Lead is among the most toxic heavy metal ion affecting the environment [1] . It may come into waterways through the combustion of fossil fuels and the smelting of sulphide ore, and into lakes and streams by acid mine drainage. Process industries, such as battery manufacturing and metal plating and finishing are also prime source of lead pollution.

Lead accumulates mainly in bones, brain, kidney and muscles and may cause many serious disorders like anaemia, kidney diseases, nervous disorders and sickness even death. In 2001, EPA published Identification of Dangerous Levels of Lead [2] and Identifying Lead Hazards in Residential Properties [3] . The World Health Organization (WHO) stated a legal limit of 50 ppb level for lead in 1995, which is decreased to 10 ppb level in 2010. The stringent requirement for lead in aquatic environment has triggered some wastewater engineer to work harder in searching for inexpensive material and technology to uptake lead pollutants from water and wastewater.

At low concentration of heavy metal concentration adsorption process is becoming an important pollutant removal method in water and wastewater treatment technology. After the revelation that activated carbon can adsorb organic and inorganic contaminants the advent of new technology is shifting towards sustainable technology that use more economic and environmentally friendly materials as a replacement of activated carbon.

The re-use of natural waste materials that arise through various industrial processes for additional purposes, rather than simple disposal, makes both environmental and commercial sense [4] . There also has been a recent focus on agricultural and food industry wastes such as tea, coffee grounds and rice hull as alternatives to mem- brane filtration, synthetic ion-exchange resins or activated carbon for treating heavy metal-containing waste waters [5] [6] . Tannin-containing materials such as exhausted coffee contain metal-binding polyhydroxy poly- phenol functional groups and they are available in large quantities from the manufacture of instant coffee [4] [7] . While there have been several studies of metal ion adsorption by tea and coffee the detailed chemistry causing their affinity for different metal ions is not yet well-known [8] [9] .

Adsorption of metal ions and other solutes on solid surfaces can be described by simple isotherms such as the Langmuir or Freundlich equations [9] [10] . In this approach, the extent of adsorption of lead ion (Pb²⁺) per unit mass of solid adsorbent is described by simple isotherm equations that are based on conceptual models similar to those used for homogeneous equilibria. For this study the experimental data was applied to Langmuir isotherm model only due to the consistency in explaining the existence of particle concentration effect in the previous study [11] . In the Langmuir model the adsorption of a solute M by a surface S− is considered to be a result of the equilibrium surface reaction

where [M − S] is the concentration of adsorbed solute M, C_e is the concentration of unadsorbed M and [S−] is the concentration of free surface sites, all at equilibrium. This formalism gives rise to the Langmuir adsorption isotherm

where G = [M − S] is the adsorption density, i.e. the concentration of adsorbed M per unit mass of solid and G_m is the maximum concentration of adsorbed M.

In Equation (2), the adsorption properties are inherently described by the equilibrium constant in Equation (1) which may be identified with the free energy of adsorption of M by the solid. Normally, K and G_m may be determined simultaneously from measurements of G and C_e under different conditions.

This batch adsorption study was meant to be a comparison study set up using different concentration of adsor- bents at various concentration of Pb²⁺. Granulated Activated Carbon (GAC) and nano metal oxide of MnO₂ were used as comparisons to agricultural by-product materials of tea leaves and coffee residues.

In most experiments each set of Pb²⁺ concentrations ranging from 19 to 291 µmol∙L⁻¹ were equilibrated with 3 (three) different concentration of adsorbent C_s except for coffee residue which used a wider range of concen- tration as detailed in Table 1.

More variations of coffee grounds concentration were used to clearly discuss a particle concentration effect found in this particular adsorption study. In this case coffee ground concentration used was extended from the concentrations of 0.971, 2.913, 4.369, 5.825 and 8.738 g∙L⁻¹. The result was expected in such cases that different concentrations of each adsorbent C_s equilibrated with Pb²⁺ ranging from 19 to 291 µmol∙L⁻¹ fit into a single Langmuir model, i.e. single values of K and Γ_m for each type of adsorbent.

Figure 1 shows the adsorption result at different MnO₂ solid concentration C_s. The calculated curves of Γ versus C_e that resulted from this fitting are also shown in the figure. The curve was calculated using a fixed value of K = 0.0119 L∙µmol⁻¹ and a fixed value of Γ_m = 528.0 µmol g⁻¹ for all C_s. The result shows that a single Langmuir equation adequately describes the data for the 3 different solid concentrations C_s. This implies that Pb²⁺ adsorption on MnO₂ does obey the Langmuir Equation (2).

