This work consists of the removal of nickel by sulfide precipitation from industrial electroplating wastewater and characterization of the produced sludge. Tests are carried out in a perfectly stirred batch reactor on electroplating industrial solution and synthetic solution prepared in the laboratory. The aim is to evaluate the impact of complexing agents formed during precipitation of metal ions in the industrial effluent. The concentration of nickel in both solutions is 100 mg/L. The operating conditions for the sulfide precipitation process are optimized: pH, molar ratio [S=]/[Ni2+] and dosage of S= ions. For an initial pH of 5 and an equimolar ratio of [S=]/[Ni2+]:1/1, the results show that the removal efficiency of Ni2+ ions is approaching 91 and 94% for industrial and synthetic solutions, respectively. Otherwise, for the same pH value in supersaturation conditions ([S=]/[Ni2+]:1.5/1), the removal efficiency is approaching 62 and 92% for industrial and synthetic solutions, respectively. For an effective metal removal, the optimal dosage of sulfide ions was evaluated. For 33 mg/L of S=, the removal efficiency of Ni2+ is approximately 90%. The resulting sludge has been characterized by X-ray diffractometry, scanning electron microscopy, infrared spectroscopy and thermal analysis. It consists essentially of millerite and nickel oxide.
The recovery of nickel precipitate was improved when the initial pH of the solution, with stoichiometric ratio ([S = ]/[Ni2+] : 1/1), was set to 5 giving a nickel abatement rate above 98%.
Tests carried out with a ratio [S = ]/[Ni2+] equal to 1.5/1 at pH 5 showed that the excess of sulphide ions reduces the efficiency of nickel removal due to the displacement of the equilibrium of the precipitation reaction to more HS- and H2S and to the rapid increase of pH which is related to the formation of aqueous polysulfide complexes.
Sulphide precipitation can be used to precipitate nickel as millerite (NiS). The result of heat treatments and analysis of the composition and morphology shows that it is a recoverable material and guarantees compliance with environmental regulations.
Heavy metal pollution has become a major environmental problem receiving growing attention in recent years. Most industrial sectors generate contaminated wastewater with metallic elements, causing serious environmental damage (Wei et al. 2019). An example would be the electroplating industry releasing effluents mainly issuing from metal products' manufacture. Environmental contamination by heavy metals (arsenic, chromium, zinc, nickel, etc.) has been reported in various countries such as China, Morocco, Mexico, United States, India, Indonesia, Bangladesh, Thailand, etc. (Wakida et al. 2008; Babu et al. 2009; Taouil et al. 2014; Wang & Chen 2015).
A number of studies in humans and animals suggest that exposure to soluble nickel salts leads to systemic effects on the kidneys, neonatal mortality and effects on the immune system. The kidney is the main target organ for both animals and humans (Lentini et al. 2017). Several experimental and epidemiological studies have shown that nickel is genotoxic and has a fatal effect on the indigenous aquatic biota (Binet et al. 2018).
In the last few decades, many technologies have been applied to the treatment of metal-laden effluent such as ion exchange, electrocoagulation, chemical precipitation, adsorption, and solvent extraction. Their advantages and limitations have been evaluated by several authors (Coman et al. 2013; Mnasri-Ghnimi & Frini-Srasra 2019; Jerroumi et al. 2019; Pan & An 2019; Wu 2019; Sunder et al. 2020).
Hydroxide precipitation treatment is the most used technique for industrial discharges. The process is simple and cost-effective (Brbooti et al. 2011). However, a high volume and poor dewatering properties characterize the precipitated sludge, which increases handling and disposal costs. Also, optimum precipitation takes place generally at pH values between 10 and 11, which may require neutralization post-treatment (Sampaio et al. 2009). Otherwise, the application of sulfide precipitation proves to be a relevant and promising technology with various merits over hydroxide precipitation, with higher selectivity for nickel removal, faster reaction rates, better dewatering properties of sludge, and lower solubility of precipitate (Lewis 2010). Even at acidic pH, most heavy metal sulfides have very low solubilities. For example, the solubility product Ksp of NiS is 1 × 10−22 (Zelekew & Kuo 2017).
Sulfides can come from either a solid (CaS, FeS), soluble salt (Na2S, NaHS) or gaseous source (H2S). However, the dosing of S= is not always easily controllable because of the very low solubility of the produced metal sulfides. Indeed, many authors have shown the influence of sulfide ion concentration on the elimination of metals (Cu, Fe, Ni, etc.). They conclude that the removal efficiency of metals is highly dependent on the dosage of S= ions. They note that an excess of S= ions leads to the formation of polysulfide complexes and an insufficient S= dosage results in an incomplete precipitation (Cao et al. 2009).
