Chemical Oxygen Demand- Acid Waste Disposal and Recovery of Mercuric Sulphate to minimise the test impact on our environment Murendeni Stewart Mafumo Johannesburg Water No. 75 4 th Street, Houghton, Johannesburg, South Africa E-mail: murendi@gmail.com Phone: 082 379 8640, office: 011 728 7373 Abstract Johannesburg water with the mandate to provide water and sanitation services to over three million residence of the city of Johannesburg has the responsibility to monitor and ensure compliance of drinking water, environmental water and industrial effluent that is discharged to the system. Over 500 samples are monitored a month at an ISO17025:2005 accredited facilities to ensure compliance with by-laws. Johannesburg water stands by the resolutions for a better tomorrow by preserving our environment and we are committed to the reduction of hazardous material and toxic waste in our tests. Chemical Oxygen Demand (COD) test assesses all chemically oxidizable substances by using Potassium Dichromate as the oxidizing reagent and a strong acid (Sulphuric Acid) each organic compound differs in the amount of oxygen necessary for complete oxidization, the COD test reflects the effect of an effluent on the receiving stream more directly than measurement of carbon content. COD test releases a waste that contains of approximately 25% Sulphuric acid, 360mg/LCr3 + from reduced potassium dichromate, 1500 mg/l Hg2 + from Mercuric Sulphate that works as a complex reagent for interferences and 1600 mg/l Ag + from the silver sulphate catalyst, this waste is very hazardous and toxic and require very strict and expensive waste management to minimize the effect of this constituents on the environment. This paper aims to discuss and quantify the method to recover chemical oxygen demand (COD) waste and reuse the mercuric sulphate from the waste to save costs and the environmental impacts of the hazardous waste. Experiments were conducted on solutions containing mercury and silver and on actual wastes from chemical oxygen demand to assess mercury and silver recovery on a pilot scale to assess the reduction potential of the material, Reaction order, reaction rate, removal efficiency, and stoichiometry of the reaction were evaluated.
Introduction In any water system, microorganism will consume any organic & inorganic matter added to it and will produce biomass using oxygen present in the water. The oxygen required for the degradation of the organic matter biologically is called the Biochemical Oxygen Demand (BOD). The industrial and municipal waste water effluents may contain very high amounts of organic matter and if discharged into natural water bodies, it can cause complete depletion of dissolved oxygen leading to the mortality of aquatic organisms. The amount of oxygen needed to consume the organic and inorganic materials is called Chemical Oxygen Demand (COD). There exists a correlation between the COD and BOD under certain conditions and by determining the COD information about the BOD of the water/wastewater can be derived. The COD test is therefore a lot more appealing to use as an indicator of the amount of organic matters in industrial and domestic wastewater than the BOD test, as the COD test is a fast(2 hours compared to 5days for BOD), easy and efficient test between the two. The Basis of the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent and acid. COD test is not only a measure of organic carbon, a number of compounds containing Nitrogen and slightly soluble hydrocarbons are also indicated in the test. As the biomass produced by the organics is of a complex structure, it is important to have a test that will reflect the amount of pollutants in the water. Chemical oxygen demand is a cornerstone test of vital importance for assessing the quality of effluents and waste waters prior to discharge. The test predicts the oxygen requirement of the effluent and is used for monitoring and control of discharges, and for assessing treatment plant performance. The impact of an effluent or waste water discharge on the receiving water is predicted by its oxygen demand. This is because the removal of oxygen from the natural water reduces its ability to sustain aquatic life. The COD test is therefore performed as routine in most if not all laboratories of water utilities and industrial companies. Potassium dichromate is considered the best oxidant due to its strong oxidizing ability, its applicability to a wide variety of samples and ease of manipulation makes it very efficient. Sulphuric acid is used as the strong acid for the oxidation reaction. Mercuric sulphate is added as a chloride interference complexing reagent, with silver sulphate used as a catalyst for the reaction. The first step for the test is digestion; concentrated sulphuric acid (H 2SO 4) provides the primary digestion catalyst. The secondary catalyst, Silver Sulphate (Ag 2 SO 4), assists oxidization of straight-chain hydrocarbons such diesel fuel and motor oil. Heat from the digestion block (150-200 C) also acts as a catalyst. During digestion the sample s organic carbon (C) material is oxidized with the hexavalent dichromate ion (Cr 2O 2-7 ) found in potassium dichromate (K 2Cr 2O 7). The dichromate readily gives up oxygen (O 2) to bond with carbon atoms to create carbon dioxide (CO 2). 2- The oxygen transaction from Cr2O 7 to CO2 reduces the hexavalent Cr 2O 2- ion to the trivalent 7 ion. In essence a COD test determines Cr3+ the amount of carbon based materials by measuring the amount of oxygen the sample will
