Selenite adsorption using leached residues generated by reduction roasting–ammonia leaching of manganese nodules

Journal of Hazardous Materials 241–242 (2012) 486–492

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Selenite adsorption using leached residues generated by reduction roasting–ammonia leaching of manganese nodules N.S. Randhawa a , N.N. Das b , R.K. Jana a,∗ a b

Metal Extraction & Forming Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, Jharkhand, India PG Department of Chemistry, North Orissa University, Baripada 757003, Odisha, India

h i g h l i g h t s    

Leached residue generated by reduction roast–NH3 leaching of manganese nodules was used. Se loading on untreated leached residue was 6.91 mg Se/g. Heat-treatment improved Se loading capacity. Achieved the goal of successful residue utilization for selenite removal.

a r t i c l e

i n f o

Article history: Received 9 May 2012 Received in revised form 25 September 2012 Accepted 26 September 2012 Available online 5 October 2012 Keywords: Leach manganese nodule residue Selenite Adsorption Contaminated water

a b s t r a c t This study was carried out to investigate the adsorption characteristics of leached manganese nodule residue (MNR), generated from the reduction roasting–ammonia leaching process, towards aqueous selenite. Physicochemical characterization revealed that the leached residue was a complex mixture of oxides of mainly manganese and iron along with MnCO3 . Adsorption studies of the water washed leached residue (wMNR) at varying the pH, selenite ion concentration, wMNR dosage, heat treatment condition indicated that selenite uptake increased with increasing pH and heat-treatment temperature of wMNR. The maximum value of selenite uptake was obtained at pH ∼5.0 with wMNR heat-treated at 400 ◦ C and thereafter decreased on increasing the pH and heat-treatment temperature further. The adsorption data were best fitted by the Freundlich isotherm model. The derived monolayer selenite adsorption capacities increased from, Xm = 9.50 mg Se/g (for untreated wMNR) to 15.08 mg Se/g (for wMNR heat-treated at 400 ◦ C). The results of the studies may be useful for possible utilization of MNR as an adsorbent for the removal of selenite ions from contaminated water bodies. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Selenium is essential micronutrient with mixed blessings to humans, wherein it is essential at low concentrations but toxic if consumption exceeding daily recommendations (≥400 ␮g/day) [1–3]. Increasing level of selenium in environmental matrices including fresh and groundwater is, therefore, becoming a serious threat to human and living organisms. The entry of selenium into the water bodies is not only from natural origin such as erosion or leaching of seleniferous minerals but also due to mining activities of seleniferrous ores, coal, combustion of fossil fuels, and discharge of effluents from industries like glass, semiconductor and solar cells, chemical, paints/pigments, etc. [4–6]. In water, selenium exists predominately in +4 (as selenite SeO3 2− ) and +6 (as selenate SeO4 2− )

∗ Corresponding author. E-mail address: [email protected] (R.K. Jana). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.09.059

oxidation states [6]. While Se(IV) is present in mildly oxidizing, neutral pH environments and typical humid regions, the Se(VI) is the predominant form under ordinary alkaline and oxidized conditions. A number of physical, chemical and biological based methods including adsorption/ion exchange, membrane filtration, reverse osmosis, chemical reduction, solar ponds, bacterial reduction, etc. have been described for the remediation of both these oxoanions [5–10]. The adsorption process involving synthetic materials, naturally occurring minerals and industrial wastes, primarily constitute of oxides/oxyhydroxides of Fe, Mn or Al, as adsorbents have been tested for removal of Se oxyanions [4–6,11–15]. The residues generated after reductive ammoniacal leaching {SO2 –NH4 OH/(NH4 )2 SO4 } [16] and reduction-roast ammoniacal leaching {NH4 OH/(NH4 )2 CO3 } [17] of polymetallic sea nodules, primarily constitute of oxides/oxyhydroxides of Fe, Mn, Al and Si, possess reasonably good surface area (∼65–127 m2 /g) compare to many naturally occurring minerals or solid wastes (e.g. iron ores, red mud, etc.) [18,19]. The favourable constituents and good surface area has lead to several studies [20–26] relating

