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Studies on manganese-nodule leached residues
Journal of Colloid and Interface Science 277 (2004) 48–54 www.elsevier.com/locate/jcis
Studies on manganese-nodule leached residues 1. Physicochemical characterization and its adsorption behavior toward Ni2+ in aqueous system K.M. Parida ∗ , S. Mallick, B.K. Mohapatra, Vibhuti N. Misra Regional Research Laboratory, Bhubaneswar 751013, India Received 4 March 2004; accepted 21 April 2004 Available online 11 June 2004
Abstract Physicochemical characterization of manganese-nodule leached residues was carried out by chemical analyses, XRD, TG-DTA, surface area measurement, and FTIR techniques. The material is very fine-grained (<75 µm), is cryptocrystalline to amorphous in nature, and contains mainly of δ-MnO2 , quartz (α-SiO2 ), and zeolite/feldspar minerals. Physically adsorbed sulfates in the leached residue are removed by repeated water washing and the washed sample shows an appreciable increase in surface area. This is indicated by the absence of 1387 and 1099 cm−1 peaks in the IR spectrum of the washed sample. The adsorption behavior of the washed sample toward Ni2+ was recorded as a function of time, pH, temperature, and concentrations of adsorbent and adsorbate. 2004 Published by Elsevier Inc. Keywords: Manganese-nodule leached residues; δ-MnO2 ; Adsorption behavior; Quartz
1. Introduction Polymetallic manganese nodules are one a potential resources for metals such as nickel, copper, cobalt, zinc, and manganese, for which these are of great interest to mineralogists, sedimentologists, and metallurgists [1–4]. Over the past few decades many researchers from academic and R&D organizations have developed processes for extraction of metal values from terrestrial manganese ores and marine manganese nodules. Recently, a process has been developed on a pilot scale jointly by the Regional Research Laboratory, Bhubaneswar, the National Metallurgical Laboratory, Jamshedpur, and Hindustan Zinc Limited, Udaipur to recover nickel, copper, cobalt, and zinc from Indian Ocean nodules through ammonia–SO2 leaching, solvent extraction, and electrowinning techniques. Once the metal values are extracted, over 97% of the material remains as waste. Even if one goes for manganese recovery, the volume of waste may be reduced but remains * Corresponding author. Fax: +91-674-2587637.
E-mail addresses: [email protected], [email protected] (K.M. Parida). 0021-9797/$ – see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.jcis.2004.04.057
over 70%. These enormous wastes create both disposal and environmental problems. Hence, one has to look into the optimum utilization of this waste. Though a volume of literature [2–5] is available on metal extraction and adsorption behavior of Indian Ocean manganese nodules, literature on this waste is scarcely seen. It has been observed by Sahoo et al. [6] that manganese-nodule leached residues possess a high surface area and can act as an effective adsorbent for removal of nickel from synthetic solutions and industrial wastewater. The present study is a modest attempt to characterize these leached residue and study their adsorption behavior. In the first attempt, adsorption of Ni+2 onto leached residue was taken up with a view to establishing its adsorption characteristics in marine manganese nodules.
2. Experimental 2.1. Sample preparation The manganese-nodule leached residue (MNLR) sample was obtained during pilot plant testing of NH3 –SO2 leaching of Indian Ocean manganese nodules at the Regional Research Laboratory, Bhubaneswar, India. The rep-
K.M. Parida et al. / Journal of Colloid and Interface Science 277 (2004) 48–54
49
resentative sample used in the present study was prepared from ∼5 kg of material by a quartering and coning method. Then the residue was processed for removal of (NH4 )2 SO4 impurities. In a typical run about 50 g of this sample in 500 ml of water was stirred at room temperature for 4 h, filtered, washed thoroughly in distilled water, air-dried for 2 days, and kept in a dessicator for further use. Water-washed manganese-nodule leached residues are henceforth abbreviated as WMNLR. 2.2. Chemical analysis For chemical analysis of various constituents, about 1 g of the residue was digested in 20 ml of aqua regia (15 ml conc. HCl + 5 ml conc. HNO3 ) over a hot plate and heated till the volume became reduced to 5 ml. To this, 10 ml of H2 SO4 was added and the contents heated to white fumes (2–3 h). It was then diluted with distilled water and filtered through Whatman No. 542 filter paper. The residue thus obtained was used for the estimation of silica by the HF method. The residue left after HF treatment was fused with 2–3 g of KHSO4 , dissolved in 50 ml of 1:3 HCl, and added to the original filtrate and the volume was made up to 250 ml. Suitable aliquots were taken for estimation of Mn and Fe by a volumetric titration method and Al by a gravimetric method [7], while Co, Ni, Cu, and Zn were analyzed by atomic absorption spectrometry (Varian Model AA-1475) after suitable dilution. For determination of sulfate the sample digestion was done without the addition of H2 SO4 and the sample was analyzed gravimetrically by BaSO4 . Reagent grade chemicals and distilled water were used in all the experiments. 2.3. Surface and textural characterization The specific surface area and pore diameter of the leached residue samples (MNLR and WMNLR) degassed at 110 ◦ C in vacuum (1 × 10−4 Torr) were determined by nitrogen adsorption/desorption isotherms at liquid nitrogen temperature (−196 ◦ C) using a Quantasorb (Quantachrom, USA). The X-ray diffraction patterns of air-dried samples were recorded on a Phillips Semiautomatic X-ray diffractometer with autodivergent slit and graphite monochromator using CuKα radiation, operated at 40 kV and 20 mA. FTIR spectra were taken using a JASCO FTIR-5300 in a KBr matrix in the range 400–4000 cm−1 . TG-DTA of the samples (∼20 mg) was carried out in static air using a Shimadzu DT 40 thermal analyzer in the temperature range 30–1000 ◦ C at a heating rate of 20 ◦ C min−1 . Micrographs showing X-ray image mapping of different elements for unwashed and washed samples were taken using a Japanese Model (JXA-8100) EPMA. 2.4. Determination of point of zero charge (pHpzc) An acid–base titration was carried out using a 0.2-g water-washed manganese-nodule leached sample suspended
Fig. 1. Surface charge of water-washed manganese-nodule leached residues as a function of pH in the presence of KNO3 .
