Firstly you needs to know which kind/type of dyes present in the waste water, that's may possible with help of GC. Other wise review the literature's or available document on particular industry to know about the dye which is used in that industry. According to available data you may standardize your UV-vis spectrometer for knowing the concentration and intensity of a particular dye.   

In the first stage, it's important to calibrate your method using different standard concentrations of dye. For this purpose, prepare three or more solutions of dye with exactly known concentrations. Next, draw the absorption (obtained via UV-vis) as a function of concentration. finally, you can measure the unknown concentration using the calibration curve quantitatively. Note that absorption(in term of OD for example) of known samples should be around the absorption of unknown sample.

As an another way, you can use fluorescence spectroscopy to ensure that dye is adsorbed effectively. Firstly, find the absorption wavelength of the dye through UV-vis. Next, illuminate the wastewater by a laser with above wavelength. If you observe fluorescence spectrum of dye, the waterwaste contains dye molecules. I personally think this method provides better limit of detection(LOD) compared with UV-vis.

Both of above methods suffer an important fault, however. The error is due to other impurities existing in waterwaste. so that the absorption peaks may overlap together. To overcome this, it's better to use UV-vis (fluorescence in second method) spectrum of waterwaste without the dye as a reference.

cheers

There are numerous dyes (both synthetic and nature) present in wastewater effluents and its not easy to determine them in single step. Firstly, you will have to estimate roughly what are the possible dyes present in that sample. For such sample (with many dyes) going for UV-Visible analysis is not a wise approach as some of the dyes have nearer maximum wave length ranges and it will interfere in their qualitative and quantitative determination. Using UPLC-MS will be a wisest approach to determine even trace levels of dyes present. But most importantly you should have a rough idea of a sample contents before going for analysis. Even you might need to use SPE extraction technique if concentration of dyes are below LOD .   

The stock solution should be prepare in distilled water using exact concentration in (mg/L) of the contaminates (dye or trace metals), you have to prepare all working solutions by diluting the stock solution with distilled water.

The equilibrium relationships between adsorbent and adsorbate can be describe by adsorption isotherms , usually the ratio between amount of solute adsorbed per unit amount of adsorbent and concentration of solute in solution at equilibrium .The amount of dye adsorbed onto the adsorbent (at equilibrium) qe (mg/g) can be calculated according to the following mass balance equation

Where C0 and Ce are the initial and equilibrium concentration mg/L of dye( whatever of contaminate), respectively, V is the volume of the solution (L) , and W is the weight (g) of adsorbent. Also you can try to study many effects using known amount of the adsorbent (mg), pollutant concentrations and agitation (150-200) rpm then take absorbance after adsorption process.

Dear Ms. Golami,

It's either UV-spectrophotometry or GC

Good Luck 

Dear Zahrae,

it is complicated matter to solve. The optial methods as UV, VIS, florescence and adsorption equilibrium methods have already been mentioned in the above answers, including use of calibration curves of model compounds /dyes/ sugested to be present in the water  samples.

It is necessary to find the "history" of examined water samples /industrial plants, various manufactures, farms along the river banks and other polutants in upstream direction/. This can provide you with info. concerning the dyes and other contaminants needed for their quantification in water samples.

Secondly, you have to rid the samples of small organic and inorganic insoluble particles via filtration or centrifuge. They may be interfering with most of used analytical methods, especially optical ones.

From chromatographic methods HPLC /MS might be an option, too. This method, however requests modification of original sample: If water contaminants  in the samples are not volatile /most of dyes, especially azodays and inorganic water soluble ones/ you may freeze dry them and get an information of dry residuum weight per the sample volume. Then you can dissolve the residuum in ethanol, THF,  in their mixture, CH3CN or in any appropriate solvent. The quantity of  all dyes souble in the mentioned solvents can be determined via metod of  internal standard or from calibration curves of model compounds /dyes/.

Another aproach to this problem solution dwells in the sample extraction with water unmiscible organosolvent/-s/ followed with separate analyses of the extract /1/ and of extractives free water /2/.

Extract /1/ could be analysed via HPLC or HPLC/ MS  and water layer can be analysed via pH-metry, conductometry, electrochemical methods , AAS, colorimetry, and various volumetric methods. It might be interesting to determine chemical and biochemical consumption of O2 of your samples.   

Good luck, R.S.

Thank you for all your precious answers.

