Chemistry Magazine Online gives you another post with interesting chemistry news articles. Abstracts d- MnO₂ was prepared with hydrothermal treatment, and its crystalline structure was verified with X-ray diffraction spectroscopy. FTIR showed there were copious hydroxyl groups on the surface of d-MnO₂ for the adsorption. The adsorption capacity of three kinds of heavy metal ions Pb²⁺, Cd²⁺ and Cu²⁺ on the d-MnO₂ was all increased with the increase of ion concentration and the increase of pH. The adsorption isotherms of Pb²⁺ exhibited good correlation with Freundlich equations, Langmuir equations and Temkin equations, whereas equilibrium data of Cu²⁺ was best fitted to Freundlich isotherm, and Langmuir isotherm was best for Cd²⁺. Adsorption thermodynamics indicates that as the temperature in the range of 298.15 ~ 323.15K, all the adsorptions were endothermic, spontaneous reaction. Therefore, increasing temperature is good for the adsorption, and the higher the temperature, the higher the spontaneity. The competitive adsorption of binary ions indicated that d-MnO₂ showed “selective adsorption” to all three ions, Pb²⁺, Cd²⁺ and Cu²⁺. With the co-existence of another ion, the adsorption capacity of all three ions decreased, which means the competitive adsorption took place by the co-existence ion. Different ion pair showed different competitive behavior, among which Pb²⁺ exhibited the highest adsorption compatibility.
Keywords d-Manganese oxide, Lead, cadmium, copper, Adsorption, Competitive Adsorption