in 50 ml of 0.1, 0.01, and 0.001 mol dm−3 KNO3 supporting electrolyte. The pH of the stirred suspension was measured after addition of aliquots of 0.1 mol dm−3 HNO3 or NaOH from a microburette. Blank titrations of supporting electrolyte were also carried out under identical conditions. The details of the procedure are the same as those described elsewhere [8]. The value of pHpzc obtained from the plots of adsorption density of H+ or OH− ions versus pH was found to be ∼4.5. This is not unusual considering the complex nature and composition of manganese nodules. The observed pzc value was the average of the pzc values reported for MnO2 (pzc ≈ 2) [9], FeOOH (pzc ≈ 7.5) [10], Al2 O3 (pzc ≈ 8.1) [11,12], and SiO2 (pzc ≈ 2.0) [13,14] (the main composition of manganese nodules), as expected for the pzc value of mixed oxides [15–18]. The surface charge, σ 0 (C/g), was calculated using the equation σ 0 = F (H+ –OH− ) [19], where the symbols have their usual meanings. The plots of surface charge against pH are presented in Fig. 1. 2.5. Adsorption experiment Adsorption experiments were carried out in 100-ml stoppered conical flasks by mixing appropriate amounts of nickel nitrate solution and about 100 mg of the water-leached residue (2 g/L). Predetermined amounts of acetic acid and sodium acetate were added to maintain the pH of the solution. The final volume was invariably kept to 50 ml. The ionic strength was maintained at 0.1 mol dm−3 using sodium nitrate. A preliminary experiment revealed that about 3 h
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Table 1 Particle size analysis of residue Size in micrometers
wt%
>250 >150 >75 >53 <53
22.48 7.61 22.07 44.07 3.73
Table 2 Chemical composition of unwashed and washed leached residues
Major elements (%) Mn+3 Mn+2 Mn+4 Fe Al2 O3 SiO2 SO2− 4 H2 O+ LOI Trace elements (ppm) Cu Co Ni Zn
Fig. 2. Distribution of pore sizes as a function of pore diameter.
is required for the Ni2+ ion to reach the equilibrium concentration. The flasks were shaken mechanically for 4 h at 30.0 ± 0.2 ◦ C, the solution was filtered through Whatman 42 filter paper, and the concentration of the metal ion in the filtrate was determined by AAS. The pH measurements were made by an Elico digital pH meter (Model LI 120) using a combined glass electrode (Model CL 51). The pH meter was standardized with NBS buffers.
