Reclamation of Cobalt and Copper from Single- and Co-contaminated Wastewater via Carbonate and Hydroxide Precipitation

25 Wastewater containing cobalt and copper comprised of plating wash water, plant wash water, and 26 equipment cooling and wash water is generated in the electroplating industry. These metals can be 27 detrimental to humans, animals, plants, and the environment. Thus, it is necessary to treat 28 electroplating wastewater to remove these toxic metals. Carbonate and hydroxide precipitation 29 were utilized for the removal of Co(II) and Cu(II) from synthetic electroplating wastewater by jar 30 tests in this work. The effects of solution pH, precipitant-to-metal ratio, and type of precipitant on 31 the precipitation efficiency of cobalt and copper from the single- and co-contaminated systems 32 were investigated. Carbonate precipitation achieved higher removal efficiency for both target 33 metals in the single- and co-contaminated wastewater streams. Furthermore, it can operate at 34 relatively low pH range of about 7.0-8.0. Cobalt in both pollutant systems was almost completely 35 removed at pH 10.0 using both precipitant systems. Copper was found to be easily removed which 36 was possibly brought about by precipitation-adsorption mechanism. The extent of the co-removal 37 of cobalt with copper is significantly pH dependent. The effect of precipitant-to-metal ratio for 38 cobalt and copper treatment varied in single- and co-contaminated streams. Carbonate 39 precipitation led to lower sludge density than that of hydroxide precipitation. 40 47 48


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With the progressing economy, and rapid growth and development of industries including mining 51 and smelting operations, leather tanning, metal plating facilities, metal cleaning and fabrication, 52 metal finishing, battery manufacturing, electrochemical, paint and pigment industries, heavy 53 metals are being discharged into the aquatic streams to an increasing degree [1]. These recalcitrant 54 and persistent pollutants are considered toxic, carcinogenic, and non-biodegradable which pose 55 detrimental effects on biological environment and human health [2]. Thus, remediation of these 56 contaminants in water and wastewater have been of particular concern. 57 Cobalt is a significant cofactor in Vitamin B12 responsible for the proper functioning of the 58 brain and nervous system, and for blood formation. It is one of the most essential transition metals 59 beneficial to human beings. However, excessive intake may be hazardous to both humans and 60 animals. Moreover, exposure to high levels of cobalt may induce toxic effects and may cause goiter, 61 thyroid damage, diarrhea, nausea, reproductive problems, hypertension, heart disease, bleeding, 62 pulmonary diseases, hyperglycemia, hair loss, bone defects, and mutations in living cells [3,4]. adsorption are the existing treatment technologies to remove heavy metals in water and wastewater 72 4 streams [9,10]. Among these current methods, chemical precipitation is the most widely utilized 73 method in the industry particularly due to the simplicity of process control, effectivity over a wide 74 range of temperature and relatively low operating cost [9,11]. Traditionally, chemical precipitation 75 processes produce insoluble precipitates of heavy metals in the form of hydroxides, sulfides, 76 carbonates, and phosphates. Chemical precipitation mechanism involves the reaction of dissolved 77 metals in the solution with the precipitating agent producing insoluble metal precipitates.

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Consequently, very fine particles are generated, and their particle size can increase by using 79 chemical precipitants, coagulants, and flocculation leading to removal as sludge. Once the metals 80 precipitate leading to the formation of solids, they can easily be removed, and low metal precipitation processes are almost similar. They are of lower cost than sulfide precipitation [14].

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Hydroxide precipitation removes heavy metals by the addition of alkalis (caustic or lime) to 90 adjust the pH of the wastewater until the pollutant metal exhibits its minimum solubility. It is easy 91 to operate, operates at ambient conditions, and appropriate for automatic control. The most 92 significant advantage of this process is its low cost [15]. On the other hand, carbonate precipitation 93 using sodium carbonate (Na2CO3) is a low-cost chemical precipitation process with added 94 5 advantages such as its simplicity, optimum treatment occurring at less pH levels and sludges with 95 good filtration characteristics [11]. 96 Few investigations have utilized chemical precipitation method in treating co-contaminated 97 heavy metal wastewater streams, particularly containing cobalt and copper. Thus, the aim of the 98 study is to investigate the influence of pH, precipitant-to-metal ratio, and type of precipitant on the 99 co-removal of cobalt with copper from synthetic electroplating wastewater using carbonate and 100 hydroxide precipitating agents since these pollutants co-exist in the said wastewater stream.

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Single-contaminated synthetic wastewater was also treated for comparison in terms of removal 102 efficiency, and sludge volume generation.

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Volumetric method was used to determine the estimated volume of the decanted sludge. The 128 sludge volume at 30 min (SV30) was determined using an Imhoff cone (Kartell Labware, Italy).

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The sludge settling rate (SSR, cm 3 min -1 ) was determined in the reaction system given by Eq. (1).

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Cobalt and copper residues were measured by using inductively coupled plasma mass In pure water, the total copper can be depicted as follows: Substituting Eqs. (14) to (17)   The behavior of Co(II) in the co-contaminated system was also studied in terms of the effect of 316 pH on its removal in the presence of Cu(II) as presented in Fig. 5a. For both precipitants, increasing 317 the pH increased the cobalt co-removal and co-precipitation with copper. Using sodium carbonate 318 as precipitant, cobalt was removed to over 99% at pH 9.0. The same result was achieved by Safitri reaction pH of about 8.5 in the single system as depicted in Fig. 6a