1. INTROUCTION
   Among all the ores in water and soil in the nature, the most common one is metal oxides, such as Fe₂O₃�qnH₂O, 2Fe₂O₃�qH₂O, Al₂O₃�qH₂O, Al₂O₃�q3H₂O andd-MnO₂, etc. _.Due to some of their important properties such as large specific surface area, plentiful superficial functional groups and superior charge convertibility, these oxides show intense chemical reactivity with heavy metal ions. Therefore their existence is a main factor in affecting the migration, transformation and inclination of heavy metals and organics in our environments.
   Manganese oxides are able to absorb heavy metal ions, and they are also the natural oxidizing agents for certain ions such as As³⁺, Cr³⁺. They not only affect the effectivity and toxicity of some nutritious elements and pollutants, and the fate of formation and transformation of some organics; but also play an important role in adjustment of the concentrations of heavy metal ions in soil and waters in our environment^[1,2].
   There are many reports on the investigation of the adsorption of heavy metal ions with the metal oxides. Forbes ^[3] studied the factors affecting the adsorption of heavy metal ions on Fe₂O₃�qH₂O , and found that the adsorption of ions were increased with the increase of pH. Although the experimental pH values were all less than the ZPC values (pH > 8.2) of Fe₂O₃�qH₂O , different heavy metal ions showed different pH values for initial adsorption and maximum adsorption. Sun et al.^[4] investigated the adsorption of mercury ion on Fe₂O₃�qH₂O , and found that with the increase of pH, the absorbance of Fe₂O₃�qH₂O to mercury increased rapidly at first, reached the maximum, and then decreased gradually, which was totally different with the adsorptive behavior of many other heavy metal ions. With the existence of chlorides, the adsorption of mercury ions was less than that without chlorides, and exhibited higher pH values for maximum adsorptions.
   The report of Masanori^[5] indicated that both of amorphous iron oxide hydrates and the iron oxides with low crystalline showed relatively high absorption capacity, which could be related to pH of the solution, ion strength and ion concentration. And with the increase of pH, the adsorption of Cu²⁺、Zn²⁺、Mg²⁺on iron oxides increased, with the order as Cu²⁺>Zn²⁺ >Mg²⁺.
   Zhu et al.^[6] investigated the ion exchange reaction between metal copper ion and d-MnO₂ particles with pH curves and exchange isotherms. They found that the ion exchange process comprised the replacement of the univalent copper ion for the H⁺ of hydroxyl groups on the d-MnO₂ surface, and it was a multi – coordination exchange reaction. The mechanism and process were analyzed and verified with the theory of multi-coordination exchange. The three complex equilibrium constants were also determined.
   The results of investigation of Randall S ^[7] for the sorption of manganese potassium mineral materials to Cd²⁺ showed that at pH = 2.0, 2/3 of Cd²⁺ in the solution could be absorbed by the manganese potassium mineral material. The absorbed Cd²⁺ was mainly located in the cavities which were not occupied by K⁺. The decreasing of pH values in the adsorption process also indicated that Cd²⁺ mainly exchanged with H⁺. Loganathan P & Burau RG ^[8] studied the sorption of synthesized d-MnO₂ to Zn、Cu、Ni、Co at T = 24.0±0.5°C and pH = 4. Their results revealed that there were ion exchanges on the surface and also throughout the specimens. All the metal ions exchanged with H coupled groups on the surface. The heavy metal ions Zn, Cu, Ni, and Co were even able to exchange with Mn²⁺ located in the unordered layers. Moreover, Co could exchange with Mn³⁺ located in the unordered layers, with the transformation of electrons and the release of Mn ions. X-ray diffraction spectrum indicated that the crystalline structure of d- MnO₂ was unchanged after the adsorption of Co and Zn.
   In order to explain the mechanisms of adsorption on metal oxides and to quantitatively describe the process of adsorption, many scholars ^[9-11] proposed a variety of sorption models, such as surface complexion model, the valence theory of the affinity of the reactive groups on the metal hydroxides surface, the sub-stable equilibrium adsorption theory, and the forecast of adsorptive behavior by thermodynamic calculation combined with proton theory. All these models from different view of points explained the adsorptive behavior of metal oxides in some extent, however, there are still some problems remained^[12]. For example, these models are unable to describe the exact situation of adsorbed ions on the surface of metal oxides, and the errors between the models and experimental values are quite large^[13].
   d- MnO₂ is one kind of important manganese oxides. Due to its poor crystalline, large specific surface area, excessive adsorption space and copious hydroxyl groups, d- MnO₂ shows excellent adsorption capacity to many pollutants. In this paper, the adsorption capacity, adsorption thermodynamics and the competitive adsorption of some heavy metal ions such as Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ are investigated. The adsorption mechanism is also explored. You might be interested in reading some chemistry related news with a topic – Determination of resorcinol on glassy carbon electrode modified by carbon nanotube and its application in wastewater analysis.
2. EXPERIMENTAL
   2.1 Preparation and Characterization ofd-MnO₂
   2.1.1 Preparation of d-MnO₂
   KMnO₄ and MnSO₄ were mixed at the molar ratio of 6:1 and were dissolved in distilled water. The mixture was added into a PTFE reactor. The hydrothermal treatment was performed at the temperature of 160 °C and under the pressure of 2.45 ~2.94 MPa for 24 h. After the reaction, the mixture was filtered. The solid part was rinsed with distilled water repeatedly and was dried and ground for further use.