N.S. Randhawa et al. / Journal of Hazardous Materials 241–242 (2012) 486–492

to possible utilization of SO2 –ammonia leached manganese nodule residue as adsorbent for anionic/cationic polluting species and dyes. The adsorption behaviour of reduction-roast–ammonia leached residue towards heavy metal and phosphate ions has also been reported [27,28]. Recently, the catalytic efficiency of residue has been tested for degradation of methylene blue [29]. However, selenite adsorption by the leached residue generated in the reduction roasting–ammonia leaching of manganese nodules, has not been studied so far. Keeping the above in view, the present study was designed to investigate the adsorptive removal of aqueous selenite by the leached residue for its possible utilization in remediation of selenium contaminated water. 2. Materials and methods 2.1. Materials The leached manganese nodule residues (MNR) of the reduction–roast ammoniacal leaching process, used in the present study, were collected from the pilot scale trial of Indian Ocean manganese nodules at CSIR-National Metallurgical Laboratory, Jamshedpur, India. The samples were air-dried and stored in airtight bottles for characterization and other uses. The washing of the residue was done with deionized water to remove loosely associated metal ions/anions. Typically, leached residue was dispersed in deionized water (solid/liquid ratio = 1:10) and stirred for 2 h at room temperature. The resulting solid (wMNR) was separated by filtration, air-dried for several days and then used for subsequent characterization, heat-treatment and adsorption studies. Heat-treatment (calcination) of wMNR at desired temperatures was carried out in a tubular furnace fitted with programmable temperature controller. The heat treated samples were abbreviated as wMNR-300, wMNR-400, wMNR-500 for wMNR heated at 300, 400 and 500 ◦ C, respectively. A stock solution of selenite ion (1000 mg Se/L), prepared by dissolving required amount Na2 SeO3 in deionized water, was used to prepare the working solutions by suitable dilution with deionized water. 2.2. Characterization of samples For chemical analyses, a weighed quantity of MNR or wMNR was digested in acid (HCl/HNO3 mixture), dehydrated, redissolved in HCl (1:1) and filtered. The dehydrated silica was estimated gravimetrically while major and minor constituents in the filtrate were analysed by conventional wet chemical methods [30] and AAS (PerkinElmer AAnalyst 400), respectively. The amount of Mn in different oxidation states was determined by volumetric methods [30]. The distribution of particle size of MNR and wMNR was determined using a Malvern Instruments SA-CP3 particle size analyzer. X-ray diffraction patterns were recorded on a Siemens D500 Xray diffractometer using Ni filtered CuK␣ radiation (30 kVA, 15 mA) at a scanning speed of 2◦ (2)/min. Surface areas were determined by the BET method using a Quantachrome Nova 4000e instrument after degassing the sample under vacuum (2 × 10−4 Pa). Points of zero charge (pHPZC ) were determined by batch acid–base titration techniques as reported by Huang and Ostavic [31]. All pH measurements were carried out using a Toshniwal CL 54 pH meter fitted with a combined glass electrode after calibration with NBS buffers of pH 4.01, 6.86 and 9.20. The morphology and surface elements distribution of wMNR were studied by SEM (Hitachi-S3400N scanning electron microscope) and EDX analyses, respectively. 2.3. Adsorption experiments The equilibrium batch-type experiments were undertaken to study the adsorption of selenite ions. Typically, 50 mL of aqueous

487

Table 1 Chemical compositions of manganese nodule (MN), untreated (MNR) and water washed (wMNR) manganese nodule leached residue (in wt.%). Element/oxide

MNR

wMNR

Mn(T)a Mn2+ Mn3+ Mn4+ Fe(T) SiO2 Al2 O3 C S P CaO MgO Co Ni Cu Zn Moisture Loss of Ignition

25.66 14.02 4.87 6.77 9.92 17.07 4.01 5.27 0.37 1.34 0.43 4.69 0.035 0.079 0.26 0.12 7.96 15.95

25.85 13.71 4.92 7.22 10.03 17.25 4.11 5.59 0.14 0.18 0.45 4.73 0.039 0.071 0.135 0.13 6.18 17.50

a The values against Mn2+ , Mn3+ and Mn4+ represent respective wt.% out of total manganese content in the samples.

