Utilization of synthetic nano-cryptomelane for enhanced scavenging of cesium and cobalt ions from single and binary solutions

The feasibility of using nano-cryptomelane for elimination of cobalt and cesium metal ions from their single and binary solutions was studied. In this respect, the material was prepared and characterized to confirm its chemical composition and structure. Results illustrate that the synthesized nano-cryptomelane has a tunnel structure with particle size ranged between 4 and 6 nm. The material feasibility was detected by conducting a series of batch experiments for determination of the kinetic and equilibrium performance of the removal process. All characteristic Raman bands for Mn–O lattice vibrations within the (2 × 2) tunnel structure of MnO6 octahedral are observed which confirm formation of nano-cryptomelane. The specific surface area (SSA) for nano-cryptomelane was calculated and equal to 299.03 m2/g while the surface fractal information (Ds) was2.53. The process sensitivity to changes of H+ concentration is attributed to changes in structural elements-species distribution at the solid/aqueous interface. The pH optimum value was desired at pH 5 for exchange of Cs+ and/or Co2+ with K+ ions. The equilibrium studies show that Langmuir isotherm model was more fitted to the experimental data than that of Freundlich model.


Introduction
The amassing of hazardous metal ions through the food chain accompanied with a harmful effects for the human health thus to reduce these effects and for environmental protection, sequestration of these contaminates from wastewater is very important process. Estimation and removal of these toxic contaminants are essential to meet safety requirements and the environmental standards [1][2][3]. Both of cobalt and cesium ions are broadly utilized in the nuclear power plant and coal-fired power plants as well as and in various applications of industry such as metallurgical, battery manufacturing, mining, electroplating, paints, grinding wheels, petroleum, pigments, and electronics [4][5][6][7]. Higher cobalt concentration has harmful impacts on the human beings; it leads to lung irritations, asthma, pneumonia, weight loss, paralysis, diarrhea, vomiting, damage thyroid hormone and liver and nausea [8][9][10][11][12][13]. Due to similarity of cesium-134 to sodium thus ingestion of cesium leads to its deposition in the soft tissues all over the body which in turn cause internal hazards [14][15][16].
Several sorbent materials can be used for the elimination of these toxic contaminants from aqueous waste. Nano sized metal oxides (NMOs) have a high specific tendency for sorption of metal ions from liquid solutions; the most widely studied ones are aluminum oxides, titanium oxides, cerium oxides and manganese oxides. NMOs exist in different morphologies such as particles, tunnels and others. Both of size and shape of NMOs are important factors that affects greatly their sorption performance. Several efficient preparation procedures are widely studied to obtain a highly stable, shape-controlled and mono disperse NMOs [23]. Nanosized manganese oxides (NMnOs) are widely studied for environmental concerns as birnessite and cryptomelane [24,25].
Hydrous manganese oxide (HMO) and mixed-valance manganese oxides are most common NMnOs [23]. Mixedvalence manganese oxides considered as potential interesting sorbents for cations and usually present as octahedral molecular sieve (OMS). Cryptomelane considered as an example of mixed-valence octahedral molecular sieve (K-OMS-2) manganese oxide, the main oxidation states of manganese are Mn 4+ and Mn 3+ [23].The OMS-2 group building unit is MnO 6 octahedral which in turn form a single octahedral chain through participating two opposite edges after that a double chain is resulted via combination of two neighbor single chains, finally, four chains corner connected forming a one-dimensional 2 × 2 open tunnel structure with a diameter of about 460 pm [26][27][28].
Cryptomelane has two ion exchange sites; particular sites found interior tunnel structure and the unspecific sites, ion exchangeouter surface sites. The exchangeable cation, K + , is located interior tunnel structure to stabilize negative charge balance that originating from Mn 3+ in an otherwise perfect Mn 4+ O 2 structure. Several synthesis procedures have been published for synthesis of nano-cryptomelane such as sol-gel method [29] hydrothermal treatment procedure for birnessite [30] and oxidation of Mn 2+ by KMnO 4 , H 2 O 2 or K 2 S 2 O 8 in acidic refluxing conditions [31]. Nano-cryptomelane considers as selective ion exchanger due to its porous and tunnel structure, hydrophobic nature and mixed valence of manganese [28].The morphology of synthesized nanocryptomelane (rod-like, fibrous-like and nest-like morphologies) depends incredibly on the preparation method (solid state, reflux and hydrothermal method) [32].
The previous work illustrated that nano-cryptomelane can be synthesized using several methods such as sol-gel route, hydrothermal treatment procedure of birnessite and oxidation of Mn 2+ by KMnO 4 , H 2 O 2 or K 2 S 2 O 8 in acidic refluxing conditions.The main target of this work is utilization of synthesized nano-cryptomelane for retention of cesium and cobalt metal ions from single and binary solutions.

