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36 Wastewater Treatment 0 0 QUESTIONS STEP 1 Write your answers in a notebook and then compare your Highly toxic hexavalent chromium (Cr6+) is reduced to answers with those on page 90. trivalent chromium(C04).Sulfuric acid is added to lower the pH to 2.0 or lower. In an acid solution the following reduction 5.2A Why are two different pH levels sometimes necessary reactions will occur, depending on the reducing agent. for effective hydroxide precipitation of metals? Using sulfur dioxide, Cr6+ to Cr3+ 5,25 List the major limitations of the sulfide precipitation SO2 + H2O -+H2so, process. Sulfur Water Sulfurous 5.3A Why are complexed metals difficult to treat? Dioxide Acid 5.31B How can complexed metal wastewaters be treated? 3H,S03 + 2H2CrO, --.Cr2(SO,)3 + 5H20 Sulfurous Chromic Chromic Water Acid Acid Sulfate 5.4 REDUCTION OF HEXAVALENT CHROMIUM Using bisulfites, Cr6+ to Cr3+ Hexavalent chromium-bearing wastewaters are produced in 2H,CrO, +3NaHSO, +3H2S01 - Cr,(SO,), +3NaHSQ, +5H2O chromium electroplating, chromium conversion coatings, etch- Chromic Sodium Sulfuric chromic Sodium water ing with chromic acid, and in metal finishing operations carried Acid Bisuiflte Acid Sulfate Bisulfate out on chromium as a basis material. Many common plating solutions contain hexavalent chromium concentrations be- When using sodium bisulfite,the pH is dropped to 2.0 or lower tween 10 and 500 mg/L; however, concentrations over to hasten the reduction of hexavalent chromium by the bi- 100,000 mg/L can be encountered.Hexavalent chromium may sulfite. be present in the form of chromic acid, chromate, or dichro- Some shops use sodium hydrosulfite for manual additions of mate in an acid solution. To remove chromium from chemicals where it is not practical to drop the pH to 2. Hydro- wastestreams, highly toxic hexavalent chromium (Cr6+) must sulfite works well at a pH of around 4 which is a pH range be reduced to trivalent chromium (Cr 3+) which can then be where hexavalent chromium reduction by bisulfate is far too removed from the wastestream by hydroxide precipitation. slow. Hydrosulfite is not practical to use on continuous proc- Strong reducing agents used in this chemical treatment proc- esses because it is more expensive than bisulfite and is oxi- ess include sulfur dioxide, sodium bisulfite, sodium dized by air once it is in solution and allowed to stand. Sodium metabisulfite, sodium hydrosulfite and ferrous sulfate. Gas- hydrosulfite is very convenient to use when chemicals are eous sulfur dioxide is the most economical reducing agent added manually. used to reduce hexavalent chromium to trivalent chromium for large flow applications.This reaction takes place at a very low Using ferrous sulfate, Cr6+ to Cr3+ pH (around 2.0 for complete reduction), The reduction rate 2H2cr0, +6FeS0, +61-12S0, 3Fe,(SO,)3 +cr,(So,), +8H20 decreases with increasing pH and is extremely slow for pH chromic Ferrous Sulfuric Ferric chromic Water levels above 5. Acid Sulfate Acid sulfate Sulfate STEP 2 �+ The trivalent chromium (Cr3+) compounds formed in Step 1 are removed by alkaline precipitation at a pH between 8.0 and DrQx CIE lime Caustic soda (NaOH) is the preferred base, but hydrated lime (Ca(OH)2) is also used. ASO%(AVK Using caustic soda to precipitate chromic hydroxide t&6u IIT1te, 6NaOH + Cr2(SO,)3 i 2Cr(011),l + 3Na2SO1 } Sodium Chromic Chromic Sodium '�rp�Q� /Xf�T _ Hydroxide Sulfate Hydroxide Sulfate AFA91 i AFIF Using lime to precipitate chromic hydroxide 3Ca(OH)2 + Cr2(SO,)3—2Cr(OH)31 + 3CaSO, Lime Chromic Chromic Calcium L,f Sulfate Hydroxide Sulfate 71+� Reduction of hexavalent chromium requires an equalization tank if the flows fluctuate considerably; two reaction tanks can be connected in series (Figure 15). Sulfuric acid is added to maintain a pH below 2. caseous sulfur cuoxioe or suuiuon bisulfite is added to each reaction tank to produce an ORP (oxidation-reduction potential) around +250 mV (millivolts) or less. Use a color comparator or hand-held colorimeter kit to determine if all of the hexavalent chromium has been con- verted to the trivalent form. If not, increase the sulfur dioxide dose. Observe and record the ORP value at which all hexavalent chromium has been converted to the trivalent form. Set the sulfonator(SO2) or chemical feeder of sulfur dioxide to Chromium wastes are commonly treated by a two-step batch maintain the desired ORP level. Regardless of whether sulfur process. The first step is to reduce the highly toxic hexavalent dioxide or sulfite is used,the tanks should be exhausted to the chromium to the less toxic trivalent form. The trivalent form is atmosphere to prevent people being exposed to sulfur dioxide removed by hydroxide precipitation in the second step. gas (can be produced from bisulfite).