Comparison of the experimental results with the Langmuir and Freundlich models for copper removal on limestone adsorbent
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The purpose of this study was to investigate the possibility of the limestone as an adsorbed media and low-cost adsorbent. Batch adsorption studies were conducted to examine the effects of the parameters such as initial metal ion concentration C0, particle size of limestone DL, adsorbent dosage and equilibrium concentration of heavy metal Ce on the removal of the heavy metal (Cu) from synthetic water solution by limestone. The removal efficiency is increased with the increase in the volume of limestone (influenced by the media specific area). It has been noted that the limestone with diameter of 3.75 is the most effective size for removal of copper from synthetic solution. The adsorption data were analyzed by the Langmuir and Freundlich isotherm model. The average values of the empirical constant and adsorption constant (saturation coefficient) for the Langmuir equation were a = 0.022 mg/g and b = 1.46 l/mg, respectively. The average values of the Freundlich adsorption constant and empirical coefficient were Kf = 0.010 mg/g and n = 1.58 l/mg, respectively. It was observed that the Freundlich isotherm model described the adsorption process with high coefficient of determination R2, better than the Langmuir isotherm model and for low initial concentration of heavy metal. Also, when the values of amount of heavy metal removal from solution are predicted by the Freundlich isotherm model, it showed best fits the batch study. It is clear from the results that heavy metal (Cu) removal with the limestone adsorbent appears to be technically feasible and with high efficiency.
KeywordsLimestone Heavy metal (Cu) Langmuir Freundlich Batch studies
Removal of heavy metals from aqueous solution is necessary because of the frequent appearance of these metals in waste streams from many industries, including electroplating, metal finishing, metallurgical, tannery, chemical manufacturing, mining and battery manufacturing. In recent years, this problem has received considerable attention, because available heavy metals in the waste streams can be readily adsorbed by marine animals which directly enter the human food chain presenting a high health risk to consumers.
Tito et al. (2008) investigated the removal of Zn2+ from aqueous solution by using bentonite clay. The results of his study showed that Langmuir equation is adequate to describe zinc adsorption at different pH values and particles size of bentonite clay. Zinc adsorption in bentonite clay depends on both pH and particle size, where it decreases as they increase. The maximum retention capacity, 3.24 mg/g, was obtained in pH 4 using a particle size of bentonite clay less than 0.5 mm.
Mohan and Chander (2006) investigated the removal of Fe(II) from aqueous media by using chitosan and the adsorbent in both batch and continuous system. Batch experiments were carried out at initial concentration range of 10-50 mg/l and temperature range of 20–40 °C. Adsorption equilibrium data were well-fitted with Langmuir–Freundlich model, and the model parameters were recovered. The results of the study in batch experiments showed that the maximum adsorption capacity of 28.7 mg/g and removal efficiency of 93% was obtained.
Abu El Hassan and Sawsan (2014) showed the removal of some heavy metals ions (Hg2+, Pb2+ and Zn2+) from aqueous solution and (Mn2+, Ca2+) ions by adsorption process. The commercial activated carbon, silica and ceramic were used as adsorbents. The adsorption process was carried out at a pH which ranges from 5.3 to 5.5 at room temperature. The results of this work showed that 100% adsorption uptake was obtained in some cases.
Li (2008) conducted three separate experiments to assess heavy metal removal from metal aqueous solutions and synthetic landfill leachate by adsorption using low-cost natural adsorbents. Fundamental batch investigations indicated that the 4.0–4.75 mm crushed mollusk shells and the Sphagnum peat moss were the best adsorbents for cadmium and nickel removal, respectively. The results indicated that 47.9% and 42.7% cadmium and nickel removal efficiencies could be obtained under these operational conditions, respectively. The flow rate applied in this operation was 1.5 ml/min (surface loading of 27.52 cm3/cm2 day). Peat was found to have the best adsorption capacities in columns treating aerated synthetic leachate for cadmium (78.6%) and nickel (83.8%) removal efficiencies.
The objective of this study is to evaluate the efficiency of limestone adsorbent to be as adsorbed media to adsorb heavy metal (Cu) from aqueous solution as alternative media of the existing commercial adsorbents.
Adsorption isotherm and sorption efficiency
Two adsorption isotherm models used frequently are the Langmuir and Freundlich.
Because adsorption isotherms at very low solute concentrations are often linear, either the Freundlich isotherm with (1/n) equaling (1) or the Langmuir isotherm with (bCe) much greater than 1 fits the data. The value of (1/n) for the adsorption of many radionuclides is often significantly different from 1, such that nonlinear isotherms are observed as in Wilhelm (1999).
Describing the Kd in terms of this simple reaction assumes that q is in great excess with respect to Ce and that the activity of qe is equal to 1. The Kd term is valid only for a particular adsorbent and applies only to those aqueous chemical conditions (e.g., adsorbate concentration, solution/electrolyte matrix, temperature) in which it was measured. Also inherent in the Kd term are the assumptions that the system is reversible and is independent of the adsorbate concentration in the aqueous phase.
Materials and methods
Commercially, limestone types used to conduct experiments are obtained from Al-Anbar city which is located (110) km west of Baghdad—Iraq. In this research, two types of limestone have been used, western red and northern white in three different sizes which are classified according to the experience of the sieve analysis conducted on them. As a result of the sieve analysis tests conducted on the limestone, the adopted three sizes of diameter of limestone are 3.75 mm (western red) and 5.0, 9.5 mm (northern white), respectively, which were used in this research.