react with. This oxygen transaction is the source of the test s name, Chemical Oxygen Demand. The use of the COD test however is somewhat limited by the costs of reagents, especially silver and mercury salts, and by the difficult disposal of a highly acidic waste that contains roughly 25% Sulphuric acid at ph lower than 1, with 360 mg/l Cr 3+ from reduced Potassium Dichromate, 1500mg/L Hg 2+ and 1600mg/L Ag +. This waste is considered hazardous and cannot be dumped down the sewer system due to the content of heavy metals such as Cr, Ag and Hg; however treating these wastes is considered complex and expensive. What is hazardous waste? Hazardous Waste is waste that has the potential, even in low concentrations, to have a significant adverse effect on public health and the environment because of its inherent toxicological, chemical and physical characteristics. Therefore, as shown in the diagram below, something needs to be done for COD waste to minimize the test impact on the environment; Hazardous Waste requires stringent control and management to prevent harm or damage and hence liabilities. It may only be disposed of on a Hazardous Waste landfill or using expensive technology. Since the Precautionary Principle is applied, a waste must always be regarded as Hazardous where there is any doubt about the potential danger of the waste stream to man or the environment. A Hazardous Waste is also defined as an inorganic or organic element or compound that, because of its toxicological, physical, chemical or persistency properties, may exercise detrimental acute or chronic impacts on human health and the environment. It can be generated from a wide range of commercial, industrial, agricultural and domestic activities and may take the form of liquid, sludge or solid. These characteristics contribute not only to
degree of hazard, but are also of great importance in the ultimate choice of a safe and environmentally acceptable method of disposal. However we currently cannot prevent nor reduce the COD waste in its current form, therefore the need for something to be done to reduce this tests impact on the environment arises. This paper will innovatively describe the methods to recover and reuse the waste to protect our environment. An experimental procedure of metal ion precipitation in COD wastes with affordable chemical products is reported in this work. Cr (VI) was chemically reduced by adding stannous chloride, Cr (III) was precipitated as a metallic hydroxide by adding NaOH and Ag was reduced by adding 2 g/l of sodium chloride and also precipitated using sodium hydroxide. Test Method and Experimentation This is an experiment of serial precipitation, recovery and reuse of the Silver, mercuric sulphate and safe disposal of the raw material using cheap readily available chemical reagents in the laboratory. Prior to the experimentation, four typical samples of COD waste were diluted 100times and analysed on the ICP-OES to determine exactly how much of the heavy metals are in the waste (see appendix 1) In a 60 litres reaction vessel about 1.5L of a 20% solution of sodium chloride is added and the solution stirred a few minutes, this will form a white silver chloride precipitate. 500ml of the clear supernatant is allowed to run out at a time. This process is repeated about 20 times until enough silver chloride precipitate is collected. Recovery of Silver: The drained 60L vessel with the silver chloride precipitate is then placed on a laboratory bench and positioned so to drain excess water in the sink. The silver chloride precipitate is washed with tap water and the clear water transferred to a waste container with 500ml allowed to run out at a time. This process is repeated until the ph of the supernatant is greater than 4. The silver slurry is then transferred into a graduated beaker and allowed to settle, with the excess water then sucked out using either an aspirator pipe or pipette. The volume of the precipitate is noted, and then weighed by difference using a top pen balance to determine the dry mass of silver chloride. In a fume cupboard 600g technical grade caustic soda per 1000g silver chloride is added, with constant stirring, black silver slurry will develop. The slurry is then allowed to stand overnight to maintain homogeneity. 600ml of 35-40% formaldehyde solution per 1000g Silver Chloride is then slowly added to the slurry in a fume cupboard with hydrogen gas being released rapidly. After all the formaldehyde has being added with constant stirring, the slurry is allowed to stand for a few days to settle. The supernatant on the top is then sucked using a pipette or aspirator pump carefully, until all the liquids are removed. The precipitate is then washed with distilled water until the conductivity is close to that of tap water (23mS/m).