Materials and reagents
Manganese sulfate, acetic acid and potassium permanganate, KMnO 4 , were obtained from Alpha Chemika, Euromedex and El-Nasr Company for chemicals, respectively and used to prepare nano-cryptomelane. Cesium chloride and cobalt chloride were obtained from LOBA. NaOH and HCl were obtained from Fluka and used for pH adjustment.

Preparation of nano-cryptomelane
As mentioned in the previous work [28],the oxidation method was the selected procedure for preparation of nanocryptomelane under acidic conditions [33]. The preparation method involved preparing 1.0 L of 0.2 M MnSO 4 in 2 M acetic acid.The acidified MnSO 4 was added over permanganate solution (0.35 M) pre-dissolved in 800 mL deionized water over a time period of 2 h at 65 °C in 4.0 Lbeaker with vigorous stirring. An aluminum hinder was used above the beaker and a hole was done to permit a thermometer to be inserted into the beaker. After 2 h, temperature was raised to 80 °C and held for about 10 min then a dark black manganese oxide was formed. The obtained mixture was gradually cooled until reach the room temperature and left overnight for the aging process after that wet-aged for 48 h at 7 °C. The resulted dark black MNOs were centrifuged at 6000 rpm, washed many times using bi-distilled water and finally, freeze-dried for 8 days, Fig. 1.

Removal studies
10 mL of 10 -3 M Co 2+ or Cs + metal ion solution was agitated with 0.05 g nano-cryptomelane and centrifuged to separate the liquid and solid phases. Same molar ratio (1:1) of Co 2+ and Cs + was used to prepare 10 -3 M binary solution. The elemental analysis for the clear diluted liquid was approached using atomic absorption spectroscopy.

Synthesis of multi-system solution
A binary stock solution of 0.02 M ionic strength was prepared through dissolving same molar ratio, 1:1 of cesium and cobalt ions. The metal ion concentrations in the solutions were determined using an atomic absorption spectrophotometer (Buck Scientific) model 210 VGP, USA, an airacetylene flame.

Structural and surface area investigation of Nano-cryptomelane
X-ray diffraction (XRD) results were determined by CuKα radiation at 40 kV using Schimadzo X-ray powder diffractometer and processed by using Match software. Ramanscattering data were performed at Raman shift between 100 and 3600 cm −1 at room temperature with spectral resolution of 0.5 cm −1 , equipped using a Peltier-cooled charge-coupled device (1152 × 298 pixels). The excitation wavelength of the spectrum was 632.81 nm and was supplied using a He-Ne laser placed on an Olympus high-stability BXFM confocal microscope. Nitrogen adsorption-desorption isotherms of nano-cryptomelane were used for determination of SSA by means of a fully automated surface area analyzer (Quantachrome Corporation, Nova station 2010 instrument, USA).
Brunauer-Emmett-Teller (BET) model can be given as [34]: where V refers to volume of adsorbed nitrogen at equilibrium pressure P (cm 3 /g), P 0 is the saturation pressure of nitrogen, C is the characteristic constant. V m is volume of nitrogen of the monolayer coverage (cm 3 /g), can be determined from slope and intercept of the linear fitting of the plot 1/ [V(P 0 /P − 1)] against the relative pressure (P/P 0 ). Value of SSA was then determined from the calculated value of V m , volume occupied by one mole of nitrogen gas and the sample mass and Avogadro's number [35]. In this work, modified Frenkel-Halsey-Hill (FHH) model [36] is adopted for estimating value of D s of sorbents using adsorption isotherm data. The logarithmic form for the modified (FHH) model can be described as follows: where A is the power law exponent that is reliant in D s value and the adsorption mechanism. It is worth mentioning that when the pore surface dimension (D) is obtained, the relevant pressure ranged between 0 and 0.3. Then, the surface fractal information (D s ) value may be determined from the following equation:

