Aqueous Transportation And Transformation Processes Of Malachite Green And Crystal Violet:Photodegradation And Sediment Adsorption Under Simulated And Natural Conditions

Triphenylmethanes(TPMs)are broadly used as dyes,veterinary drugs,food additives,antibacterial agents,and finger-print developers in forensics etc.,which enter the environment in various ways.They have the potential of carcinogenesis,teratogenesis,and mutation,leading to vast attention to the treatment of their wastewater and aquaculture safety,but their residues,transportation,transformation,toxicity,and ecological risks in the environment are underestimated.Malachite green(MG)and crystal violet(CV)are two representative dyes/veterinary drugs which are the most widely used.Thus,it is indispensable to understand these properties,which is in favor of reducing potential risks of environment and human health.Most of current researches mainly focus on artificial elimination of these substances,such as simple oxidation,electrochemical process,biodegradation catalysis adsorption,and advanced oxidation processes(AOPs).However,the transportation and transformation process of MG and CV under simulated or natural conditions were rarely reported.As for the transformation process of MG and CV,they can be photodegraded effectively by direct and indirect photolysis.In regard with transportation processes of MG and CV,owing to the ubiquitousness of suspended sediments in natural waters,the variation of physical and chemical parameters of waters as well as the affinity of pollutants for the particles of sediments will directly influence their mobility.Therefore,this dissertation has investigated the aqueous transportation and transformation processes of malachite green and crystal violet,i.e.their photodegradation and sediment adsorption under simulated and natural conditions.The content and results of this study is as follows:(1)The hydrolysis of MG and CV under dark condition was fit by apparent first order kinetics model,and hydrolytic rate constants of MG and CV were 0.0192 h-1(R2=0.9409)and 0.0386 h-1(R2=0.9631),respectively;the reactions both followed SN1 mechanism.During hydrolysis process,MG,MG leucocarbinol,and LMG as well as CV,CV leucocarbinol and LCV can convert into each other,respectively.(2)Mutiple influential factors including pH,humic acid,Fe2+,Ca2+,HCO3-,and NO3-were investigated by the orthogonal array design under simulated sunlight in laboratory.Both the photolysis of MG and CV were fit by apparent first order kinetics model.The rate constant of photodegradation of MG was between 0.0062 and 0.4012 h-1,with half-lives 111.6 to 1.7 h and the least residue of 4.4%under optimal condition after 40 h irridiation;the rate constant of photodegradation of CV was between 0.0067 and 0.1099 h-1,with half-lives 86.6 to 6.3 h and the least residue of 9.1%under optimal condition after 40 h irridiation.The hydroxyl radical,singlet oxygen,and superoxide anion have partially or totally participated in the indirect photolysis process,leading to much higher decolorization efficiencies than those of direct photolysis and hydrolysis.Solar irradiation led to complete decolorization.(3)The pretreatment method of samples by solid phase extraction(SPE)was employed,precise and effective qualitative analytical methods with LC-iontrap-MS,LC-TOF-HRMS,and GC-MS were applied,53 products of MG and 65 products of CV were identified,and mechanisms were proposed with the aid of calculation of Frontier Electron Density(FEDs).(4)Main degradation pathways of MG during natural photolysis under different conditions were the decomposition of conjugated structures,N-demethylation reactions,HO·addition reactions,the removal of benzene ring,and the ring-opening reaction.Main degradation pathways of CV were singlet oxygen addition,N-demethylation reaction,hydroxyl addition reaction,decomposition of conjugated structures,the removal of benzene ring,and the ring-opening reaction.Afterwards,small organic molecules such as 4-(dimethylamino)benzaldehyde,4-aminobenzoic acid,other carboxylic acids,alcohols,and amines were engendered,which is not likely to cause severe hazards to the environment.(5)The XRD spectrum and XRF result illustrated that the Yangtze River sediment was mainly composed of silicon dioxide.SEM micrographs and TEM results illustrated the structure of chip multilayers and other amorphousness.The specific surface area of this sediment was 6.46 m2/g,pH was 7.51,cationic exchange capacity(CEC)was 13.38 Cmol/kg,and the capacity of organic matter was 12.37 g/kg.(6)The results of sediment adsorptions of MG and CV showed with increasing pH,the dye uptake increased.Not only did pH affect the adsorption behavior,the selftransformation process of dyes also contributed to the decolorization process.Na+showed a very weak competition between MG,CV dye cations and negative sites on the surface of the sediment,and it did not influence the adsorption process distinctly,leading to the insignificance of its ion intensity;while Ca2+also demonstrated a weak competition with MG,but a middle strong competition between CV and negative sites on the surface of the sediment,and it played a decisive role when affecting adsorption capacity.The competition on dye cations between NOMs and the sediment led to the decrease of adsorption capacities.Meanwhile,NOMs were responsible for chelation and complexation,decreasing the adsorption capacities.Temperature did not play a decisive role when affecting adsorption behavior.Low temperature favored the adsorption of dyes,indicating that the adsorption process was exothermic in nature.The intensity of stirring affected the dispersion of dye molecules in the aggregate solution,and the effective collisions between the dye cations and the sediment binding sites.(7)The results of adsorption kinetics showed that for MG,the correlation coefficient value of the pseudo-second-order kinetic model is higher(R2=0.999)than that(R2=0.954)of the pseudo-first-order kinetic model;and as for CV9 the correlation coefficient value of the pseudo-second-order kinetic model is higher(R2=0.991)than that(R2=0.977)of the pseudo-first-order kinetic model.These results suggested that the pseudo-second-order model better represents the adsorption kinetics.(8)The results of thermodynamics showed that for MG,the correlation coefficient for the Langmuir isotherm(R2=0.998)was slightly higher in comparison with the value obtained for the Freundlich isotherm(R2=0.996).The monolayer adsorption capacity qm was 199 mg/g(25℃),indicating that the adsorption process may be combination of monolayer and multilayer adsorption.While as for CV,the correlation coefficient for the Langmuir isotherm(R2=0.999)was a little higher in comparison with the value obtained for the Freundlich isotherm(R2=0.971).The monolayer adsorption capacity qm was 58.8 mg/g(25℃),indicating that monolayer adsorption was more important than multilayer adsorption.The negative values of ΔG0 confirmed the feasibility of the process and spontaneous nature of adsorption.The negative value of ΔH0 confirmed the exothermic process.The negative values of ΔS0 also revealed the decrease of randomness at the solid/liquid interface during the adsorption on the sediment.All in all,these results would provide scientific data for environmental pollution control,food safety,and application of triphenylmethane dyes in the future,offer proposals for human health and environmental protection,and provide reference of transportation and transformation for other pollutants in the environment.