solution of sodium selenite of the desired concentration was mixed with appropriate amount of the adsorbent (wMNR) in a series of 100 mL stoppered conical flasks. The initial selenite ion concentration and the amount of adsorbent employed varied over the ranges 0.5–50 mg Se/L and 0.2–10.0 g/L, respectively. The solutions were adjusted to the desired pH value by the addition of dilute NaOH or HCl as necessary. The flasks were shaken in a water shaker bath at 30.0 ± 0.1 ◦ C at a constant shaking rate (100 strokes/min) until the selenite solution reached equilibrium. A preliminary experiment revealed that ∼12 h was necessary to attain such equilibrium. Samples taken from these flasks at regular intervals were centrifuged and the concentrations of selenite ions in the filtrate determined by hydride generation atomic absorption spectrometry (HG-AAS) using 0.2% NaBH4 solution as the reducing agent and 10% HCl as carrier reagent. The amount of selenite ions adsorbed was determined from the ratio of selenite ions in the solution and in the particulate phases using the following equation: qe =

(Ci − Ce )V W × 1000

(1)

where qe , Ci , Ce , V and W represent the amount of selenite ions (Se) adsorbed onto the solid at equilibrium (mg Se/g), the initial concentration of selenite ions (mg/L), the final equilibrium concentration of selenite ions (mg/L), the volume of the solution (mL) and the amount of adsorbent employed (g), respectively. 3. Results and discussion 3.1. Characteristics of leached residue The chemical analyses data of MNR and wMNR are listed in Table 1. It is evident that the leached residue mainly contained Mn, Fe, Si and Al. On washing MNR with deionized water, marginal changes in the weight percentages of Mn, Fe, SiO2 , C, CaO and MgO were noted, while the losses of adsorbed metal ions such as Cu2+ and Ni2+ along with S and P were quite significant. Hence, it is reasonable to believe that the S and P in the MNR, generated by roasting of sea nodules with LD oil, are mostly present in the soluble forms. The morphology of wMNR particles under scanning electron microscope is shown in Fig. 1, which revealed that shapes of the particles are irregular and particles are congregated moderately. The point analysis by EDX also revealed the Mn and Fe as major constituent. The wt.% of Si is found to be very less presumably due

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Table 2 Adsorption parameters derived from Langmuir and Freundlich isotherms along with experimentally obtained maximum Se uptake of some relevant adsorbents. Sample description

Natural hematite Natural goethite Hematite GRI-300a Mn nodules (raw) Fe-Mn hydrous oxide Fe/SiO2 binary oxide Al/SiO2 binary oxide Al oxide coated sand Tropical soil Peanut shell Dry Wet Aged magnetite wMNR wMNR-300 wMNR-400 wMNR-500 a b

Surface area (m2 /g)

0.38 2.0 8.40 49.4 107.5

Initial [Se], mg Se/L

pH

– 9.17

0.28–39.0 0.28–39.0 31.58 5–50 26.1 0–500 0–236 0–236 0–158 0–60

4.0 4.0 3.5 4.5 5.2 6–8 5.0 5.0 4.9–8.4 4.5

72.0 – – 66.7 73.5 91.7 45.3

25–250 25–250 0.25–10.0 0.5–50 0.5–50 0.5–50 0.5–50

1.50 1.50 4.0 5.0 5.0 5.0 5.0

Max. uptake cap (exptl.) (mg Se/g)

1.80 7.04 6.50 –

– – –

Langmuir

Freundlich

K (mgSe/g)

Ref.

Xm (mg Se/g)

b (l/g)

1/n (l/g)

0.39 0.523 4.11 7.55 – 26.7–18.5 20.7 32.7 1.08–0.76 0.145b

– – – 0.56 – 6.83–0.725 0.076 0.055 223–1.38 17.3b

– – – – – –

– – – – – –

[4] [4] [33] [34] [15] [5] [6] [6] [35] [36]

23.76 32.30 1.92–2.38 9.50 14.89 15.08 6.87

0.266 0.0276 – 0.064 0.053 0.097 0.060

– – – 1.62 1.20 1.39 2.77

– – – 0.751 0.761 0.706 0.742

[7] [7] [2] This study This study This study This study

GRI-300 = goethite rich iron ore heat treated at 300 ◦ C. In presence of Cl− background.