3. Results and discussion 3.1. Chemical characteristics The air-dried residue was sieved into different size fractions. The size distribution (dry sieving) is presented in Table 1 and it is seen that a major portion of the solid is in the <75-µm fraction. Physicochemical characterization and adsorption studies were carried out taking this fraction of sample. The specific surface areas of unwashed and waterwashed samples were found to be 195 and 269 m2 /g, respectively. The increase in surface area in the washed sample may be due to removal of entrapped and surface-adsorbed sulfate. The distribution of pore size as a function of pore diameter, Fig. 2, shows that pore diameter values range from 35 to 143 Å and the samples contain mainly mesopores, along with micropores. The chemical analysis results of MNLR and WMNLR samples are shown in Table 2. The major constituents of the residue are Mn, Fe, SiO2 , and Al2 O3 . It may be seen from
Unwashed leached residues
Washed leached residues
5.68 0.65 15.99 11.61 5.1 16.80 11.23 8.86 6.41
6.34 0.70 18.40 13.40 7.28 19.64 0.56 9.45 7.86
2.97 0.46 2.56 0.58
4.07 0.69 3.87 0.76
the table that after recovery of Cu, Ni, and Co from manganese nodules only insignificant amount of such metals are retained in the leached residues. After washing of the leached residue sample it was observed that nearly 97% of sulfate could be removed simply by leaching with water. So it is presumed that most of the sulfates are present in entrapped (physically absorbed) form rather than in bound form. 3.2. Mineralogical characteristics 3.2.1. XRD analysis The XRD patterns of the 110 ◦ C-dried washed and unwashed leached residue samples are presented in Fig. 3. The illustration shows that both the samples are cryptocrystalline to amorphous in nature, with limited sharp and diffused broad peaks. The peaks observed at 2.22 and 2.46 Å are due to δ-MnO2 and the peaks at 3.34 and 3.21 Å indicate the presence of quartz (α-SiO2 ) and zeolite/feldspar, respectively. In addition to these above phases, occurrence of minor kaolinite (7.20 Å) is observed in the original residue sample (unwashed). The absence of this phase in the washed sample reveals that the finely dispersed kaolinite particles get removed during washing. 3.2.2. DTA and TG analysis The DTA pattern of the manganese-nodule leached residue sample shows three endothermic peaks (Fig. 4a). The peaks at 70 and 100 ◦ C in the DTA curve correspond to a weight loss of physisorbed water and the peak at 135 ◦ C may be due to the decomposition of (NH4 )2 SO4 . The exothermic peak at 380 ◦ C could be attributed to a sulfate phase. The minor shoulder at 780 ◦ C is attributed to structural water loss from a hydrous iron oxide/manganese phase. The original leached residue sample undergoes a total loss of ∼25% due to physisorbed, chemisorbed, and decomposed of oxide phases, etc., while in the case of the water-washed sample,
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Fig. 3. XRD pattern of leached residues before and after washing (a—manganese-nodule leached residue; b—water-washed manganese-nodule leached residue).
Fig. 5. IR spectra of the manganese-nodule leached residue (a) and the washed sample (b).
stretching vibration of the OH group. The peak observed at 2363 cm−1 is probably due to MnOOH. The peak appeared at 1635 cm−1 is due to the bending mode of vibration of water (H–O–H). A very sharp absorption band at 1387 cm−1 could be due to the N–H stretching vibration in NH+ 4 ion. The peak appeared at 1099 cm−1 may be attributed to the −1 to the Si–O or SO2− 4 of (NH4 )2 SO4 and that at 1026 cm Si–O–Al of silicate minerals. But in the water-washed sample (Fig. 5b) the peaks at 1387 and 1099 cm−1 do not appear, with all other peaks being retained.
Fig. 4. TG-DTA pattern of the manganese nodules and washed samples (a—DTA pattern of manganese-nodule leached residue; b—DTA pattern of water-washed leached residue; ai—TG pattern of leached residue; bi—TG pattern of water-washed leached residue).
only one endotherm is observed at 70 ◦ C and the total loss is ∼30% (Fig. 4b). 3.2.3. Infrared spectroscopy The IR spectra as presented in Fig. 5a show strong absorption bands at 3422 cm−1 , which is due to the O–H
3.2.4. Electron probe microanalyses The X-ray image maps of different elements under the electron microprobe indicates that the residue particles are very fine in size (5 to 50 µm), semicircular to circular or irregular/elongated in shape, and enriched in both Mn and Fe phases. From the illustrations (Figs. 6 and 7), the presence of four silicate phases has been brought out (quartz, goethite, kaolinite, and Fe–Al silicate). The original unwashed sample exhibits the presence of sulfate mostly attached to ferromanganese phases, while the SEM micrographs of the washed sample (Fig. 7) show the presence of sulfate in traces, indicating considerable removal of the sulfate phase. Other micrographs in both Figs. 6 and 7 show more or less similar patterns of element distribution. The image maps of Ni, Zn, and Cu in both Figs. 6 and 7 indicate the near absence of these elements.
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Fig. 6. Scanning electron micrograph of original leached-residue sample: (A) X-ray image map of Fe; (B) secondary electron image of morphological view of residue sample; (C) X-ray image map of Si; (D) X-ray image map of Al; (E) X-ray image map of Mn; (F) X-ray image map of Ni; (G) X-ray image map of Zn; (H) X-ray image map of S; (I) X-ray image map of Cu.
Fig. 8. Effect of time on adsorption of nickel MNLR.
Fig. 9. Effect of pH on adsorption of nickel WMNLR. Fig. 7. Scanning electron micrograph of washed leached-residue sample: (A) X-ray image map of Mn; (B) secondary electron image of morphological view of residue sample; (C) X-ray image map of Fe; (D) X-ray image map of Si; (E) X-ray image map of S; (F) X-ray image map of Ni; (G) X-ray image map of Zn; (H) X-ray image map of Al; (I) X-ray image map of Cu.
3.3. Absorption of nickel on water-washed manganese-nodule leached residues 3.3.1. Effect of time The equilibrium period for nickel adsorption was determined, keeping the initial pH of the solution at 4.5. The
results are presented in Fig. 8, which shows that equilibrium is reached in ∼4 h. 3.3.2. Effect of pH The effect of pH on adsorption of nickel is shown in Fig. 9. It can be observed that the extent of adsorption onto the residue increases with increased pH of the solution up to pH 5 and thereafter no significant improvement takes place. It is also seen that there is no adsorption of nickel at pH 1. Beyond pH 6 hydrolysis of nickel is observed.
