed at900r/min for20min.The effect of the experiment parameters on the adsorption ofReactive Orange1(RO1) and Reactive Red24(RR24) onto CGMSs werestudied. These parameters included the adsorption technology, such astemperature, time and pH, the adsorbent properties, ionic strength,shaking speed and recycling application. It was found that (1) theadsorption capacity increased with increasing shaking speed anddecreased with increasing the dosage of electrolyte;(2) The optimalconditions were pH2～3, temperature of30℃, adsorption for80min.Meanwhile, the dye removal rate can achieve to98.7%and99.1%forRO1and RR24at the condition of initial dye concentration of0.4g/L,the CGMSs concentration of0.8g/L and1.0g/L, respectively;(3)Theadsorption capacity decreased by only8.6%of RO1and9.7%of RR24 separately after the CGMSs was used for7times. These resultssuggested that CGMSs can be used repeatedly without losing adsorptioncapacities significantly. So the CGMSs are proved to be a kind ofpromising excellent adsorption material.In order to illustrate the mechanism of CGMSs adsorbing reactivedyes, the adsorption kinetics and isotherm models as well asthermodynamic parameters were researched. The results indicated thatthe adsorption kinetics followed the pseudo-second-order model. Boththe external mass transfer and the intraparticle diffusion had the ratelimiting influences on the removal process, but intraparticle diffusionwas the rate determining step. The isotherm models revealed that theadsorption process of reactive dyes onto CGMSs fited well with theLangmuir model which suggested the adsorption was monomolecularlayer chemisorption. The mean adsorption energy (E) calculated fromthe Dubinin-Radushkevich isotherm manifested that the adsorption wasmainly chemisorption as well. The positive values of ΔH indicated thatthe adsorption was endothermic and the process would benefit from thetemperature increasing. The positive values of ΔS implied that therandomness increased at the microsphere/solution surface. It can beexplained that the randomness gains by water desorption were exceedthe randomness reduces by dye adsorption. Whereas, the negative ΔGvalues showed that the adsorption was spontaneous in the experimentalconditions.The CGMSs are taken as the reactive wash-off agent, which are theinnovation of this research. The function of CGMSs wash-off can becompleted by the vagrant dye shifting from fabric to CGMSs whichbreak the dye balance between fabric and dye solution by adsorption.The CGMSs have the properties of pH buffering, resistance to thehardness water and the electrolyte as a wash-off agent. The efficiencyof wash-off can be improved with the increase of chitosan content andthe decrease of diameter of CGMSs. With regard to the technologycondition, the optimal wash-off parameters were as follows: the CGMSsdosage was0.2g/L, temperature was95℃and pH was4washing for15 min. The efficiency of CGMSs can rival routinely detergent in colorfastness, while the COD of CGMSs wash-off residue accounts for1/16of that of detergent wash-off residue. Compared with the traditionallytechnology, CGMSs method saved once room temperature wash-off andonce acid wash-off. The use of CGMSs will sharply alleviate the burdenof dyeing wastewater and can save water and energy and cut the cost ofdyeing process.The dyes unfixed to fabric are still capable of dyeing, meanwhile,the pH of dye desorption from CGMSs coincides with that of reactivedye dyeing technics. In order to value the unfixed dyes recycling, theapplication of CGMSs loaded reactive dye in cellulose fabric dyeingtechnics were studied. The results showed that CGMSs loaded reactivedyes were suitable to low liquor ratio dyeing with the superior levellingproperty to the general dyeing technics. The best RR24dyeingproduction will be achieved in condition that the liquor ratio was1:30,time was70min, temperature was70℃, sodium sulfate was30g/L andsodium carbonate was20g/L. For the RBE19dye, the optimal conditionwas temperature of60℃, sodium carbonate of15g/L and the othersections were the same as RR24.Chitosan and gelatin are the natural renewable bioresources. TheCGMSs showed excellent performances on adsorption and desorption ofreactive dyes. It can achieve many good effects such as water-saving,energy-saving and reducing COD of effluents using CGMSs as awash-off agent for reactive dyes instead of the traditional surfactantcombinations. The regeneration of CGMSs can be obtained at the sametime with the reutilization of unhydrolyzed reactive dyes in dyeingprocess. This wash-off agent designed on the basis of adsorptionmechanism expanded the way of wash-off for reactive dyeing, providinga new technology to resolve the problem of reactive dye dyeingwastewater and environmental pollution. |
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