to reason that Si phases containing particle are finer and partially covered by Mn and Fe phases. Particle size analyses showed that the leached residue contained very fine particles with mean particle diameters (d50 ) of 17.8 ␮m. The BET surface areas of MNR and wMNR are found to be 60.9 and 66.7 m2 /g, respectively. The marginally higher surface area of wMNR is presumably due to an increased number of accessible pores on washing adsorbed species out from MNR. On heating, the surface area of wMNR gradually increases up to 400 ◦ C and then decrease on further increase of temperature (Table 2). The increase of surface area is mainly due to decomposition of MnCO3 leading to Mn oxide/oxyhydroxide as also observed previously from XRD and FT-IR studies [27]. The point of zero charge of wMNR (pHPZC = 6.5) is found higher than that of sea nodule (4.5) [15]. The X-ray diffraction patterns of air-dried MNR and wMNR are presented in Fig. 2. The prominent peaks at 2 (28.36, 41.08, 53.20, 61.0 and 71.02), (36.60 and 40.12) and (34.54, 48.60 and 59.48) are assigned mainly to MnCO3 (File No. 44-1472), Mn2 SiO4 (File No. 35-0748) and Mn2 SiO3 (OH)2 ·H2 O (File No. 471869) phases, respectively. On washing with distilled water, no

Fig. 1. Scanning electron micrograph of wMNR with EDX.

changes in the positions of the characteristic peaks are observed from those of MNR. 3.2. Effect of time and heat treatment of wMNR on selenite ion adsorption Time variation adsorption studies with initial selenite concentration of 10 mg Se/L at fixed pH of 5.0 and adsorbent dosage of 2 g/L show that adsorption is relatively slow, with equilibrium being attained only after 12 h (Fig. 3). Equilibrium selenite ion concentration remained unchanged when contact time was extended up to 30 h. The wMNR was heat-treated at different temperatures as thermal activation of naturally occurring minerals or wastes invariably has a positive effect on the adsorption of different anions [27]. Selenite ion adsorption is found strongly dependent on the temperature employed in the heat treatment of wMNR as shown in Fig. 3. The uptake of selenite ion is increased with increasing temperature of heat-treatment, attains a maximum value at ca. 400 ◦ C

Fig. 2. Powder X-ray diffraction patterns of manganese nodules residue (MNR) and washed manganese nodules residue (wMNR).

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Fig. 3. Effect of contact time on the adsorption of selenite ions at pH ca. 5 with initial selenite concentration (10 mg Se/L) and adsorbent dose (1.0 g/L) by untreated wMNR (1), wMNR-300 (2), wMNR-400 (3) and wMNR-500 (4).

and then decreases drastically on further increase in the heattreatment temperature. This may be attributed to either an increase in the number of active components (oxides/oxyhydroxides) in wMNR due to loss of weight or structural changes in wMNR with increasing temperature. Another possibility of increased selenite adsorption could be due to partial conversion of MnCO3 in wMNR to oxide/oxyhydroxide of Mn (e.g. MnOOH) on calcination at ≥400 ◦ C [27]. Drastic decrease in the extent of selenite ion adsorption by wMNR-500 is most likely due to a significant loss of surface hydroxyl groups and the transformation of the material to the respective higher oxides. 3.3. Effect of pH The effect of varying the pH on the adsorption of selenite ions was studied at a fixed selenite ion concentration (10 mg Se/L) and a fixed dosage (2 g/L) of untreated or heat-treated wMNR as the adsorbent. As evident from Fig. 4, the adsorption of selenite increases with increasing pH, attains a maximum value at pH ∼5.0 and then decreases progressively on further increase in pH up to ∼8.0 in all the samples studied. In aqueous solution, the distribution of selenite species is a function of pH according to dissociation equilibrium of selenite ion (Eqs. (2) and (3)): K1

H2 SeO3 H+ + HSeO3 − (pK1 = 2.62) K2

HSeO3 − H+ + SeO3 2− (pK2 = 8.32)

(2) (3)

This indicates that HSeO3 − is the most abundant anionic species at pH < 8.32, while SeO3 2− is prevalent at pH values above 8.32. The oxide/oxyhydroxide constituents of wMNR readily form metal–hydroxo complexes in solution and their subsequent acidic or basic dissociation at the solid/solution interface leads to the development of a positive or negative charge on the wMNR surface. The extent of surface charge is also dependent pH and pHPZC of wMNR. Different mechanisms such as electrostatic attraction/repulsion, chemical interaction and ligand exchange are often proposed for adsorption of anions on oxide/oxyhydroxides based adsorbents [4–6,11,15,25,28,32]. The ligand exchange adsorption involving exchange of surface hydroxyl group with selenite ion

489

Fig. 4. Effect of pH on the adsorption of selenite ions with initial selenite concentration (10 mg Se/L) and adsorbent dose (1.0 g/L) by untreated wMNR (1), wMNR-300 (2), wMNR-400 (3) and wMNR-500 (4).