   2.1.2 Characterization of d-MnO₂
   The specimen was analyzed with DX-2000 X-ray diffraction spectrometer. Data was also recorded on BRUKER FTIR spectroscopy with KBr pellet method.
   2.2 Adsorption thermodynamics of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂
   0.01g d-MnO₂ was weighed and was put into centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. Certain amount of PbCl₂ standard solutions，CdCl₂ standard solutions or CuCl₂ standard solutions was added. The concentration of Pb²⁺ were 0.242，0.435，0.580，0.725，0.870，1.014，1.159，1.304mmol L^-1 respectively. And the concentration of Cd²⁺ were 0.05，0.10，0.15，0.30，0.50，0.75，1.00，1.50mmol·L^-1and the concentration of Cu²⁺ were0.25，0.60，0.90，1.50，2.10，3.00，3.30mmol·L^-1 respectively. The final volume of the solutions was controlled to 30 mL. The specimens were oscillated at 25 °C、35°C、50°C for 24 h and were then centrifuged. Finally, the mixture was filtrated and the equilibrium concentrations of Pb²⁺、Cd²⁺、Cu²⁺ were measured by analyzing the liquid part with Agilent 3510 Atomic absorption spectrophotometer.
   2.3 Adsorption of Heavy Metal Ions on d-MnO₂
   2.3.1 Adsorption of Pb²⁺ on d-MnO₂
   8×0.01g d-MnO₂ was weighed and was put into centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. PbCl₂ standard solutions were added and the concentrations of Pb²⁺ were 0.242，0.435，0.580，0.725，0.870，1.014，1.159，1.304mmol L^-1 respectively. The final volume of the solutions was controlled to 30 mL. The specimens were oscillated at 25 °C for 24 h and were then centrifuged. Finally, the mixture was filtrated and the equilibrium concentrations of Pb²⁺ were measured by analyzing the liquid part with Agilent 3510 Atomic absorption spectrophotometer.
   2.3.2 Adsorption of Cd²⁺ on d-MnO₂
   8×0.01g d-MnO₂ was weighed in centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. CdCl₂ standard solutions were added and the concentrations of Cd²⁺ were 0.05，0.10，0.15，0.30，0.50，0.75，1.00，1.50mmol L^-1 respectively. The final volume of the solutions was adjusted to 30 mL. The specimens were oscillated at 25 °C for 24 h and were then centrifuged. The mixture was filtrated and the liquid part was analyzed with Agilent 3510 Atomic absorption spectrophotometer.
   2.3.3 Adsorption of Cu²⁺ on δ-MnO₂
   7×0.01g d-MnO₂ was weighed in centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. CuCl₂ standard solutions were added and the concentrations of Cu²⁺ were 0.25，0.60，0.90，1.50，2.10，3.00，3.30mmol L^-1 respectively. The final volume of the solutions was controlled to 30 mL. The specimens were oscillated at 25 °C for 24 h and were then centrifuged. The mixture was filtrated and the liquid part was analyzed with Agilent 3510 Atomic absorption spectrophotometer.
   2.3.4 Adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ at different pH
   0.01g d-MnO₂ was weighed in centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. Certain amount of PbCl₂ standard solutions，CdCl₂ standard solutions or CuCl₂ standard solutions was added and the concentration of Pb²⁺、Cd²⁺ and Cu²⁺ was controlled to 0.58mmol L^-1、0.10 mmol L^-1 and 0.60 mmol L^-1 respectively. pH values were regularized with 0.2 mol L^-1 KOH solution or with 0.2 mol L^-1 HCl solution. Therefore, a series of solutions with pH values in the range of 2 ~ 7 were prepared. The final volume of the solutions was controlled to 30 mL. The specimens were oscillated at 25 °C for 24 h and were then centrifuged. The mixture was filtrated and the liquid part was analyzed with Agilent 3510 Atomic absorption spectrophotometer.
   2.4 Competitive adsorption for binary ions
   0.01g d-MnO₂ was weighed in centrifugal tubes (50 mL) respectively. KCl (0.01 mol L^-1) was used as supporting electrolyte. PbCl₂ standard solutions were added and the concentration of Pb²⁺ was controlled to 1 mmol L^-1, and then Cu²⁺ or Cd²⁺ was added respectively. The concentrations of Cu²⁺ in above Pb²⁺-Cu²⁺ solutions were 0, 0.3, 0.6, 0.9, 1.5, 2.4, 3.0 mmol L^-1 respectively, whereas the concentrations of Cd²⁺ in the Pb²⁺-Cd²⁺ solutions were 0, 0.1, 0.15, 0.3, 0.5, 0.75, 1.0, 1.5 mmol L^-1. The solutions of Cu²⁺-Pb²⁺, Cu²⁺-Cd²⁺, Cd²⁺-Cu²⁺, and Cd²⁺-Pb²⁺ were prepared respectively with the same protocol. For Cu²⁺-Pb²⁺solutions, Cu²⁺ concentration was 1 mmol L^-1 and Pb²⁺ concentrations were 0, 0.2, 0.3, 0.5, 0.75, 1.0, 1.2, 1.5, 2.0 mmol L^-1 respectively. The concentrations of Cd²⁺ for Cu²⁺-Cd²⁺ solutions were 0,0.1, 0.15, 0.3, 0.5, 0.75, 1.0, 1.5 mmol L^-1. The concentrations of Cd²⁺ for Cd²⁺-Cu²⁺ solutions was 1 mmol L^-1 and the Cu²⁺ concentrations were 0, 0.3, 0.6, 0.9, 1.5, 2.4, 3.0 mmol L^-1. And the concentrations of Cd²⁺ for Cd²⁺-Pb²⁺ solutions was 1 mmol L^-1 and the Pb²⁺ concentrations were 0,0.2, 0.3, 0.5, 0.75, 1.0, 1.2, 1.5, 2.0 mmol L^-1.The final volume of the solutions was controlled to 30 mL. The specimens were oscillated at 25 °C for 24 h and were then centrifuged. The mixture was filtrated and the liquid part was analyzed with Agilent 3510 Atomic absorption spectrophotometer.
3. RESULTS AND DISCUSSION
   3.1 Characterization ofd-MnO₂
   The X-ray diffraction spectrum of d-MnO₂ was shown in Figure 1. From the figure we can see that there are three distinct peaks at 12.44°, 25° and 37°, with intensity as 720, 534 and 418, respectively. These data matches the standard spectrum very well. The diffraction peaks are relatively broad, which indicates that the grain size of d-MnO₂ crystals is quite small, and its crystalline capacity is poor. Thus d-MnO₂ can provide more surface area for adsorption.

