| Chromium-containing residue and organic pollutants-containing salt waste are two typical types of industrial hazardous waste.Cr-containing residue is widely generated from the industrial production process.It contains large amounts of toxic and easily-migrated hexavalent chromium?Cr?VI??,which has caused serious environmental pollution and posed a great threat to human health.Detoxification of Cr?VI?-containing residue usually involves the reduction of carcinogenic Cr?VI?to the less toxic and less mobile Cr?III?.However,the long-term stability of the reduced Cr?VI?-containing residue in the environment is difficult to be guaranteed.Cr?VI?often reoccurs in the reduced site,which is called the“re-yellowing phenomenon”,posing a risk of serious secondary pollution.In principle,two causes lead to Cr?VI?reoccurrence:the release of unreduced Cr?VI?and oxidation of Cr?III?.It is generally believed that the release of unreduced Cr?VI?is the reason for the re-yellowing phenomenon,while the effect of Cr?III?oxidation was always neglected.On the other hand,organic pollutants-containing salt waste has a large output and is highly toxic,but it lacks practical treatment methods.Firstly,this paper,starting with the oxidation of trivalent chromium,proved that a small amount of manganese oxide can effectively and continuously oxidize Cr?III?solids to produce toxic Cr?VI?under alkaline conditions,and discovered and measured different pathways of Cr?III?oxidation.Secondly,in order to prevent the environmental risk of Cr?III?oxidation,crystal regulation technique?with sulfuric acid and hydrothermal condition?was developed to fully separate and extract Cr?VI?in the Cr?VI?-containing residue.Finally,the crystal regulation technique was extended to the treatment of the organic pollutants-containing salt waste.With the addition of regulator?acetone?,the growth of sodium chloride crystals in this waste was regulated and the organic pollutants?pyridine derivatives?were removed.The main research contents and innovation points are as follows:?1?Cr?OH?3 oxidation at alkaline p H?9–11?with?-Mn O2 under oxic/anoxic conditions was investigated.Results revealed the three pathways for Cr?III?oxidation under oxic conditions:1)oxidation by oxygen,2)oxidation by?-Mn O2,and 3)catalytic oxidation by Mn?II?.Oxidations in the latter two were efficient,and oxidation via Pathway 3 was continuous and increased dramatically with increasing p H.With the analysis of XRD,XPS,SEM,and XAFS conducted in the solid products,feitknechtite??-Mn OOH?and hausmannite?Mn3O4?were the reduction products and catalytic substances.Additionally,a kinetic model was established to describe the generation rates of Cr?VI?and the relative contributions of each pathway at a specific time.The simulation outcomes showed Cr?VI?was mainly formed via Pathway 2?>51%?over a short timeframe?10 days?,whereas in a longer-term?365 days?,Pathway 3 predominated the oxidation?>78%?with an increasing proportion over time.?2?Sulfuric acid?H2SO4?was used under hydrothermal treatment to regulate the crystal growth of the host phase of Cr?VI?in Cr?VI?-containing residue,i.e.calcium sulfate,and consequently the Cr?VI?was effectively extracted.The results show that the extraction rate of Cr?VI?is greater than 99.8%using 0.5M H2SO4 at 120?for 3 hours.Combined with the means of XRD,SEM,EDS,Cs-STEM,XPS,and UV-Via,the final products with different hydrothermal temperature,hydrothermal time and acid concentration were studied,and the roles which H2SO4 played in the regulation process were interpreted further.It showed the main regulation function of H2SO4 in the process of separating and extracting Cr?VI?mainly includes:?1?accelerating the dissolution-growth process of calcium sulfate crystals;?2?providing sulfate matrix for the growth of calcium sulfate crystals;?3?converting Cr O42-to Cr2O72-;?4?Lowering the detoxification temperature of the Cr?VI?-containing residue.?3?Acetone was introduced as a crystal regulator at room temperature to treat the by-product organic pollutants-containing salt waste in the production process of 2-chloropyridine,and XRD,SEM,GC-MS,FTIR,and TOC demonstrated the dual effects of acetone as following:1)acetone and Na Cl form aqueous two-phase systems?ATPS?to remove organic pollutants;2)acetone regulates Na Cl crystals to recrystallize in cubic morphology with a smooth surface,which enhanced the removal of pollutants.Besides,the acetone-water phase split facilitated the separation of organic pollutants as well as the reuse of acetone.After treatment,Na Cl was effectively recovered?recovery rate,99.12%?,and the TOC value of its saturated solution was reduced from 806.2 to 145.4 mg/L,below the limits?200 mg/L?for chlorine gas and caustic soda production in chlorine alkali plant. |
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