The problem of resource scarcity has encouraged research on waste valorization. Industrial sludge usually contains heavy metal, which enhances its dangerousness and treatment difficulty (Shi et al. 2018). Sulfide sludge is commonly generated from sulfide precipitation of nickel in wastewater. Therefore, the valorization of this material would have a higher industrial interest. Currently, the main disposal routes include landfills, incineration and pyrolysis (Xu et al. 2015). These traditional methods have resulted in wasting of resources and risk of secondary pollution of the environment (Panepinto et al. 2016). The present work aims to improve the efﬁciency of nickel elimination by sulfide precipitation and establish a series of tests on electroplating effluent with the goal of optimizing the relevant parameters such as the pH, the molar ratio [S=]/[Ni2+] and the dosage of sulfides. The same sulfide precipitation protocol is applied to synthetic water. In this paper, we report the results of characterization of metallic sludge by several techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR) and thermal analysis (TGA/DTA), in order to provide detailed information about the crystallographic structure, chemical composition and physical properties of sulfide sludge.
MATERIALS AND METHODS
Sulfide precipitation tests were performed in a pyrex-reactor. The treated volume of nickel-rich effluent was 500 mL (100 mg/L of Ni2+). Synthetic nickel-rich waters were prepared from NiCl2, 6H2O. Sodium sulfide salts (Na2S) were used as the source of sulfide ions (S=) and added to the volume of nickel-rich solution. The mixture was stirred at moderate speed (30 rpm) with a magnetic stirrer. The pH adjustment was carried out with 0.1 M HCl or 0.1 M NaOH. All of the chemicals were of analytical grade supplied by Scharlau and all solutions were prepared using distilled water.
The first test studied the effect of initial pH (3, 5, 6, 7) of the mixture (nickel-rich solution and sulfide solution) with stoichiometric ratio between nickel and sulfide (1[S=]:1[Ni2+]) on the nickel removal.
The second test studied the effect of molar ratio between nickel and sulfide (1.5[S=]:1[Ni2+]) on the nickel removal. The initial pH of the mixture was fixed on the optimum value giving the maximal nickel ions' removal rate in the first test.
The third test studied the effect of sulfide ions' concentration on the nickel removal.
During each test, a 20 mL sample was taken every 2 minutes for the first 10 minutes of the reaction, then, every 5 minutes for the next 30 minutes, and every 15 minutes for the last 30 minutes. The samples were filtered using membrane filters (0.45 μm) and stored with a drop of concentrated nitric acid (70%) (Aldrich) for nickel analyses. The nickel concentration was measured using an atomic absorption spectrometer (AA-7000).
The sludge generated from sulfide precipitation at optimal conditions was a black powder whose mass weight was approaching 48.93 mg. It was filtered and dried at 150 °C for 24 hours before characterization. The X-ray diffraction analysis of the sludge was performed by D8 ADVANCE powder diffractometer coupled to a computer operating system. The spectra were recorded at 10° < 2θ < 80°. The diffractograms were processed using HighScore software which is based on data from JCPDS files (Joint Committee of Powder Diffraction Spectrometry).
The infrared absorption spectra were recorded using a SHIMADZU spectrometer (IR Affinity_1S, Fourier Transform Infrared Spectrophotometer) on a solid film in the form of a KRr pellet, where the treated compound was in suspension.
The DTA/TGA thermal analysis device was performed by SHIMADZU type (DTG – 60H). The tests were carried out with a heating/cooling rate of 10 °C/min. The scanning electron microscopy device was performed by FEI SEM Quanta type.
RESULTS AND DISCUSSION
Test 1: effect of pH on nickel removal in a mixture with an equimolar ratio 1[S=]:1[Ni2+]
In order to ensure a good precipitation of nickel ions, it is necessary to study the effect of different initial pH values on the nickel removal rate. The pH values have been defined based on many studies on sulfide precipitation (Reis et al. 2013; Li et al. 2019).
Diagrams illustrated in Figure 1 show the variation of Ni2+ ions concentration of synthetic and industrial solutions and their pH with time.
For both solutions, pH increases over time because of the consumption of S= to NiS at one time, and the release of H2S gas, accelerated by the mechanical agitation, at the other time.