Figure 1 X-ray diffraction pattern for d-MnO₂

FTIR spectrum for d-MnO₂ was shown in Figure 2. The absorbance around 505 cm^-1 (fingerprint district) corresponds to O-Mn-O characteristic stretching vibration, indicating the existence of MnO₂ crystalline lattice. The FTIR spectrum of δ-MnO₂ also showed typical absorbance in the range of 3600 ~ 3000 cm^-1, corresponding to the stretching vibration of H-O-H of water and hydroxyl groups, whereas the peak at 1624 cm^-1 corresponds to the bending of H-O-H. The simultaneous appearance of these peaks indicates the existence of bonded water for the unit cell of MnO₂ crystals.

Figure 2 FTIR spectrum of d-MnO₂

3.2 Adsorption thermodynamics of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂
The adsorption isotherms of Pb²⁺, Cd²⁺ and Cu²⁺ on δ-MnO₂ at 298.15K, 308.15K and 323.15K are shown in Figure 3, 4, and 5. From the figures, we can see that the adsorption of all three heavy metal ions is increased with the increase of temperature, indicating that all the adsorptions are endothermic reactions. Therefore, increasing temperature is helpful for the adsorption.

Figure 3 Adsorption isotherms of Pb²⁺ at different temperatures

Figure 4 Adsorption isotherms of Cu²⁺ at different temperatures

Figure 5 Adsorption isotherms of Cd²⁺ at different temperatures

Table 1 gives the Langmuir isotherm parameters of Pb²⁺, Cd²⁺ and Cu²⁺ resulting from the fitting of the Langmuir equation. The Langmuir parameters show that as temperature increases from 298.15K to 323.15K, the saturate adsorption G_max of Pb²⁺ is increased from 1.614 mmol g^-1 to 1.855 mmol g^-1, whereas G_max of Cu²⁺ increased from 1.484 mmol g^-1 to 1.626 mmol g^-1, and G_max of Cd ²⁺ is increased from 0.7407 mmol g^-1 to 0.9556 mmol g^-1. From Table 1 we can see that equilibrium constants are also increased with the increase of temperature. All of these data indicate that with the increase of temperature, the adsorptive affinity of δ-MnO₂ to Pb²⁺, Cd²⁺ and Cu²⁺ is increased, and so is the adsorption capacity.

Table 1 Langmuir isotherm parameters

Temperature (K)	Langmuir isotherm parameters
		（mmol·g-1）	K	R2
Pb2+	298.15	1.614	79.45	0.9600
	308.15	1.776	140.8	0.9522
	323.15	1.855	299.4	0.9961
Cu2+	298.15	1.484	51.82	0.9616
	308.15	1.544	68.91	0.9696
	323.15	1.626	70.95	0.9624
Cd2+	298.15	0.7407	52.73	0.9728
	308.15	0.8702	78.18	0.9813
	323.15	0.9556	109.0	0.9808

               [1]

         [2]

Table 2 Thermodynamic parameters

Temperature (K)	Thermodynamic parameters
		（KJ·mol-1)	（KJ·mol-1)	（KJ·mol-1·K-1)
Pb2+	298.15	-10.40	32.99	149.1
	308.15	-12.68
	323.15	-15.31
Cu2+	298.15	-9.617	6.397	55.47
	308.15	-10.39
	323.15	-11.46
Cd2+	298.15	-9.824	17.28	92.48
	308.15	-11.16
	323.15	-12.59

Table 3 Adsorption heat caused by different force (KJ mol^-1) ^[14]

Van der Waal	Hydrophobic groups	Hydrogen bonding	Coordinate groups	Dipole-dipole	Chemical bonding
4―10	5	2―40	40	2―29	>60