Results and discussion
As mentioned in the past work, particle size of the synthesized nano-cryptomelane was ranged between4 and 6 nm [28].

Raman spectroscopy
Raman scattering spectrum of nano-cryptomelane is illustrated in Fig. 3. Seven Raman bands are detected at 183, 286, 330, 386, 512, 574, and 753 cm −1 which are specific bands of the Mn-O lattice vibrations within the (2 × 2) tunnel structure of MnO 6 octahedral [39]. The low-frequency Raman bands at 183 and 286 cm −1 are ascribed to an external vibration that derives from the translational motion of the MnO 6 octahedral; the Raman bands at 330 and 386 cm −1 are specific to the Mn-O bending vibrations; The Raman bands located at 512 and 574 cm −1 may attributed to displacement of the oxygen atoms relative to the manganese atoms along the octahedral chains. The high-frequency Raman band at 753 cm −1 corresponds to the anti-symmetric Mn-O stretching vibrations. Figure 4a shows N 2 adsorption-desorption isotherm of nanocryptomelane. At low relative pressure region (P/P 0 < 0.1), nano-cryptomelane display a steep uptake of nitrogen, on the other side at relatively higher pressure the hysteres is loop between 0.8 and 1P/P 0 suggests N 2 capillary condensation in mesopores, representing the existence of mesoporous structures in nano-cryptomelane [40]. Figure 4b shows the linear fitting for the plot of 1/[V(P 0 /P−1)] against relative pressure (P/P 0 ) where V m value was detected using the slope and the intercept data. The value the SSA compared to other scavengerswas illustrated in Table 1. Value of D s was calculated using slope of the fitting curve of plot of ln(V/V m ) versus ln(ln(P 0 /P)) and detected to be 2.53.

Effect of initial effluent H + ion concentration (pH)
The elimination efficiency of cesium and cobalt ions from single and binary solutions was studied at pH range of 1 to 6; the alkaline conditions were excluded to avoid complications of precipitation. The data reveals that sorption capacities of cesium and cobalt ions from single and binary systems increase with increasing pH of the effluent and the pH optimum value was 5.0, Fig. 5a. This may be attributed to the fact that at low pH values, a competition between H + ions and cations for nano-cryptomelane exchange sites. The zero point charge of nano-cryptomelane, pH zpc , was detected and equal to 4.0.The surface of nano-cryptomelane possess a negative charge above pH zpc and positive below the pH zpc . Therefore, the sorption of cobalt and cesium ions at pH greater than 4 should be high, Fig. 5b. The relation  The slopes of the straight lines are corresponding to the valence of sorbed metal ions Fig. 5c. Values of n for cesium and cobalt metal ions either in single solutions or in binary solutions were about 1 which reflects that both of them are sorbed as monovalent species. The speciation of Co 2+ and Cs + at different pH values in the aqueous solution are illustrated in Fig. 5d which illustrates that at the pH optimum value, pH 5, both of Co 2+ and Cs + are sorbed as monovalent species. Nano-cryptomelane has a higher affinity towards Cs + more than Co 2+ which may attributed to fact that the atomic radius of Co(OH) + is larger than that of Cs + thus the movement of monovalent cobalt species from the bulk of solution to the surface of nano-cryptomelane is slower than that of Cs + [41,42].

