Contents lists available at ScienceDirect Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe Comparison of heavy metal removals from aqueous solutions by chemical precipitation and characteristics of precipitates Quanyuan Chen a,b,c, ⁎ , Yuan Yao a , Xinying Li a , Jun Lu a , Juan Zhou a,b , Zhaolu Huang a a School of Environment Science and Engineering, Donghua University, Shanghai, 201620, PR China b Shanghai Institution of Pollution Control and Ecological Security, Shanghai, 200092, PR China c State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua University, Shanghai, 201620, PR China ARTICLE INFO Keywords: Heavy metal Precipitation Sludge characteristics Particle size Solid–liquid separation ABSTRACT Typical chemical precipitation methods using lime (Ca(OH) 2 ), soda ash (Na 2 CO 3 ) and sodium sulfide (Na 2 S) for removals of heavy metals (i.e. Zn (II), Cu (II) and Pb (II)) from aqueous solutions were compared by jar tests. A focus was especially given to particle size differences and chemical phase conversion of precipitates. A removal of 99.99% from aqueous solutions with three precipitants was achieved for copper and zinc at an initial con- centration of 100 mg/L. And lead was efficiently removed (99.75%) by sodium sulfide. In contrast, the maximal lead removals with lime or soda ash precipitation were only 76.14% and 97.78%. The mean particle size of precipitates was in the range of 55 nm–45 μm, depending on properties of precipitants and heavy metal to precipitant ratios. The settling performance of the sludge derived from precipitation was dominated by particle size and Zeta-potential of precipitates. It was observed that ultra-fine copper sulfide particles resulted from the precipitation were around 55 nm and did not settle in 12 h due to electrostatic repulsion force between particles. The main compounds in the sludge obtained from precipitation were metal hydroxides and metal sulfides. However, spontaneous dehydration of metal hydroxide, oxidation of sulfide and atmospheric carbonation were identified by means of XRD and thermal analyses, which is invaluable to the disposal and utilization of the sludge. 1. Introduction Heavy metal-bearing wastewater discharged into environment have disastrous impacts on surface water, ground water, sediment and soil [1,2]. To remove heavy metals from the contaminated wastewater, various treatment approaches such as chemical precipitation, coagula- tion/flocculation, ion flotation, adsorption, ion exchange and mem- brane filtration have been employed [3–6]. Of these methods, chemical precipitation is most widely practiced in industry, mainly for the sim- plicity of process control, effective over a wide range of temperature and low cost of operation. It employs pH adjustment to convert heavy metal ions to hydroxide, sulfide, carbonates or other less soluble com- pounds, which then can be removed by physical means such as sedi- mentation, flotation or filtration. These processes depend on the size, density and surface charge of particles to be removed [7–9]. Common inorganic precipitants used for heavy metal precipitation are lime (Ca(OH) 2 ), caustic soda (NaOH), soda ash (Na 2 CO 3 ), sodium bicarbonate (Na(HCO 3 ) 2 ), sodium sulfide (Na 2 S) and sodium hydro- sulfide (NaHS). The selection of these chemicals should be able to meet the maximum contaminant level (MCL) standards or to reach the de- sired final concentration required for recycling [10–13]. Heavy metals exhibit divergent precipitation behavior at different pH, as the pH af- fects the solubility of metals [14–18]. On the other hand, pH is de- pendent on pollutants in wastewater and the dosage of precipitants, which should be constantly monitored and controlled during the pre- cipitation process. The further studies on the removal effectiveness for heavy metals (at a common concentration of around 100 mg/L) and the detailed comparison of heavy metal precipitation with typical pre- cipitants are required. As legislation for metal discharge to environment becomes more stringent and metal resources become increasingly scarce, heavy metal sludge may need to be recyclable [19]. The detailed understanding of chemical compounds formed in precipitation as well as possible con- version of heavy metal compounds during sludge processing is in an urgent need, which is invaluable to the disposal and utilization of the sludge [20]. In the present work, precipitations of heavy metals namely zinc, copper and lead from aqueous solutions with lime (calcium hydroxide), https://doi.org/10.1016/j.jwpe.2018.11.003 Received 15 May 2018; Received in revised form 30 October 2018; Accepted 3 November 2018 ⁎ Corresponding author at: School of Environment Science and Engineering, Donghua University, Shanghai, 201620, PR China. E-mail address: qychen@dhu.edu.cn (Q. Chen). Journal of Water Process Engineering 26 (2018) 289–300 2214-7144/ © 2018 Elsevier Ltd. All rights reserved. T