This observation was also revealed by researchers who worked on the treatment of synthetic nickel solution. They report that the pH of the nickel solution increased to approximately 9 as the reaction occurred (Karbanee et al. 2008; Reis et al. 2013). For initial pHs of 3 and 5 we note a remarkable decrease of nickel ions concentration for both solutions (Figure 1(b) and 1(d)), but for pHs of 6 and 7, this decrease is low. The yields of abatement at 25 minutes of reaction are presented in Table 1.
|pH .||3 .||5 .||6 .||7 .|
|pH .||3 .||5 .||6 .||7 .|
For industrial effuent, as can be seen in Figure 1(b), the nickel sulfide precipitation increased when pH reached 5. At this pH value, above 90% nickel ions were precipitated. Normally, in acidic solutions, S= is sufficiently present. Then, it can react easily with Ni2+ (reaction (3)). In the basic environment, reaction (2) is dominant; the preponderant species are HS− and H2S. Thus, if the precipitation of NiS is not sufficient, all the more H2S could evaporate with agitation as cited above. Nickel ions form complexes such as NiHS+, Ni2(HS)3+ and Ni3(HS)5+, which probably involve the formation of NiS (Rickard et al. 1996). Some studies have argued that only HS− ions are responsible for the precipitation of NiS because S= ions are not present in sufficient concentration in the pH interval of 0–14 (Lewis & Van Hille 2006).
In both solutions (synthetic and industrial), the chemical precipitation of nickel was better at an initial pH of 5. However, the kinetics of reaction remained different. It was relatively weak for the industrial solution. This can probably be explained by the formation of complex molecules between Ni2+ and organic or mineral entities present in the industrial solution, thereby reducing accessibility of cations presumably because of their chelation, as confirmed by some authors (Wang et al. 2005; Jiang et al. 2008; Aghazadeh-Ghomi et al. 2018).
Test 2: effect of the ratio 1.5 [S=]:1[Ni2+] on the removal of Ni2+ at pH 5
Tests carried out with a ratio [S=]/[Ni2+] equal to 1.5/1 at pH 5 showed that the excess of sulfide ions reduced the efficiency of nickel removal (Figure 2).
The elimination yields of nickel ions, obtained in super saturation conditions after 25 minutes of reaction were 62% for the industrial solution (compared with 91% for tests carried out with a ratio 1[S=]/1[Ni2+]) and 92% for the synthetic solution (compared with 94% for 1[S=]/1[Ni2+]).
The decrease of nickel removal efficiency can be explained by a redissolution of the precipitate NiS, as demonstrated by some authors (Hille et al. 2005), or by the displacement of the equilibrium to more HS− and H2S. This tendency, possibly enhanced by the rapid increase of pH, is related to the formation of aqueous polysulfide complexes or to the low stability of the fine sulfide particles. Studies performed by some authors on a synthetic solution ([Ni2+] = 200 mg/L) of NiSO4 in a ratio [S=]/[Ni2+] of 1.5/1 show that excess sulfide produces nickel–sulfide complexes (stable at pH values above 7) (Uther et al. 1999; Karbanee et al. 2008). For the 1.5[S=]/1[Ni2+] ratio, the pH increases rapidly in the industrial effluent, after 2 minutes of reaction, from 5 to 7.89. This results probably from the production of H2S gas in large quantities because of the added amount of S= ions and its evacuation by stirring. Thereafter, pH stabilizes and reaches values identical to those obtained in tests carried out with a ratio of 1[S=]/1[Ni2+].
Test 3: influence of sulfide concentration on the removal of Ni2+ at pH = 5
Figure 3 shows the effect of sulfide concentration on the final concentration of nickel and the final pH of the industrial effluent.
The residual Ni2+ ions concentration decreases to a minimum corresponding to S= concentration of 33.33 mg/L (elimination yield of 84%) and a final pH of 5.6, then it increases, denoting a redissolution because of the increase of sulfide ions.
Examination of conductivity results shows:
first, a decrease from 2.5 mS/cm to 0.7 mS/cm when the concentration of S= increases (34 mg/L), which could be explained by the rise of concentration of Ni2+ by precipitation;
then, an increase with the sulfide dose to reach 2.7 mS/cm approximately owing to the re-ionization, in the form of complexes [Nix(HS−)y] (y−x) −;
a minimum value of 0.7 mS/cm coinciding with that of the residual concentration of Ni2+.
Characterization of nickel sulfide sludge
The structure of the sludge issuing from the industrial effluent at optimal conditions of precipitation was studied using X-ray diffraction technique. The sludge has been heat treated at 190 °C, 350 °C and 600 °C. At 190 °C, the XRD diagram shown in Figure 4(a) illustrates an amorphous structure or poorly crystallized precipitate of nickel monosulfide. It should be noted that the number of diffraction lines is 7, with low intensities. It seems that the precipitate corresponds to the millerite (NiS). This could be supported by the works of some authors who have reported that the precipitate of nickel sulfide is an amorphous material (Dusastre et al. 1997; Jeong & Manthiram 2001). Similarly, another study reports identical results for a mixture of sulfide precipitates and confirms that they consisted of an amorphous NiS (Donges 1942).
Recent studies suggest that potential forms of nickel sulfide formed at low temperatures are β-NiS (millerite), NiS2 (vaesite), Ni3S4 (polydymite) and Ni3S2 (heazlewoodite) (Bijmans et al. 2009; Sampaio et al. 2010; Reis et al. 2013). Authors suggest that the majority of these precipitates have a structure of millerite which is transformed into Ni3S4 (polydymite), accompanied by the production of the form Ni3S2 (heazlewoodite).