leading to formation surface complexes is given by Eqs. (4) and (5). S· · ·OH + H+ + SeO3 2− → S· · ·SeO3 − + H2 O +

S· · ·OH + 2H + SeO3

2−

−

→ S· · ·HSeO3 + H2 O

(4) (5)

where S· · ·OH is the proton-specific surface sites, i.e., a surface hydroxyl group; S· · ·SeO3 − and S· · ·HSeO3 are the adsorbed selenite species. It is seen from Fig. 4 that a substantial amount of selenite adsorbed at pH above pHPZC (6.5) of wMNR even if the surface of wMNR becomes negatively charged. This indicates that the adsorption is not solely controlled by the electrostatic interaction between the negatively charged selenite and the oxide/oxyhydroxide surface of wMNR. In addition, the equilibrium pH of selenite containing solution is always found ∼1.0 unit higher than the initial pH presumably due to consumption of H+ ions as per Eqs. (4) and (5). Both these effects favour selenite adsorption on wMNR through ligand exchange mechanism. 3.4. Effect of adsorbent dosage The results of selenite adsorption on wMNR with varying amounts of leached residues (0.2–10 g/L) at fixed pH ∼5.0 and initial selenite ion concentration (10 mg Se/L) are illustrated in Fig. 5. The percentage selenite ion adsorption is increased with the increasing dose of wMNR (untreated or heat-treated). For example, the percentages of selenite adsorption increased from 25% to 88% and 45.3% to 99.9% in the case of wMNR and wMNR-400, respectively as the adsorbent dosage is increased from 0.2 g/L to 10 g/L. With increased adsorbent dose, the number of available active components increases and in turns increases the selenite ion adsorption [27]. 3.5. Adsorption kinetics Among the commonly used kinetic equations for adsorption of anionic species on different adsorbents [4–6,11,15,27,28,34], the data were found better fitted to Lagergren pseudo-first-order rate model [15,27] as expressed in Eq. (6). ln(qe − qt ) = ln qe − k1 t

(6)

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Fig. 5. Effect of the wMNR dosage on the adsorption of selenite ions at a pH value of ca. 5.0 with initial selenite concentration (10 mg Se/L) by untreated wMNR (1), wMNR-300 (2), wMNR-400 (3) and wMNR-500 (4).

where qe and qt are the amounts of selenite ions adsorbed per unit weight of adsorbent (mg Se/g) at equilibrium and at a time t (min), respectively. The plots of ln(qe − qt ) versus t obtained in the present study were almost linear (R2 ≥ 0.98) for all the adsorption data obtained for heat treated or untreated wMNR, thereby indicating the applicability of the Lagergren equation to the present experimental data. The pseudo-first-order adsorption rate constants (k1 ), derived from least squares fitting of selenite adsorption onto untreated wMNR and wMNR-400, are found to be 6.28 × 10−3 and 7.73 × 10−3 min−1 , respectively at pH ∼ 5.0 with an initial selenite ion concentration of 10 mg Se/L and adsorbent dosage of 1.0 g/L. These values are comparable to those observed earlier for adsorption of phosphate ion of wMNR [27]. 3.6. Adsorption isotherms The classical Langmuir and Freundlich models are most widely used to describe the equilibrium established between the adsorbate on the adsorbent and that in solution. The Langmuir model assumes that maximum adsorption corresponds to the formation of a saturated monolayer of adsorbate on the adsorbent surface, with uniform adsorption energy and no trans-migration of the adsorbate in the surface plane. On the other hand, the Freundlich model is an empirical equation employed to describe multi-site adsorption onto heterogeneous surfaces. The linearized form of the Langmuir equation may be written as: 1 Ce Ce = + 0 qe bQ 0 Q

(7)

where Ce is the equilibrium selenite ion concentration in solution (mg Se/L), qe is the equilibrium selenite ion uptake per unit mass of adsorbent (mg Se/g), Q0 is the maximum adsorption capacity (mg Se/g) and b is the Langmuir constant (adsorption energy). The Freundlich equation may be expressed as: ln qe = ln Kf + n ln Ce

(8)

where qe and Ce are as described above while Kf and n are the Freundlich constants which are considered to be relative indicators of the adsorption capacity (mg Se/g) and the adsorption intensity, respectively. The selenite adsorption data with varying initial concentrations, keeping all other parameters constant, were fitted to both isotherms. The adsorption parameters, derived from the