K.M. Parida et al. / Journal of Colloid and Interface Science 277 (2004) 48–54
The increase in adsorption of metal ions with a rise in pH may be partly attributed to the formation of more MOH+ species. A simple calculation using the values of Kh and pH showed that the concentration of NiOH+ species formed at pH 6 and Ni+2 concentration (0.02 mol dm−3 ) is 7.9 × 10−7 mol dm−3 . Thus the formation of NiOH+ , species leading to higher adsorption, may not be a significant factor. The other important factor that might contribute to the higher adsorption of metal ions with increased pH is the pzc value of the manganese nodule (pzc = 4.5). At any pH below the pzc, the surface of the metal oxide is positively charged, and above the pzc, it is negatively charged. Thus, a lowering of the positive surface charge with an increase in the pH makes it easier for the cation to closely approach the surface, which in term results in higher adsorption of nickel ions. The nickel adsorption below pHpzc may be due to the combined effects of both chemical and electrostatic interaction between the manganese-nodule leached-residue oxide surface and the nickel ions.
53
Fig. 10. Effect of temperature on adsorption of nickel.
3.3.3. Effect of temperature The adsorption of nickel onto manganese-nodule leached residues is illustrated in Fig. 10 as a function of temperature. The percentage of adsorption of nickel onto WMNLR increases slightly with increased temperature from 30 to 50 ◦ C, which may be explained as follows: (1) if the sorption of metal ions is endothermic, which is the case for many divalent metal cations [20,21], then adsorption is favored at higher temperatures; (2) at a given pH the surface charge of MnO2 /goethite is decreased and the extent of hydrolysis of the metal cation is increased [22–24]. Both these factors will reduce the electrostatic repulsion between the surface and the adsorbing species, leading to higher adsorption. 3.3.4. Effect of adsorbent and adsorbate concentration The percentages of nickel adsorption with varying amounts of leached residues and nickel concentrations are illustrated in Figs. 11 and 12, respectively. As expected, the amount of nickel adsorption increases with increased adsorbent (nodule) concentration, whereas it decreases with increased adsorbate concentration, indicating that the adsorption is dependent upon the availability of the binding sites. To determine the adsorption capacities of the samples the experimental data points (Fig. 12) were fitted to the Langmuir equation [25], C/X = 1/bXm + C/Xm , where X indicates the amount of nickel adsorbed per unit weight of the sample, C represents the nickel concentration in equilibrium solution, b denotes a constant related to the energy of adsorption, and Xm is the adsorption capacity of the sample. Fig. 13 depicts the Langmuir plot (C/X vs C) for the WMNLR sample. The correlation coefficients in the case are higher than 0.95.
Fig. 11. Effect of adsorbent concentration on adsorption of nickel.
4. Conclusions 1. Polymetallic manganese-nodule leached residues are very fine-grained (mainly mesopores along with micropores), are cryptocrystalline to amorphous in na-
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ture, and constitute δ-MnO2 , quartz, kaolinite, and zeolite/feldspar minerals. 2. The entrapped sulfate in this sample can be removed by sample washing with water. 3. There is an appreciable increase in surface area after removal of sulfate from the leached residue. 4. The water-washed manganese-nodule leached residues can be used for effective removal of Ni2+ in aqueous solution at 4.0 pH.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Fig. 12. Effect of concentration on adsorption of nickel on WMNLR. [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 13. Langmuir plot of nickel adsorption on manganese-nodule leachedresidue.