Effect of contact time and temperature
Corresponding to Figs. 7a and 8a and Tables 2 and 3 the experimental data indicate that sorption process more fitted to pseudo-second-order reaction which indicates that sorption process is chemisorption [44].Sorption of Cs + and Co 2+ from single and binary solutions onto nano-cryptomelane  Fig. 6. Experimental data illustrate that the removal performance of Cs + and Co 2+ either from single or binary solutions increase by increasing the time of contact and equilibrium is obtained after about 45 min for all solutions. At beginning of the cation elimination process, the quantity sorbed increased rapidly which may be attributed to presence of more sorbing sites, after 45 min, equilibrium was obtained and sorbing sites are approximately saturated. Results show that the quantity sorbed of Co 2+ and Cs + increased with raising temperature which reflects the endothermic nature of sorption process. The percent of metal ion elimination may be detected using the following equation.
where C i and C t are metal ion concentration in solutions at the initial state and equilibrium time, respectively, (mg/L). Experimental data were analyzed to determine rate constants of sorption reaction using the non-linear forms of pseudofirst-order Eq. (9) and pseudo-second-order kinetic models Eq. (10) [29].
where K x is the rate constant, x = 1 pseudo-first-order, x = 2 pseudo-second-order, q e is the amount sorbed at equilibrium and can be determined as follow [43]: Freundlich, Eq. (12), and Langmuir, Eq. (15), isotherm models for multi-component sorption process were used to analyze the experimental data to describe the nature of the sorption sites.
where C e,z is the equilibrium concentrations of the component z, K F,z is Freundlich constant for the component z indicating sorbent capacity, mg/g, (mg/L) 1/n and n is a constant describe the heterogeneity of the surface.
For the sorption of Co 2+ and Cs + in a binary system, Freundlich equation becomes; where C e,x and C e,y are the equilibrium concentrations of the components x and y, respectively, q m,x , mg/g, is the maximum monolayer sorption capacity for the component x, q e,x is the quantity sorbed of component x, mg/g, at equilibrium in a multicomponent system, b x and b y , L/mg, describe the affinity of sorbent towards the sorbates x and y, respectively, y = 1, 2, 3, …, N, and N is the number of components in the experimental isotherm.
The extended Langmuir equation for the competitive sorption of Co 2+ and Cs + in a binary system becomes [45]: (14) q e,Co = K F,Co C 1∕n Cs e,Cs + C 1∕n Co e,Co . The nonlinear fitting of the experimental data to Freundlich and Langmuir isotherm models are shown in Figs. 9 and 10 which illustrated that both models could represent the behavior of sorption process. The correlation coefficients of Langmuir are slightly higher than these of Freundlich model (Tables 4 and 5) [46,47].

Conclusion
The performance of synthesized nano-cryptomelane for the sequestration of cobaltandcesium ions from their single and binary solutions was studied. The following specific (17) q e,Co = q m,Co b Co C e,Co conclusions could be drawn from the above mentioned investigations: (1) Nano-cryptomelane was prepared using oxidation method and the characteristic results indicated the formation. (2) The synthesized nano-cryptomelane possesses a tunnel structure with particle size ranged between 4 and 6 nm.   (5) The SSA of nano-cryptomelane was determined and equal to 299.03 and D s was calculated and equal to 2.53. (6) The pH optimum value for sequestration of cesium andcobalt ions was 5, the percent uptakes take the order Cs + > Co 2+ (7) Kinetic studies illustrate that sorption process more fitted to non-linear pseudo-second-order model.
(8) Single system solutions show higher removal percentage of cobalt and cesium than that of the binary solution. (9) This work recommends nano-cryptomelane as an effective sorbent material for the elimination of cobalt and cesium metalions from single and binary solutions. (10) Langmuir isotherm model fits the data reasonably well and it is more applicable than Freundlich isotherm. a Fig. 7 a Non-linear plot of pseudo-first-order kinetic model for sorption of Cs + and Co 2+ onto nano-cryptomelane at different temperatures. b Residual errors for non-linear regression of Kinetic data for cesium and cobalt in single and binary solutions in pseudo first order model.          Fig. 10 Non-linear plot of extended Langmuir isotherm plots for sorption of Cs + and Co 2+ onto nano-cryptomelane at different temperatures Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.