The resultant precipitate is then placed on a large porcelain dish and dried at 105 C, cooled and broken into small particles. The dish is then placed on a furnace at 375 C for two hours. The cake is broken down into smaller portions and placed on a furnace again, this time at 600 C for at least an hour. The pure content is then cooled and broken up and stored in a container for purity testing and future use. Mercuric Sulphate Recovery When the 200 Litre vessels is full from the previous experiment, 500g stannous chloride solution per 600ml waste is added to the waste with care and constant stirring, this will form a white precipitate of mercurous chloride. from the bottom tap, about 500ml is allowed to run out into a beaker, the clear supernatant is then ran off into another drum, tap closed and content thrown back to the beaker until sufficient mecurous chloride has been collected. The precipitate is then washed by adding tap water in the reaction vessel with constant vigorous stirring, about 500ml allowed to run out at the bottom at a time. This process is repeated until the ph of the supernatant is greater than 4. The slurry is then transferred into a graduated beaker and allowed to settle, with the excess water then sucked out using either an aspirator pipe or pipette. The volume of the precipitate is noted, and then weighed by difference using a top pen balance to determine the dry mass of mercurous chloride. In a fume cupboard 350g technical grade caustic soda is added per 1000g mercurous chloride to the reaction mixture and with constant stirring the dissolved caustic soda will turn the reaction mixture to black slurry that is then allowed to stand overnight. The following day 250ml of 37% formaldehyde per 1000g mercurous chloride is carefully added to the slurry and for the next few days the reaction is given an occasional stir then allowed to settle. The reaction vessel is then filed with tap water and the slurry rinsed until the conductivity is at ±23mS/m. The mixture is then transferred to a 5L Erlenmeyer flask; excess water removed and allowed to settle. In a fume cupboard and very carefully 1300ml of 65% nitric acid per 1000g mercurous chloride is added to the slurry, 360ml conc. Sulphuric is also added carefully with constant stirring and the mixture allowed to stand for at least 2days. The slurry is then placed on a centrifuge at 5000rpm for 10minutes, the solution is then filtered and the filtrate transferred to a 25L plastic bucket. The ph of the filtrate is then adjusted using a 20% caustic soda solution to ph 3, a sparingly soluble yellow precipitate of mercuric sub-sulphate, Hg(HgO) 2 SO 4 will form. The precipitate is then allowed to settle and excess water removed. The sparingly soluble mercuric sub-sulphate is then washed with distilled water until the conductivity of the supernatant becomes constant at ±5mS/m The precipitate is then transferred to a large glass container and dried at 105 C for atleast 2days. The cake then crushed enough to pass through a 1mm sieve. With the resultant being mercuric sulphate that is then subjected to purity test and stored for re-use. By-products: The waste generated from the precipitation was neutralized and analyzed for ph and conductivity before disposal at room temperature.
Results Appendix 1: Using an ICP-OES four typical COD waste samples were analyzed to determine the exact amount of reagents to be added; Discussion: Volume and toxicity reduction of liquid waste is the main objective of hazardous waste treatment and it is usually carried out by physiochemical, biological and thermal process. This experiment employed a reduction-precipitation as a physiochemical treatment process based on the formation of a sparingly soluble molecule due to very low Kps values; however it was noted that the Cr (VI) concentration must be decreased to avoid this ion interfering with the reduction-precipitation process. This was done by adding 37% formaldehyde to the reaction mixture. COD residual treatment was carried out through a series of precipitation reactions in which insoluble salt is formed from the ions present, Silver Chloride was successfully formed from silver by adding 20% sodium chloride and as discussed by T Mañunga, 2010 a 99% reduction occurred with the solution insoluble in both acid and alkali. Likewise, mercury was precipitated as mercurous chloride using a stannous chloride solution. This precipitation as described by Holm, 1996 takes place at ph value lower than 3 and produce fumes of nitrogen dioxide which is volatile, toxic, corrosive and has a strong odour. The objective of this study was to present an efficient and environmentally friendly methodology for treating COD waste generated at Cydna Labs. The sequence of chemical reactions in which heavy metals are eliminated as hazardous solid residues was achieved using commonly available reactants. It was observed during the experimentation that the dosage of 20% Sodium Hydroxide was sufficient to precipitate the required amount of silver chloride from the waste, as initially assumed, waste from the Cydna Laboratories of Johannesburg water consisted of high amount of mercury than many other studies done in the past, the amount of formaldehyde was then increased to sufficiently compensate for the heavy