[23] [24] [25]
D.W. Fuerstenau, K.N. Han, Miner. Process. Technol. Rev. 1 (1983) 1. A.K. Ghosh, R. Mukhopadhaya, Ind. J. Marine Sci. 20 (1991) 17. P. Rao, Marine Geol. 74 (1987) 151, and references therein. H. Momoi, M. Nakamoto, K. Kamata, Miner. Geol. 42 (1992) 155. K.M. Parida, P.K. Satpathy, J. Colloid Interface Sci. 181 (1996) 456– 462. R.N. Sahoo, S.C. Das, B.R. Reddy, P.C. Rath, R.P. Das, Hydrometallurgy 62 (2001) 185–192. N.H. Furman, Stand. Methods Chem. Anal. 1 (1962) 951. K.M. Parida, Adsorpt. Sci. Technol. 5 (1988) 257. S.B. Kanugo, K.M. Parida, J. Colloid Interface Sci. 98 (1984) 245. D.P. Rodda, B.B. Johnson, J.D. Wells, J. Colloid Interface Sci. 161 (1993) 57. M.M. Ghosh, C.D. Cox, J.R. Yuan-pan, Environ. Prog. 13 (1994) 79. S.S. Noh, J.A. Schwarz, J. Colloid Interface Sci. 130 (1989) 157. G. Pignet, G. Martin, H. Bougault, J. Fr. Hydrol. 10 (1979) 179. R.O. James, Healy, J. Colloid Interface Sci. 40 (1972) 42. G.A. Parks, Am. Chem. Soc. Monogr. 67 (1967) 121. I. Fox, M.A. Malati, J. Chem. Technol. Biotechnol. l57 (1993) 93. C.P. Huang, E.A. Rhoads, J. Colloid Interface Sci. 131 (1982) 289. N. Peinemann, A.K. Helmy, Appl. Clay Sci. 8 (1994) 389. S. Ardizzone, L. Formaro, J.F. Laurent, Electrochim. Acta 10 (1965) 947. P.H. Tewari, A.B. Campbell, W. Lee, Can. J. Chem. 50 (1972) 1642. J.F. Hodgson, H.R. Geering, M. Fellows, Soil Sci. Soc. Am. Proc. 28 (1964) 39. D.D. Perin, B. Dempsey, in: Buffers for pH and Metal Ion Control, Chapman & Hall, London/New York, 1974, p. 104. J.P. Gabano, P. Etienne, J.F. Laurent, Electrochim. Acta 10 (1965) 947. A. Kozawa, J. Electrochem. Soc. 106 (1959) 552. S.M. Chaudhari, Tech. thesis, Indian Institute of Technology, Kanpur, 1985.
Studies on manganese-nodule leached residues 1. Physicochemical characterization and its adsorption behavior toward Ni2+ in aqueous system K.M. Parida ∗ , S. Mallick, B.K. Mohapatra, Vibhuti N. Misra Regional Research Laboratory, Bhubaneswar 751013, India Received 4 March 2004; accepted 21 April 2004 Available online 11 June 2004
Abstract Physicochemical characterization of manganese-nodule leached residues was carried out by chemical analyses, XRD, TG-DTA, surface area measurement, and FTIR techniques. The material is very fine-grained (<75 µm), is cryptocrystalline to amorphous in nature, and contains mainly of δ-MnO2 , quartz (α-SiO2 ), and zeolite/feldspar minerals. Physically adsorbed sulfates in the leached residue are removed by repeated water washing and the washed sample shows an appreciable increase in surface area. This is indicated by the absence of 1387 and 1099 cm−1 peaks in the IR spectrum of the washed sample. The adsorption behavior of the washed sample toward Ni2+ was recorded as a function of time, pH, temperature, and concentrations of adsorbent and adsorbate. 2004 Published by Elsevier Inc. Keywords: Manganese-nodule leached residues; δ-MnO2 ; Adsorption behavior; Quartz
1. Introduction Polymetallic manganese nodules are one a potential resources for metals such as nickel, copper, cobalt, zinc, and manganese, for which these are of great interest to mineralogists, sedimentologists, and metallurgists [1–4]. Over the past few decades many researchers from academic and R&D organizations have developed processes for extraction of metal values from terrestrial manganese ores and marine manganese nodules. Recently, a process has been developed on a pilot scale jointly by the Regional Research Laboratory, Bhubaneswar, the National Metallurgical Laboratory, Jamshedpur, and Hindustan Zinc Limited, Udaipur to recover nickel, copper, cobalt, and zinc from Indian Ocean nodules through ammonia–SO2 leaching, solvent extraction, and electrowinning techniques. Once the metal values are extracted, over 97% of the material remains as waste. Even if one goes for manganese recovery, the volume of waste may be reduced but remains * Corresponding author. Fax: +91-674-2587637.
E-mail addresses: [email protected], [email protected] (K.M. Parida). 0021-9797/$ – see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.jcis.2004.04.057
over 70%. These enormous wastes create both disposal and environmental problems. Hence, one has to look into the optimum utilization of this waste. Though a volume of literature [2–5] is available on metal extraction and adsorption behavior of Indian Ocean manganese nodules, literature on this waste is scarcely seen. It has been observed by Sahoo et al. [6] that manganese-nodule leached residues possess a high surface area and can act as an effective adsorbent for removal of nickel from synthetic solutions and industrial wastewater. The present study is a modest attempt to characterize these leached residue and study their adsorption behavior. In the first attempt, adsorption of Ni+2 onto leached residue was taken up with a view to establishing its adsorption characteristics in marine manganese nodules.