Thermic treatment at 350 °C affects the crystalline structure of the nickel sulfide, as observed in Figure 4(b). The X-ray spectrum shows the same lines appearing in that of the precipitate treated at 190 °C, but with more intense peaks. Diffraction peaks in Figure 4 were identified using the standard XRD of NiS from the JCPDS database (JCPDS – No.75-0613).
The observed positions and planes of the formed product are in good agreement with the standard values of JCPDS database (No. 47-1049), indicated for NiO.
At high temperature, metallic elements have probably similar affinities for sulfide and have much higher affinities for oxygen, which explains the oxidation of sulfides in the presence of oxygen.
In a work on selective oxidation and sulfation of nickel concentrate, as a new technique for nickel production, it has been argued that in a roasting temperature ranging from 500 °C to 800 °C, the resulting product would be NiO (Yu et al. 2014). A recent study on the manufacture of nickel sulfides reports that in the presence of oxygen, the atoms of sulfides are rapidly replaced by the oxygen atoms to form nickel oxide (Balayeva et al. 2016).
Scanning electron microscopy
The characterization of the morphology of the thermally treated millerite at 190 °C, 350 °C and 600 °C was carried out using the SEM technique with similar dimensions.
The precipitate treated at 190 °C is poorly crystallized (Figure 5(a)). Otherwise, when the temperature is 350 °C, it appears as luminous aggregates with high polydispersity (Figure 5(b)). The grains or nanocrystals are characterized by a porous structure and are formed by agglomeration of small particles having irregular shapes and sizes, which gives the product a large exchange surface and can be used in electrocatalytic activities (Lin 2015). SEM photomicrographs support the morphological aspects of the millerite revealed in the previous sections.
For nickel sludge treated at 600 °C, micrographs in Figure 5(c) show that this compound has a structure formed from an assembly of aggregates similar to that observed in the literature for nickel oxide in its thin form.
The identification of chemical structure and functional groups and bonds present in composants are well explored by infrared spectroscopy (Alamgholiloo et al. 2020).
The wide peak at 3,465.5 cm−1 corresponds to the valence vibration of the O-H bond of water. The band which appears around 1,600 cm−1 is attributed to the angular deformation of H-O-H. As for the band located at 1,140 cm−1, it characterizes the valence vibrations of the SO42−. The least intense peak located at around 600 cm−1 reveals the presence of the Ni-S bond (Figure 6).
In Figure 7, we observe characteristic bands of the vibration of the hydration water located at 3,480 cm−1 and 1,600 cm−1. The band that appears around 1,150 cm−1 is attributed to the valence vibration of the SO42− molecule. As for the band located around 1,600 cm−1, it is attributed to the angular deformation of H-O-H. The less intense peak around 600 cm−1 reveals the presence of an Ni-S bond. The band located around 500 cm−1 is probably due to the valence vibration of the Ni-O bond.
On the TGA curve, we detected three transformations (Figure 8):
The first and second mass loss, between 43 °C and 426 °C, are due to water departure. At 426 °C the sludge dehydrates completely.
The DTA diagram highlights the following observations:
A first exothermic peak around 170 °C probably corresponds to an allotropic transformation, transition from millerite apha (NiSα) to millerite beta (NiSβ).
A second exothermic peak with low amplitude, located at 411 °C, is probably due to the crystallization of NiS.
Nickel sulfide is known as a high performance catalyst. This material has a wide industrial application in hydrogenation reactions, in petrochemical and refining processes for the conversion of diolefins into mono-olefins to prevent the formation of gums and resins (Olivas et al. 1998). It has been proven as a bifunctional cathode electrocatalyst material for non-aqueous lithium-air batteries (LABs). NiS catalysts give a much better performance in terms of the specific capacity, discharge/charge overpotentials, rate dischargeability and cycling performance (Ma et al. 2015). NiO appears in the manufacturing of several products such as adhesives, pigments for ceramics and glasses, and antiferromagnetic layers (Danjumma & Yakubu 2019).
The conducted research on the sulfide precipitation treatment of nickel-rich water has provided comprehension of the phenomenon occurring during the process. The results illustrated that the nickel removal efﬁciency depends on several parameters, such as the pH of the solution, the molar ratio [S=]/[Ni2+] and the S= dosage. The removal yield achieved is approaching 90% for 25 minutes of treatment in the industrial effluent. The initial pH of 5 gave the best yields of nickel removal in the synthetic and industrial solutions. The effect of the sulfide dose on the removal of nickel from the industrial effluent was evaluated. XRD, IR, SEM, and TGA/DTA analysis reveal that the majority of products formed are NiS and NiO.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.