Guidelines for the reduction of dyes released from pulp and paper mills: appendix 3

Appendix 3: Proposed method for measuring dye retention on fibres^[4]

Equipment

• Spectrophotometer 
• Fibreglass filter paper (1.5-mm nominal porosity).
• Deionized water
• Büchner vacuum flask
• Stirrer hot plate
• Beakers
• Graduated cylinder
• Analytical balance measuring to at least four decimal places
• Pipettes
• pH meter
• Conductivity meter

Products

• Dye to be tested
• Fresh, undyed (never dried) pulp

Procedure

1. Preparation of dye solution
   1. Dilute the dye to 0.2 g/L (Ci) with deionized water.
      
2. Use the spectrophotometer to determine the maximum absorption wavelength and develop a “concentration-absorbance” curve using that value. Establish the minimum dye concentration that can be measured with this instrument (minimum measurable value).
   
3. Pulp retention measure
   1. Dilute 2.00 g of dry pulp in deionized water until you achieve a 1% consistency.
   2. Agitate at 50°C for 2 minutes or until no fibre flocs remain.
   3. Filter with the Büchner and filter paper and keep the filtrate in a beaker.
   4. Measure the filtrate absorbance (A int) at the wavelength established in Step 2 in order to estimate the interference caused by suspended solids and the filtrate colour.
   5. Measure the quantity of dry pulp on the filter paper.
   6. Calculate the pulp retention. A 99% minimum value is required to minimize interference with the absorption measure. For lower retention, increase the quantity of pulp or use a lower-porosity fibreglass filter.
   
   
4. Dye retention measure
   1. Weigh 2.00 g of dry pulp.
   2. Add 180 mL of deionized water.
   3. Add 20 mL of the dye solution (Vi) to the pulp to obtain a concentration of 0.002 g pure dye/g dry pulp.
      • The final consistency of the pulp should be 1%.
   4. Agitate at 50°C for 2 minutes or until no fibre flocs remain.
   5. Filter with the Büchner and filter paper.
   6. Measure the filtrate volume (Vf).
   7. Measure the filtrate absorbance (Af).
   8. Calculate the corrected absorbance (A corr) = Af - A int (from Step 3-d).
   9. Calculate the dye concentration in the filtrate (Cf) with the “concentration-absorbance” curve (Step 2), using the corrected absorbance (A corr).
   10. Calculate the quantity of (pure) dye in the filtrate (Df) = Vf × Cf [g].
   11. Calculate the initial quantity of (pure) dye (Di) = Vi × Ci [g].
   12. Measure the pH and conductivity of the filtrate (for reference only).
   13. Calculate the dye retention = (Di - Df) / Di × 100%.
   14. Repeat Step 4 twice. Record the mean retention, pH and conductivity.

Notes

1. The initial dye quantity per gram of pulp used is based on the hypothesis that the lowest value the spectrophotometer can measure in the filtrate is 1 ppm of dye and that the dye retention is approximately 90%. The quantity of dye may need to be adjusted should these hypotheses not apply. 
2. Instruments used should be calibrated.
3. Results should be presented to the appropriate significant decimal places.
4. The minimum measurable value of the instruments should be determined.
5. If a measurement falls below the minimum measurable value for an instrument, then the minimum measurable value should be used or reported.
6. Factors that may affect dye retention include: conductivity (salts), temperature, pH, quantity of anionic trash in water, pulp type, dye type, pulp consistency, and initial concentration.

^[4] The method was reviewed and tested by a laboratory associated with the University of British Columbia.