2. Experimental 2.1. Sample preparation The manganese-nodule leached residue (MNLR) sample was obtained during pilot plant testing of NH3 –SO2 leaching of Indian Ocean manganese nodules at the Regional Research Laboratory, Bhubaneswar, India. The rep-
K.M. Parida et al. / Journal of Colloid and Interface Science 277 (2004) 48–54
49
resentative sample used in the present study was prepared from ∼5 kg of material by a quartering and coning method. Then the residue was processed for removal of (NH4 )2 SO4 impurities. In a typical run about 50 g of this sample in 500 ml of water was stirred at room temperature for 4 h, filtered, washed thoroughly in distilled water, air-dried for 2 days, and kept in a dessicator for further use. Water-washed manganese-nodule leached residues are henceforth abbreviated as WMNLR. 2.2. Chemical analysis For chemical analysis of various constituents, about 1 g of the residue was digested in 20 ml of aqua regia (15 ml conc. HCl + 5 ml conc. HNO3 ) over a hot plate and heated till the volume became reduced to 5 ml. To this, 10 ml of H2 SO4 was added and the contents heated to white fumes (2–3 h). It was then diluted with distilled water and filtered through Whatman No. 542 filter paper. The residue thus obtained was used for the estimation of silica by the HF method. The residue left after HF treatment was fused with 2–3 g of KHSO4 , dissolved in 50 ml of 1:3 HCl, and added to the original filtrate and the volume was made up to 250 ml. Suitable aliquots were taken for estimation of Mn and Fe by a volumetric titration method and Al by a gravimetric method [7], while Co, Ni, Cu, and Zn were analyzed by atomic absorption spectrometry (Varian Model AA-1475) after suitable dilution. For determination of sulfate the sample digestion was done without the addition of H2 SO4 and the sample was analyzed gravimetrically by BaSO4 . Reagent grade chemicals and distilled water were used in all the experiments. 2.3. Surface and textural characterization The specific surface area and pore diameter of the leached residue samples (MNLR and WMNLR) degassed at 110 ◦ C in vacuum (1 × 10−4 Torr) were determined by nitrogen adsorption/desorption isotherms at liquid nitrogen temperature (−196 ◦ C) using a Quantasorb (Quantachrom, USA). The X-ray diffraction patterns of air-dried samples were recorded on a Phillips Semiautomatic X-ray diffractometer with autodivergent slit and graphite monochromator using CuKα radiation, operated at 40 kV and 20 mA. FTIR spectra were taken using a JASCO FTIR-5300 in a KBr matrix in the range 400–4000 cm−1 . TG-DTA of the samples (∼20 mg) was carried out in static air using a Shimadzu DT 40 thermal analyzer in the temperature range 30–1000 ◦ C at a heating rate of 20 ◦ C min−1 . Micrographs showing X-ray image mapping of different elements for unwashed and washed samples were taken using a Japanese Model (JXA-8100) EPMA. 2.4. Determination of point of zero charge (pHpzc) An acid–base titration was carried out using a 0.2-g water-washed manganese-nodule leached sample suspended
Fig. 1. Surface charge of water-washed manganese-nodule leached residues as a function of pH in the presence of KNO3 .
in 50 ml of 0.1, 0.01, and 0.001 mol dm−3 KNO3 supporting electrolyte. The pH of the stirred suspension was measured after addition of aliquots of 0.1 mol dm−3 HNO3 or NaOH from a microburette. Blank titrations of supporting electrolyte were also carried out under identical conditions. The details of the procedure are the same as those described elsewhere [8]. The value of pHpzc obtained from the plots of adsorption density of H+ or OH− ions versus pH was found to be ∼4.5. This is not unusual considering the complex nature and composition of manganese nodules. The observed pzc value was the average of the pzc values reported for MnO2 (pzc ≈ 2) [9], FeOOH (pzc ≈ 7.5) [10], Al2 O3 (pzc ≈ 8.1) [11,12], and SiO2 (pzc ≈ 2.0) [13,14] (the main composition of manganese nodules), as expected for the pzc value of mixed oxides [15–18]. The surface charge, σ 0 (C/g), was calculated using the equation σ 0 = F (H+ –OH− ) [19], where the symbols have their usual meanings. The plots of surface charge against pH are presented in Fig. 1. 2.5. Adsorption experiment Adsorption experiments were carried out in 100-ml stoppered conical flasks by mixing appropriate amounts of nickel nitrate solution and about 100 mg of the water-leached residue (2 g/L). Predetermined amounts of acetic acid and sodium acetate were added to maintain the pH of the solution. The final volume was invariably kept to 50 ml. The ionic strength was maintained at 0.1 mol dm−3 using sodium nitrate. A preliminary experiment revealed that about 3 h
50
K.M. Parida et al. / Journal of Colloid and Interface Science 277 (2004) 48–54
Table 1 Particle size analysis of residue Size in micrometers
wt%
>250 >150 >75 >53 <53
22.48 7.61 22.07 44.07 3.73
Table 2 Chemical composition of unwashed and washed leached residues
Major elements (%) Mn+3 Mn+2 Mn+4 Fe Al2 O3 SiO2 SO2− 4 H2 O+ LOI Trace elements (ppm) Cu Co Ni Zn
Fig. 2. Distribution of pore sizes as a function of pore diameter.
is required for the Ni2+ ion to reach the equilibrium concentration. The flasks were shaken mechanically for 4 h at 30.0 ± 0.2 ◦ C, the solution was filtered through Whatman 42 filter paper, and the concentration of the metal ion in the filtrate was determined by AAS. The pH measurements were made by an Elico digital pH meter (Model LI 120) using a combined glass electrode (Model CL 51). The pH meter was standardized with NBS buffers.
