The rapid development of industry brings benefits but also problems. The concentration of total phosphorus in industrial wastewater cannot be ignored.
Phosphorus is mostly inorganic compound phosphorus, which is mainly composed of orthophosphate and fused ring phosphate from detergents. A small part of total phosphorus is organic compound phosphorus. General sewage treatment must convert various forms of phosphorus into orthophosphate Phosphate is then removed.
In the “Integrated Wastewater Discharge Standard” (GB 8978-1996), the first-level standard for the maximum allowable discharge quality concentration of total phosphorus in wastewater is 0.5 mg/L, but if the phosphorus content in the water is too high (for example, more than 0.2 mg/L), will cause the excessive reproduction of algae, and the harmful gas produced by the decomposition of organic matter accumulated at the bottom under anaerobic conditions and the biotoxin produced by some plankton will also harm fish.
Moreover, eutrophication water contains nitrate and nitrite. People and animals who drink water with these substances exceeding a certain standard for a long time will also be poisoned and cause disease.
In this paper, our intelligent graphite digestion instrument is used, and the high-purity graphite block is wrapped around the heating, which is safe, fast, and evenly heated. The experimental results are satisfactory, method for reference:
Experimental part
Principle of the method:
Heating at 120°C, the potassium persulfate solution oxidizes and decomposes the phosphorus-containing compounds in the sample into soluble phosphates. Under acidic conditions, orthophosphate reacts with ammonium molybdate and potassium antimony oxytartrate to generate phosphomolybdenum heteropolyacid, which is reduced by ascorbic acid to turn into a blue complex. Use a 1 cm cuvette at a wavelength of 700 nm to measure the absorbance with the reagent blank solution as a reference, and the content of total phosphorus (calculated as P) is directly proportional to the absorbance.
Instrument:
Digestion heater:
Reagent:
Potassium persulfate 50 g/L
Water sample collection and storage:
Water samples were acidified with sulfuric acid to pH ≤ 1, stored and tested within 24 hours.
Graphite digestion instrument digestion steps:
Accurately draw 25 mL of water sample into a 50 mL Teflon digestion tube, add 4 mL of potassium persulfate solution, cover it, put the digestion tube into an intelligent graphite digestion apparatus preheated to 120°C, and digest for 30 min. After the digestion is complete, take out the digestion tube and cool it to room temperature, transfer it to a 50 mL stoppered glass colorimetric tube, add the chromogenic reagent and measure its absorbance.
Results and discussion:
Detection limit
The “Comprehensive Wastewater Discharge Standard” (GB 8978-1996) has a primary standard of 0.5 mg/L and a secondary standard of 1.0 mg/L for the maximum allowable discharge mass concentration of total phosphorus in wastewater. It can be seen from the table that the detection limit of the graphite digestion method meets the requirements of the national standard.
2. Comparison of precision and accuracy
The determination was repeated 6 times, and the relative standard deviation (RSD) was calculated. The results of the method were all within the standard value range.
3. Add standard substance and recovery experiment
0.400 mg/L phosphorus standard solution was added to three kinds of water samples with different mass concentrations, treated with graphite digestion method, and the recovery rate of standard addition was determined. The results met the laboratory quality control requirements of “Technical Specifications for Surface Water and Sewage Monitoring”.
Conclusion
The graphite digestion method is to put the sample into the intelligent graphite digestion instrument preheated to 120°C, and the digestion is completed in 30 minutes; The traditional high-pressure steam digestion method has many steps and takes a long time to digest. In contrast, the graphite digestion method is efficient and fast.
Our countertop of the intelligent graphite digestion instrument is made of polytetrafluoroethylene, which has strong acid and alkali resistance and is not easy to burn the staff. Our intelligent graphite digestion instrument custom-made hole number is simple and fast to process samples, and is very suitable for the analysis of total phosphorus in large quantities of wastewater. It meets the requirements of total phosphorus analysis in “Determination of total phosphorus in water quality by ammonium molybdate spectrophotometric method” (GB/T 11893-1989).