Adsorption of Pb²⁺ on GAC in aqueous solution was investigated using the procedure described above for MnO₂ at the same concentration of solid C_s as used fortea leaves in the range 2.913 - 8.738 g∙L⁻¹. The result is shown in Figure 2 with the curve was calculated using a fixed value of K = 0.0158 L∙μmol⁻¹ and a fixed value of Γ_m = 134.0 μmol∙g⁻¹ for all C_s. Again in this case, the results are reasonably well-described by a single Langmuir equation for different values of the solid concentrations C_s. This implies that Pb²⁺ adsorption on GAC does obey the Langmuir Equation (2).

Figure 3 shows the adsorption results for Pb²⁺ at different coffee solid concentrations C_s. The results are presented as the adsorption density Γ as a function of the measured concentration of unadsorbed metal ion C_e. For a given value of solid concentration C_s, the data were fitted to the linearized form of Langmuir isotherm, Equation (2), using least-squares regression; the calculated curves are shown in the figure. Separate estimates of K and Γ_m values were obtained for each C_s value, with results shown in Table 2.

The results shown in Figure 3 were very different from the previous two results, when using GAC and MnO₂ as adsorbents. The results revealed that while the Langmuir Equation (2) adequately describes the adsorption equilibrium for a given C_s value, the best-fit values of both K and G_m vary with C_s. This is not consistent with the simple Langmuir equation, which should give the same values of both parameters at different adsorbents con- centrations.

A number of authors had reported that the parameters K and/or G_m could vary when measurements weremade at different concentrations of solid. Examples included quinoline adsorption by calcium montmorillonite [12] , 2,2-bipyridine onto Na⁺-kaolinite [13] , phosphate by hydrous aluminium oxides [14] and zinc by goethite [15] [16] . This dependence on C_s violates the simple assumptions of the Langmuir theory.

Macchi [10] reported that the adsorption of Hg²⁺ by coffee grounds shows anomalous behaviour when the solids concentration C_s is varied. However, they did not attribute this to an effect of C_s on adsorption, but on the competitive effect of soluble organic substances leached out of the coffee. In fact the particle concentration ef- fect on coffee grounds, when adsorbing heavy metal ions, was affected by particle aggregation [11] .

The adsorption results using tea leaves adsorbent show different behaviour from that showed with coffee residue in Figure 3. There is very much less dependence of the adsorption curve on the concentration of solid. As shown in Figure 4, a single Langmuir model using fixed values of K = 0.0119 L∙μmol⁻¹ and Γ_m = 5.0

μmol∙g⁻¹ for all C_s values fitted the data reasonably well, given the scatter of the points at 2.913 g/L. This sug- gests that lead adsorption on tea leaves approximately follows the simple Langmuir equation.

Pb²⁺ adsorption at low concentration onto various solid surfaces using either nano metal oxide of MnO₂, or granulated activated carbon (GAC) or agricultural by-products such as tea leaves and coffee residue is unarguably promising.

Comparing all the adsorption capacities calculated using single Langmuir equation the Pb²⁺ adsorption using nano metal oxide of MnO₂ shows the highest adsorption capacity with the estimated Γ_m = 528.0 μmol∙g⁻¹ at a fixed equilibrium constant K = 0.0119 L∙μmol⁻¹. This could be due to the largest surface area per mass of MnO₂.

The Pb²⁺ adsorption by coffee residue is subject to “particle concentration effect” in which the adsorption density decreases as the concentration of solid adsorbent C_s is increased. The Pb²⁺ adsorption onto tea leaves, MnO₂ and GAC shows less dependency to the concentration of solid adsorbent C_s, especially at lower metal ion concentrations. In the particular case of Pb²⁺ adsorption on MnO₂ there appears to be no dependence on C_s and the single Langmuir equation is followed.

The laboratory experiment was conducted at Marine and Freshwater Chemistry Laboratory, University of Otago, New Zealand where the author was receiving the University of Otago, Division of Science Award. Preparation of manuscript writing and presentation was supported by Singapore Polytechnic (TIEFA) Vote Number: 11- 27801-36-R185. The author acknowledged Professor Keith A. Hunter for his assistance in developing the non- linear Langmuir isotherm model using NLReg special software.

Handojo DjatiUtomo,11, (2015) Lead Adsorption onto Various Solid Surfaces. Natural Resources,06,152-158. doi: 10.4236/nr.2015.63014