metal concentration. When the reaction for the precipitation of silver chloride was started the reaction mixture times were increased to 20 minutes and constant stirring was monitored; And the reaction; Ag 2 SO 4 + NaCl AgCl + Na 2 So 4 The reaction time in which maximum Cr(VI) reduction is achieved varies and in the case of most of our samples that averaged 546mg/L Cr(VI) a 30 minute stirring and reaction time was required to achieve the full precipitation. The immediate formation of large particles was observed when adding NaCl to the reaction mixture for the precipitation of silver chloride. However, further studying is needed to determine the percentage reduction and the amount of time the phase separation is obtained and fully settled. Difficulty in adjusting the ph to a slightly neutral value proportional to sample dilutions was observed during this study, excess amounts of caustic soda in precipitation of both silver and mercury was therefore recommended. the dosages as described by T.W Lory were sufficient for the recovery and reuse of Silver and Mecuric sulphate, the precipitation was observed immediately, it is however important as the studies continue to monitor the stirring speed and time. Conclusion The use of the chemical oxygen demand (COD) test is somewhat limited by the costs of reagents, especially silver and mercury salts, and by the difficult disposal of a highly acid waste that contains roughly 0.01 M concentrations of silver (Ag), mercury (Hg), and chromium (Cr). A method for separately removing silver and mercury to low levels and in forms suitable for chemical recovery and reuse was presented after a pilot scale project. Silver is precipitated as silver chloride (AgCl). Mercury is reduced as Hg to a sparingly soluble yellow precipitate of mercuric sub-sulphate, Hg(HgO) 2 SO 4 which after drying is resultant as Mercuric Sulphate. Neutralization is not required and little heat is generated during the chemical recovery process. Waste may be disposed to sewer systems with sufficient dilution to raise the ph. The proposed methodology for treating COD residues generated in Cydna laboratories is a feasible and environmentally sound alternative allowing institutions to comply with regulations regarding maximum allowable heavy metal discharge in liquid waste. Reduction and selective precipitation concentrates the ions present in COD liquid residues into a solid residue; the by-product is a sludge which can be raw material for an Ag, Hg and Cr recovery/extraction process. The pilot scale project has proved the efficiency of the proposed method as the recovery and reduction of the material is proving to be viable in all respects, the material that is collected and reusable will be continually tested and used for COD analysis.
Recommendations This study is a continual study that requires and consists of four steps: Precipitation, Recovery, Reuse and Disposal of by-products Therefore it is recommended that this study is conducted on a full scale basis, with the dosages and methodology preliminary showing initially predicted results and trends. The process of treating COD waste is a very expensive process, therefore this method is highly recommended within Johannesburg Water as it will save money in terms of waste treatment and as the recovery of mercuric sulphate is conducted on a full scale this reagent will be then reusable in the laboratory, the silver recovered is not one that is used within Cydna laboratories therefore it can either be sold or taken to the waste treatment company to dispose of. This method is one that uses readily available reagents, material and is one that can be done after a few months after sufficient waste is collected. The recycled material will then have to be tested each time for purity. As this was a pilot study therefore more work is needed to determine the Reaction order, reaction rate, removal efficiency, and stoichiometry of the reaction. The purity of the mercuric sulphate also needs to assessed before the material can be tested on actual COD analysis. Removal efficiencies of better than 99% for silver and mercury need to studied and shown to be attainable. The thickness of the mercury film deposited from single-solute mercury solutions needs to be studied for proportion to the ratio of initial mercury concentrations to reductant surface area. References 1. J.P Gould, M.Y Masingale, M Miller, Recovery of silver and mercury from COD samples by iron cementation 1984 2. intermediate chemistry 6 th edition T.W Lowry and A.C Cavell 3. City of Johannesburg water services by-laws (notice 835 of 21 May 2004) 4. A.D Eaton, L.S Clesceri etc al. Standard methods for the examination of water and wastewater 21 st edition 5. T Mañunga, Treatment of COD analysis liquid wastes generated in environmental laboratories 2010 6. R.T Holm, Treatment of spent chemical oxygen demand solutions for safe disposal WMRC Reports: Waste Management and research centre, university of Illinios, 1996.