3. Results and discussion 3.1. Chemical characteristics The air-dried residue was sieved into different size fractions. The size distribution (dry sieving) is presented in Table 1 and it is seen that a major portion of the solid is in the <75-µm fraction. Physicochemical characterization and adsorption studies were carried out taking this fraction of sample. The specific surface areas of unwashed and waterwashed samples were found to be 195 and 269 m2 /g, respectively. The increase in surface area in the washed sample may be due to removal of entrapped and surface-adsorbed sulfate. The distribution of pore size as a function of pore diameter, Fig. 2, shows that pore diameter values range from 35 to 143 Å and the samples contain mainly mesopores, along with micropores. The chemical analysis results of MNLR and WMNLR samples are shown in Table 2. The major constituents of the residue are Mn, Fe, SiO2 , and Al2 O3 . It may be seen from
Unwashed leached residues
Washed leached residues
5.68 0.65 15.99 11.61 5.1 16.80 11.23 8.86 6.41
6.34 0.70 18.40 13.40 7.28 19.64 0.56 9.45 7.86
2.97 0.46 2.56 0.58
4.07 0.69 3.87 0.76
the table that after recovery of Cu, Ni, and Co from manganese nodules only insignificant amount of such metals are retained in the leached residues. After washing of the leached residue sample it was observed that nearly 97% of sulfate could be removed simply by leaching with water. So it is presumed that most of the sulfates are present in entrapped (physically absorbed) form rather than in bound form. 3.2. Mineralogical characteristics 3.2.1. XRD analysis The XRD patterns of the 110 ◦ C-dried washed and unwashed leached residue samples are presented in Fig. 3. The illustration shows that both the samples are cryptocrystalline to amorphous in nature, with limited sharp and diffused broad peaks. The peaks observed at 2.22 and 2.46 Å are due to δ-MnO2 and the peaks at 3.34 and 3.21 Å indicate the presence of quartz (α-SiO2 ) and zeolite/feldspar, respectively. In addition to these above phases, occurrence of minor kaolinite (7.20 Å) is observed in the original residue sample (unwashed). The absence of this phase in the washed sample reveals that the finely dispersed kaolinite particles get removed during washing. 3.2.2. DTA and TG analysis The DTA pattern of the manganese-nodule leached residue sample shows three endothermic peaks (Fig. 4a). The peaks at 70 and 100 ◦ C in the DTA curve correspond to a weight loss of physisorbed water and the peak at 135 ◦ C may be due to the decomposition of (NH4 )2 SO4 . The exothermic peak at 380 ◦ C could be attributed to a sulfate phase. The minor shoulder at 780 ◦ C is attributed to structural water loss from a hydrous iron oxide/manganese phase. The original leached residue sample undergoes a total loss of ∼25% due to physisorbed, chemisorbed, and decomposed of oxide phases, etc., while in the case of the water-washed sample,
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Fig. 3. XRD pattern of leached residues before and after washing (a—manganese-nodule leached residue; b—water-washed manganese-nodule leached residue).
Fig. 5. IR spectra of the manganese-nodule leached residue (a) and the washed sample (b).
stretching vibration of the OH group. The peak observed at 2363 cm−1 is probably due to MnOOH. The peak appeared at 1635 cm−1 is due to the bending mode of vibration of water (H–O–H). A very sharp absorption band at 1387 cm−1 could be due to the N–H stretching vibration in NH+ 4 ion. The peak appeared at 1099 cm−1 may be attributed to the −1 to the Si–O or SO2− 4 of (NH4 )2 SO4 and that at 1026 cm Si–O–Al of silicate minerals. But in the water-washed sample (Fig. 5b) the peaks at 1387 and 1099 cm−1 do not appear, with all other peaks being retained.
Fig. 4. TG-DTA pattern of the manganese nodules and washed samples (a—DTA pattern of manganese-nodule leached residue; b—DTA pattern of water-washed leached residue; ai—TG pattern of leached residue; bi—TG pattern of water-washed leached residue).
only one endotherm is observed at 70 ◦ C and the total loss is ∼30% (Fig. 4b). 3.2.3. Infrared spectroscopy The IR spectra as presented in Fig. 5a show strong absorption bands at 3422 cm−1 , which is due to the O–H
3.2.4. Electron probe microanalyses The X-ray image maps of different elements under the electron microprobe indicates that the residue particles are very fine in size (5 to 50 µm), semicircular to circular or irregular/elongated in shape, and enriched in both Mn and Fe phases. From the illustrations (Figs. 6 and 7), the presence of four silicate phases has been brought out (quartz, goethite, kaolinite, and Fe–Al silicate). The original unwashed sample exhibits the presence of sulfate mostly attached to ferromanganese phases, while the SEM micrographs of the washed sample (Fig. 7) show the presence of sulfate in traces, indicating considerable removal of the sulfate phase. Other micrographs in both Figs. 6 and 7 show more or less similar patterns of element distribution. The image maps of Ni, Zn, and Cu in both Figs. 6 and 7 indicate the near absence of these elements.
