Study On The Removal And Recovery Of Phenols From High Strength Phenolic Wastewater

Phenol and its derivatives are the important chemical materials and intermediates, which are widely used in the areas of medicines, spices, dyes, synthetic rubber and plastics, as well as a solvent. However, they are also with high toxicity and poor biodegradability, easy to cause a serious problem for human healthy and ecological balance, therefore, researchers have developed a variety of technologies for phenolic wastewater treatment, such as chemical methods, physical methods and biological methods; while each method has its own application range. In our paper, the wastewater with phenol content of27200g/L and COD value of70600mg/L was studied, it has been known the water cannot be degraded by biological methods due to the high phenol content, and can hardly either be extracted in our preliminary experiments. For this situation, considering from the properties of large amounts of Cl-and H+in wastewater, a halogenation pretreatment was proposed to achieve the recovery of phenols. The process was conducted by adding into the wastewater to react with Cl-and H+to produce Cl2, which then combined with phenol and converted to the insoluble chloro-phenols, achieving the recovery in the form of precipitate. Studies on the feeding mode and feeding quantity of NaC103, it can be concluded the dosage of NaClO3should be performed by multiple feed with small quantity, when the dosage reaches33g/L,98%of phenol has been recovered and trichlorophenol is confirmed the predominant product with the content of98.2%by GC-MS analysis, simultaneously coexist a small amount of dichlorobenzoquinone and tetrachlorophenol. At this point, the COD of supernatant can be also reduced98%. The determination of Cl2by a double platinum electrode in chlorination process shows the "current method" can be used as a sign to indicate the chlorination end.The wastewater after chlorination still contains a small amount of chlorophenols with poorer biodegradability. By comparision, the Fenton oxidation technology with simple operation process was chosen to do further treatment. To clearly understand the principles and changes of solution properties during the Fenton process, phenol solution which prepared by using pure phenol agent was studied firstly, including the reaction conditions and the influences of chloride ions in the procedure. Through determination and analysis on pH, bromine value and COD index in the chlorination process, a possible mechanism for Fenton degradation was discussed. The experimental methods and conclusions are as follows:1) Online detection and study on the potential changes in Fenton process shows it can be used as an indicator to judge whether the reaction happens and how the reaction rate. 2) With the potential change as a criterion, studies on the Fenton degradation for phenol solution in initial pH at0-5and catalyst Fe2+addition in range of0.5-10mmol/L indicate that the allowed acidic condition for Fenton reaction occurrence is pH>1. Since the initial stage of the reaction is a process that acidity increases rapidly, therefore, the initial pH of solution is determined to be at3in later experiments. The catalyst concentration influences the reaction rate, from the perspective of H2O2utilization, its concentration is chosen to be1mmol/L.3) Phenol solution with different content of Cl-as object, the Fenton degradation under various H2O2dosages was studied. Determination on the Cl-content indicates Cl-is oxidized to Cl2during the reaction process, and which reacts with organics through substitution and addition form. Measurements on the bromine number and COD, as well as the thermogravimetric analysis to the precipate show that when H2O2addition is in the range of nH2O2/nphenol=2.5-6.5, phenol solution with high strength Cl" has organic precipitate produced, and because of this, the COD of supernatant decreases more obviously than that of the low Cl-content. When the H2O2dosage reaches nH2O2/nphenol=17, phenols in all kinds of solutions have been open-loop completely, the moment the solution has also become colorless and transparent; while, their COD has not yet been removed completely, the values are measured almost in the range of300-400mg/L, reduced by95%, indicating the organics obtained via ring opening of phenol have not been degraded thoroughly. The H2O2amount required for complete degradation is nH2O2/nphenol=23in our experiment, the value is1.6times of the theoretical H2O2requirement (nH2O2/nphenol=14); the degradation process needs4-5days to complete by tracking the pH change, showing the organic compounds which cause remaining5%COD is difficult to be degraded at room temperature. Based on above results, we infer the possible mechanism of Fenton degradation for phenol may be as follows:first, phenol is oxidized and the ring is opened by Fenton reagent, forming the long-chain organic acids (mainly be dicarboxylic acid); then the long-chain organic acids are oxidized and the double bond is broken to generate small molecule acid, such as formic acid, which is difficult to be degraded continuously under normal temperature, and the process is simultaneous with the process of ring-opening for residual phenol; finally, it is the process of slow degradation for formic acid. For solution containing chloride ion, the mechanism of phenol degradation is similar, the Cl2and chlorinated organics generated in the process finally still exist in the form of Cl-in the case of excess H2O2addition.Based on the studies above on the Fenton degradation, the filtrate obtained after chlorination was treated by using the method. The results show that24.2wt%H2O2dosage reaches15mL/L, chlorophenols in wastewater can be degraded effectively, and after treatment the solution is colorless and transparent, COD has already reduced to72mg/L. Feasibility analysis on the combined process shows the proposed process for the specific wastewater treatment is feasible, not only has advantages of simple operation and low cost, may also become a profit project under rational use of the recycled products.