Thermodynamics parameters are also calculated from Gibbs equations (Equation [1] and [2]) and the results are listed in Table 2. All the DG°s are negative values; and the higher the temperature, the lower DG°. This indicates that all the adsorptions of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ are spontaneous reaction, and the higher the temperature, the higher the spontaneity. From Table 2 we can also find that all the DH°s are positive values, which once again verifies that all the adsorption are endothermic, and increasing temperature is good for adsorption. At the meantime, since adsorption enthalpy is also an important criteria for physical adsorption and chemical adsorption, from Table 2 and Table 3 we can tell that the adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ comprises physical adsorption and chemical adsorption simultaneously. And since the DH° value of Cu²⁺ is the smallest one, the extent of chemical adsorption of Cu²⁺ should be the least compared to those of the other ions.
3.3 Adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ 
The adsorption behavior of d-MnO₂ to Pb²⁺, Cu²⁺ and Cd²⁺ is described in Figure 6a, 6b and 6c. The results show that both the adsorption Pb²⁺ and Cu²⁺ on d-MnO₂ (Figure 6a and 6b) are well interrelated with the ion concentrations, i.e., the adsorption capacity is increased with the increase of ion concentrations. Both isotherms rise rapidly at first and then flat over, and reach saturation at last. The adsorption capacity of Cu²⁺ on d-MnO₂ is weaker than that of Pb²⁺, perhaps due to the fact that at the experimental conditions, Pb²⁺ is more easily hydrolyzed to form PbOH⁺ and coordinated with hydroxyl groups on d-MnO₂ surface.
The adsorption capacity of Cd²⁺ is less than those of Pb²⁺ and Cu²⁺ (Figure 6c). The adsorption reaches equilibrium quickly and the saturate adsorption is only around 0.80mmol·g^-1.

Figure 6a Absorption isotherm of d-MnO₂ to Pb²⁺

Figure 6b Adsorption isothermal of d-MnO₂to Cu²⁺

Figure 6c Adsorption isothermal of by d-MnO₂ to Cd²⁺ 

Table 4a Isotherm constants and related coefficient for adsorption of Pb²⁺

Equations	k	Gm	n	a	R2
Langmuir	0.412	294.18	―	―	0.9533
Freundlich	148.56	―	4.7778	―	0.9672
Temkin	119.51	―	―	136.58	0.9937

Table 4b Isotherm constants and related coefficient for adsorption of Cu²⁺

Equations	k	Gm	n	a	R2
Langmuir	15.50	99.01	―	―	0.8888
Freundlich	33.07	―	4.95	―	0.9514
Temkin	32.06	―	―	20.86	0.9025

Table 4c Isotherm constants and related coefficient for adsorption of Cd²⁺

Equations	k	Gm	n	a	R2
Langmuir	1.769	74.627	―	―	0.9504
Freundlich	41.049	―	7.364	―	0.8401
Temkin	23.526	―	―	30.108	0.8520

The experiment data was also fitted with Langmuir equation (1/G = 1/G_m + (1/G_mk) × (1/c)), Freundlich equation (lgG = (1/n) lgc + lgk) and Temkin equation (G = a + k logc), and the results were shown in Table 4a, 4b and 4c. We find that Temkin model is the best fitted equation for adsorption of Pb²⁺, whereas Langmuir equation matches adsorption of Cd²⁺ very well, and Freundlich equation is the best fitted one for Cu²⁺. By comparing the isotherm parameters of Langmuir equation, we can see that Pb²⁺ has the highest adsorption capacity among the three ions.

3.4 Adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ at different pH
The effect of pH on the adsorption behavior of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ is shown in Figure 7a, 7b and 7c respectively. With the increase of pH values, the adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ is all increased gradually; at pH = 4~5, the adsorptions almost reach saturation. The reason for low adsorption capacity at low pH is mainly due to the fact that the competitive adsorption of H⁺ inhibited the complexion of the heavy metal ions with d-MnO₂. With the increase of pH, the concentration of H⁺ is decreased and the selective adsorption of δ-MnO₂ to the heavy metal ions is intensified ^[15,16]. Meanwhile, with the increase of pH, the negative charges on the surface of manganese oxide hydrate become copious, and the exclusive adsorption of the heavy metal ions becomes steadier. Thus pH should be considered a main factor which can affect the adsorption of heavy metal ions onto manganese oxide.