Different sizes of roughing filter media as in Wegelin (1996)
Roughing filter description
First compartment (mm)
Second compartment (mm)
Third compartment (mm)
Experimental conditions for the limestone used in the present study
Western Red No. 2
Northern White No. 2
Northern White No. 3
Solid weight (g)
(Solid + water) weight(g)
Water weight (g)
Standard specification for limestone dimension stone as in Ref. ASTM C568
As a result of the density analysis test conducted on the chosen limestone and compare it with ASTM C568, “Standard Specification for Limestone Dimension Stone,” it is shown that the used limestone has been within the limited standard (low density).
Chemical characterization of the limestone used in the present study
Loss on ignition (LOI) (%)
Classification of limestone by calcium carbonate content as in Bloodworth (2002)
Very high purity
Copper solution preparation
A synthetic water has been prepared by mixing the effluent from the water tank with the highest solubility copper compound (copper nitrate) in three different concentrations (40 mg/L, 24 mg/L and 8 mg/L). A synthetic wastewater has been prepared by dissolving (1, 3 and 5) g of copper nitrate into 125 l of water tank and mixed well for a homogeneous solution and to ensure melting of copper nitrate in water. Atomic absorption spectroscopy (AAS) was used for the quantitative determination of chemical elements employing the absorption of optical radiation (light) by free atoms in the gaseous state. In analytical chemistry, the technique is used for determining the concentration of a particular element (Cu) in a sample to be analyzed. AAS can be used to determine over 70 different elements in solution or directly in solid samples.
The batch study has been conducted to establish the removal pattern of heavy metals using limestone. In this experiment, different volumes of limestone calculated based on the weight 20, 60, 100, 140 and 180 g are used in a specific volume of heavy Cu solution (120 ml of synthetic Cu solution) which are kept in polyethylene bottles. The experiment has been conducted at different Cu concentrations (40, 24, and 8 mg/L), which is shaken by an orbital shaker at 300 rpm for 60 min, which it allows for all the surface area of the adsorbent to come in contact with the model water containing heavy metals. Afterward, the solution has then been left to settle for 90 min before testing Cu concentration by an atomic absorption spectrophotometer.
Results and discussion
Effect of volume of limestone on removal efficiency of copper
A trend of increment in efficiency capacity with increment in adsorbent dosage is observed, and the maximum efficiency is at weight of 180 g used in this experiment. The increment in adsorption capacity with increase in adsorbent dosage has been expected, since number of adsorbent particles increases, and thus, more surface areas were available for metals attachment. It is plausible to suggest that with higher dosage of adsorbent, there would be great availability of exchangeable sites for metal ions.
Effect of volume of limestone on effluent copper concentration
Effective size of limestone in batch studies
Ranges of removal efficiency for different sizes of limestone and influent copper concentrations
Influent copper concentration (mg/L)
Calculated values for the different initial concentrations and Limestone sizes to plot Langmuir adsorption isotherm
Limestone size (mm)
Y = 49.52X + 11.52
Y = 23.26X + 71.93
Y = 5.904X + 342.9
Y = 27.90X + 80.34
Y = 69.49X + 46.26
Y = 94.15X + 135.5
Y = 68.18X + 73.63
Y = 55.63X + 83.70
Y = 136.80X + 100.20
Calculated values for the different initial concentrations and limestone sizes to plot Freundlich adsorption isotherm
Limestone size (mm)
Y = 1.055X − 3.950
Y = 0.761X − 4.051
Y = 0.396X − 5.125
Y = 0.546X − 4.661
Y = 0.853X − 4.641
Y = 0.907X − 4.975
Y = 0.396X − 5.031
Y = 0.572X − 4.954
Y = 0.943X − 5.189
The Freundlich isotherm only applies to data obtained at low values of Ce (concentration of contaminant in the equilibrium solution). Mckay et al. (1982) indicated that the value of n between 2 and 10 is a good adsorption. The calculated n value for the adsorption of copper was 1.58 showing good efficiency for copper adsorption by limestone adsorbent.
The removal efficiency of (Cu) is increased with the increase in the volume of limestone. It has been influenced by the media specific area or the surface area.
The most effective size of limestone for removal of copper from synthetic solution is with a diameter of 3.75 mm.
There is a significant difference in removal rate (90–99%) with decreasing copper concentration from 7.04 to 1.72 ppm.
The average values of the empirical constant and adsorption constant (saturation coefficient) for the Langmuir equation were a = 0.022 mg/g and b = 1.46 l/mg, respectively.
The average values of the Freundlich adsorption constant and empirical coefficient were Kf = 0.010 mg/g and n = 1.58 l/mg, respectively.
It was observed that the Freundlich isotherm model described the adsorption process with high coefficient of determination R2, better than the Langmuir isotherm model and for low initial concentration of heavy metal.
When the values of amount of heavy metal removal from solution are predicted by the Freundlich isotherm model, it showed best fits the batch study.
It is clear from the results that heavy metal (Cu) removal with the limestone adsorbent appears to be technically feasible and with high efficiency.
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