Fig. 6. Effect of coexisting anions on selenite adsorption by wMNR-400 at pH ca. 5.0 with initial selenite concentration (10 mg Se/L) and adsorbent dose (1.0 g/L).

least-square fittings of the Langmuir and Freundlich equations, are listed in Table 2. The correlation coefficients show that the adsorption data obtained in the present study are slightly better fitted to the Freundlich isotherm (r2 = 0.991–0.996) than the Langmuir isotherm (r2 = 0.817–0.968), thereby demonstrating the heterogeneous nature of the adsorbent surface. A comparison of derived monolayer adsorption capacities (Xm = 6.91–15.17 mg Se/g) with those reported for other relevant adsorbents is given also in Table 2. It is evident from the table that the adsorption capacities in the present study are relatively higher than those obtained with several naturally occurring minerals [4,14,15,33,34] but invariably lower than those observed with synthetic materials [5,6,36] under comparable conditions. The moderate adsorption capacity is, thus, favourable for further study to use wMNR in adsorptive removal of selenite from contaminated water. 3.7. Effect of competitive anions The effect of some commonly occurring anions on adsorption of selenite by wMNR-400 is shown in Fig. 6. It is seen that the effects of Cl− , NO3 − and SO4 2− ions are negligible to very small up to 100 mg/L but decrease up to 20% when the competing anion concentration is raised to 200 mg/L. On the other hand, the CO3 2− shows much larger effect even at lower concentration (50 mg/L). The overall effect of competing ions on adsorption of selenite follows the order Cl− < NO3 − < SO4 2− < CO3 2− which is very similar to those observed in earlier studies [15,27,36]. Significant adsorption of selenite on wMNR, even in the presence of competing anions, indicates its selectivity towards selenite and partially fulfils the criteria requires for its possible use in treatment of contaminated water. 3.8. Desorption studies Since the adsorption of selenite is strongly dependent on pH of the solution, its desorption from wMNR can be effected by controlling the pH of the eluent. The desorption of selenite from selenite loaded wMNR-400 was carried out under varying pH and results obtained are presented in Fig. 7. It is seen that desorption of selenite progressively increases with increasing pH of the eluent and ∼92% of the adsorbed selenite is desorbed at pH ∼10.0 or above. This indicates that selenite adsorption on wMNR-400 is

N.S. Randhawa et al. / Journal of Hazardous Materials 241–242 (2012) 486–492

[4]

[5]

[6]

[7] [8] [9]

[10]

[11]

Fig. 7. Desorption of selenite from loaded wMNR on treatment with water at varying pH.

[12]

[13]

not completely reversible and the selenite ions that are strongly adsorbed through surface complex are not desorbed.

[14] [15]

4. Conclusions [16]

The potential of water-washed leached residue (wMNR), generated from the selective extraction of Cu, Ni and Co in the reduction–roasting ammoniacal leaching process of Indian Ocean sea nodules, for the removal of selenite ions from aqueous solution was studied. The adsorption of selenite on wMNR was pH dependent and with a reasonable surface area was found to be effective as an adsorbent for selenite ions. However, the capacity of wMNR as an adsorbent decreased with increasing pH of the aqueous solution after attaining a maximum at ca. 5.0 and decreasing thereafter as the pH value increased further. On calcination up to 400 ◦ C, the maximum value of selenite ion uptake onto wMNR increased from 9.50 mg Se/g to 15.17 mg Se/g, but drastically diminished to 6.91 mg Se/g at 500 ◦ C calcination temperatures. Adsorption studies in the presence of commonly occurring coexisting anions such as Cl− , NO3 − , SO4 2− and CO3 2− showed that the adsorption efficiency was unaffected up to a concentration of 100 mg/L. However, at a higher concentration of co-existing anions (200 mg/L), selenite ion adsorption diminished by 20%. More than 90% of the adsorbed selenite ions could be desorbed at pH ∼10, indicating the regeneration capability of wMNR. Overall, the results of this study demonstrate one of the possible uses of reduction roast ammonia leached manganese nodule residue.

[26]

Acknowledgements

[27]

The authors are thankful to Director, CSIR-NML for giving permission to publish this paper. Thanks are also due to InhouseProject Support Group (iPSG) of CSIR-NML for funding the work.

[28]

[17] [18] [19]

[20]

[21]

[22]

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