Figure 7a Absorption of Pb²⁺ onto d-MnO₂ at different pH

Figure 7b Adsorption of Cu²⁺ onto d-MnO₂ at different pH

Figure 7c Adsorption of Cd²⁺ onto d-MnO₂ at different pH

3.5 Competitive adsorption of binary ions
By setting the Cu²⁺ concentration as a constant and changing the concentrations of Pb²⁺ in the Cu²⁺– Pb²⁺ binary component solutions, the effect of Cu²⁺ on the adsorption of Pb²⁺ on d-MnO₂ is described in Figure 8a. Similarly, by setting the Pb²⁺ concentration as a constant and changing the concentrations of Cu²⁺ in the Pb²⁺– Cu²⁺ binary component solutions, the effect of Pb²⁺ on the adsorption of Cu²⁺ on d-MnO₂ can also be investigated (Figure 8b). It is obvious that because of the co-existence of Pb²⁺, the adsorption of Cu²⁺ was decreased greatly. And the co-existence of Cu²⁺ can also inhibit the adsorption of Pb²⁺, but not as effective as the effect of Pb²⁺ to Cu²⁺.

 

Figure 8 Binary component competitive sorption of Pb²⁺and Cu²⁺ (a) adsorption isotherm of Pb²⁺ affected by Cu²⁺; (b) adsorption isotherm of Cu²⁺ affected by Pb²⁺.

The effect of co-existence of Cd²⁺ to Cu²⁺, and that of Cu²⁺ to Cd²⁺ was obtained at the same time with the aforementioned method and the results are described in Figure 9a and 9b. The figures show that due to the co-existence of competitive ion, the adsorption capacity is decreased. With or without the co-existence of Cd²⁺, the adsorption of Cu²⁺ was double of the adsorption of Cd²⁺. However, the competitive adsorption capacity of Cd²⁺ to Cu²⁺ is much higher than that of Cu²⁺ to Cd²⁺.

Figure 9 Binary component competitive sorption of Cu²⁺and Cd²⁺ (a) adsorption isotherm of Cu²⁺ affected by Cd²⁺; (b) adsorption isotherm of Cd²⁺ affected by Cu²⁺

Figure 10a and 10b give the adsorption isotherms of Pb²⁺ with/without Cd²⁺ and the adsorption isotherms of Cd²⁺ with/without Pb²⁺. The figures show that at low concentrations of the metal ions, the co-existence of Pb²⁺ causes the adsorption of Cd²⁺ decrease much more than that of the adsorption of Pb²⁺ with the co-existence of Cd²⁺, whereas at high concentrations of the metal ions, the effect of Cd²⁺ to Pb²⁺ is almost the same as that of Pb²⁺ to Cd²⁺. To get the most interesting chemistry news articles, follow chemistrymag.org.

Figure 10 Binary component competitive sorption of Pb²⁺and Cd²⁺ (a) adsorption isotherm of Pb²⁺ affected by Cd²⁺; (b) adsorption isotherm of Cd²⁺ affects by Pb²⁺

4. CONCLUSIONS
   1) The adsorption of Pb²⁺, Cd²⁺and Cu²⁺on d-MnO₂ is increased with the increase of ion concentrations. At low ion concentration, the adsorption isotherms rise rapidly. With the increase of concentration of ions, the curves flat over and the adsorption reach saturation. At equilibrium, the rank of adsorption capacity of the three heavy metal ions on d-MnO₂ is: G_max(Pb²⁺) > G_max(Cu²⁺)> G_max(Cd²⁺).
   2) The adsorption of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ is all increased with the increase of pH. Thus pH value is a main factor which affects the adsorption of the heavy metal ions on δ-MnO₂.
   3) The adsorption isotherms of Pb²⁺, Cd²⁺ and Cu²⁺ show good correlations with Freundlich equation, Langmuir equation and Temkin equation respectively, among which Temkin equation fitted for adsorption isotherm of Pb²⁺ best, Freundlich equation fitted best for Cu²⁺, and Langmuir equation gives the best fitting for Cd²⁺. These indicate that the adsorption of Pb²⁺ and Cu²⁺ on d-MnO₂ shows more physical-chemical adsorptive behavior, whereas the adsorption of Cd²⁺ shows more chemical adsorptive behavior.
   4) Thermodynamic studies indicate the adsorptions of Pb²⁺, Cd²⁺ and Cu²⁺ on d-MnO₂ are endothermic reactions, and thus increasing temperature is helpful for the adsorption. The adsorptions are also spontaneous reactions, and the higher the temperature, the higher the spontaneity.
   5) The competitive adsorption of binary ions indicates that d-MnO₂ shows “selective adsorption” to all three ions, Pb²⁺, Cd²⁺ and Cu²⁺. With the co-existence of another ion, the adsorption capacity of all three ions decreased, which means the competitive adsorption take place by the co-existence ion. Different ion pair shows different competitive behavior, among which Pb²⁺ shows the highest adsorption compatibility.

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