Hi Zahrae,

I just want to add some points here, you may want to consider the possible interactions that might happened between dye-dye or dye-other contaminants that may affect the signature wavelength (WL) of each of the dyes in the real waste water solutions. The interactions may caused the WL to be shifted, suppressed or magnified (synergistic effect) and therefore, could result in wrong misinterpretation of the measured data. This is a common issue related to UV spectro operation that i have observed in my lab.

Multi-component concentration analysis involving the interacted components are very tricky and therefore UV might not be the best solution. Unless you are 100% sure that all the components didn't react or take influence on each other. You may need to consider a use of other advanced analytical tools mentioned by our friends above.

Good luck! 

I checked and scanned the real wastwater using UV-Vis, but there is no peak to prepare the calibration curve based on that. However, it can be seen that for example if I select a random vavenumber, even there is no peak there,  by dilution of samples the intensity of that peak gradually decreased, but it is not any specific visible wavenumber for the sample.

I tried to check the sample at different vavenumbers as well, but same thing was observed for all selected wavenumbers.

is it possible to consider one point without any peak as a reference for the 100% real wastewater (C0) and then prepare the calibration curve based on this wavenumber? 

Dear Zahra,

Maybe your idea could work, however, you will get somewhat mixed overlapped signal /maximum/ in your UV spectra at the selected wavelength. The result may be considered for informative only, comprising absorbances of all absorbing functional groups of the dyes present in the wastwater.

You could try to record spectra of wastewaters in alkaline evironment. If your contaminants contain ionizable functional group/-s/, mostly phenolic -OH, these groups will be dissociated in alkaline environment to phenolate anion, and the absorption maximum  will be shifted towards longer wavelength /300 to 340 nm/, depending on the molecular structure of the dye. Also the absortivity coefficient of phenolate structures is higher as compared to that of phenolic structures. 

As for me, I would prefer chromatographic /HPLC/ analysis, in this case. A´propo, is the concentration of dyes in wastwater in the range of UV/Vis detection? Dark color of  wastewater may result from the presence of microscopic or colloidal particles of mud sediments and of rot biological material.

Good luck.

Dear Professor Rastislav, 

Thank you for your comprehensive response to my question. I will try this method.

Regards, 

I couldn't realize exactly about your problem. But from your description, it seems you had got some waste water sample from somewhere which had been containminated by some kinds of dyes. And you try to analysis it to make sure the exact types of dye containminated and the appo. concentration for those types of dye.

So,  FirstlyYou should identify which types of dye containminated qualificately. You may use the appropriate solvent (such as ether)to extract organic phase to prevent the interference of water soluable containminate, and follow by analysis by LC Mass and other spectroscopy methods to identify the structure for each component. Since some compounds of dye may contain ionized functional group, you should neutralize the waste water before extraction to make sure the dye containminate have been exctrated as completely as possible. After the exact containminates of dye had been identified,  you may use the quantitation analytical method we usually use. And I don't think you need to consider the precision of results for waste water analysis.

From your pre-test, colorous compounds was absorbed, and the black color of aquous solution was dispeared. It means the principal colored compounds can be asorbed to your absorbent. If you make quantitative sampling, you may use the apropiate solvent to re-dissolve the compounds absorbed to the known concentration of organic solution for further analysis.

Since you had checked and scanned the real wastwater using UV-Vis, but there is no peak to prepare the calibration curve based on that. It means the compounds of dye have strong broden bands in UV-VIS spectrum. Quantitative analysis of single component with calibration curve on single wavelenth  or Internal standard may be OK.  It is difficult to make the calibration  for multi-components without specific absorption peaks, and even use multi- wavelength calibration. You can either check the  whole spectrum of each dye identified, try to find the unique wavelenth for each component, and do multi-wavelenth calibration; or use the chemometric analytic technique (linear or  nonlinear regression) to solve the composition for each component.

I agree the suggestion from Pr. Rastislav SOLÁR that chromatograpy method could be the best choice for you.

Good Luck  

Dye intensity or dye concentration in a real waste water can be measured by colorimetry by measuring optical density. First find out lambda max. for that dye solution then keep it constant , prepare various concentrations of dye , plot a standard graph of optical density vs conc. .Them measure O.D. of unknown water solution containing dye & by extrapolation fine out the conc. of unknown dye solution.

Salam

I have the same problem with real wastewater. Could you explain me what way you chose to report color absorbance values? 

I think this link is useful, 

http://www.instanano.com/characterization/theoretical/concentration-calculation-from-uv-vis-absorbance/

From my experience you can use ADMI unit to measure mixture of colur from solution such as textile wastewater..