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Fig. 6. Scanning electron micrograph of original leached-residue sample: (A) X-ray image map of Fe; (B) secondary electron image of morphological view of residue sample; (C) X-ray image map of Si; (D) X-ray image map of Al; (E) X-ray image map of Mn; (F) X-ray image map of Ni; (G) X-ray image map of Zn; (H) X-ray image map of S; (I) X-ray image map of Cu.
Fig. 8. Effect of time on adsorption of nickel MNLR.
Fig. 9. Effect of pH on adsorption of nickel WMNLR. Fig. 7. Scanning electron micrograph of washed leached-residue sample: (A) X-ray image map of Mn; (B) secondary electron image of morphological view of residue sample; (C) X-ray image map of Fe; (D) X-ray image map of Si; (E) X-ray image map of S; (F) X-ray image map of Ni; (G) X-ray image map of Zn; (H) X-ray image map of Al; (I) X-ray image map of Cu.
3.3. Absorption of nickel on water-washed manganese-nodule leached residues 3.3.1. Effect of time The equilibrium period for nickel adsorption was determined, keeping the initial pH of the solution at 4.5. The
results are presented in Fig. 8, which shows that equilibrium is reached in ∼4 h. 3.3.2. Effect of pH The effect of pH on adsorption of nickel is shown in Fig. 9. It can be observed that the extent of adsorption onto the residue increases with increased pH of the solution up to pH 5 and thereafter no significant improvement takes place. It is also seen that there is no adsorption of nickel at pH 1. Beyond pH 6 hydrolysis of nickel is observed.
K.M. Parida et al. / Journal of Colloid and Interface Science 277 (2004) 48–54
The increase in adsorption of metal ions with a rise in pH may be partly attributed to the formation of more MOH+ species. A simple calculation using the values of Kh and pH showed that the concentration of NiOH+ species formed at pH 6 and Ni+2 concentration (0.02 mol dm−3 ) is 7.9 × 10−7 mol dm−3 . Thus the formation of NiOH+ , species leading to higher adsorption, may not be a significant factor. The other important factor that might contribute to the higher adsorption of metal ions with increased pH is the pzc value of the manganese nodule (pzc = 4.5). At any pH below the pzc, the surface of the metal oxide is positively charged, and above the pzc, it is negatively charged. Thus, a lowering of the positive surface charge with an increase in the pH makes it easier for the cation to closely approach the surface, which in term results in higher adsorption of nickel ions. The nickel adsorption below pHpzc may be due to the combined effects of both chemical and electrostatic interaction between the manganese-nodule leached-residue oxide surface and the nickel ions.
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Fig. 10. Effect of temperature on adsorption of nickel.
3.3.3. Effect of temperature The adsorption of nickel onto manganese-nodule leached residues is illustrated in Fig. 10 as a function of temperature. The percentage of adsorption of nickel onto WMNLR increases slightly with increased temperature from 30 to 50 ◦ C, which may be explained as follows: (1) if the sorption of metal ions is endothermic, which is the case for many divalent metal cations [20,21], then adsorption is favored at higher temperatures; (2) at a given pH the surface charge of MnO2 /goethite is decreased and the extent of hydrolysis of the metal cation is increased [22–24]. Both these factors will reduce the electrostatic repulsion between the surface and the adsorbing species, leading to higher adsorption. 3.3.4. Effect of adsorbent and adsorbate concentration The percentages of nickel adsorption with varying amounts of leached residues and nickel concentrations are illustrated in Figs. 11 and 12, respectively. As expected, the amount of nickel adsorption increases with increased adsorbent (nodule) concentration, whereas it decreases with increased adsorbate concentration, indicating that the adsorption is dependent upon the availability of the binding sites. To determine the adsorption capacities of the samples the experimental data points (Fig. 12) were fitted to the Langmuir equation [25], C/X = 1/bXm + C/Xm , where X indicates the amount of nickel adsorbed per unit weight of the sample, C represents the nickel concentration in equilibrium solution, b denotes a constant related to the energy of adsorption, and Xm is the adsorption capacity of the sample. Fig. 13 depicts the Langmuir plot (C/X vs C) for the WMNLR sample. The correlation coefficients in the case are higher than 0.95.
Fig. 11. Effect of adsorbent concentration on adsorption of nickel.
4. Conclusions 1. Polymetallic manganese-nodule leached residues are very fine-grained (mainly mesopores along with micropores), are cryptocrystalline to amorphous in na-
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ture, and constitute δ-MnO2 , quartz, kaolinite, and zeolite/feldspar minerals. 2. The entrapped sulfate in this sample can be removed by sample washing with water. 3. There is an appreciable increase in surface area after removal of sulfate from the leached residue. 4. The water-washed manganese-nodule leached residues can be used for effective removal of Ni2+ in aqueous solution at 4.0 pH.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Fig. 12. Effect of concentration on adsorption of nickel on WMNLR. [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 13. Langmuir plot of nickel adsorption on manganese-nodule leachedresidue.
[23] [24] [25]
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