Cornulaca monacantha (CM) a desert plant has been utilized as adsorbent material for the removal of Hg (II) ions from contaminated water after treatment with acrylamide in alkaline medium to form carbamoylethylated Cornulaca monacantha (CECM). Three levels of CECM having different nitrogen content were prepared. The CECM samples were characterized by estimating the nitrogen content. The ability of CECM to adsorb Hg (II) was investigated by using batch adsorption procedure. The data of the adsorption isotherm was tested by the Langmuir, Freundlich and Temkin models. The removal of Hg (II) onto CECM particles could be well described by the pseudo-second order model. The adsorption rate of mercury was affected by the initial Hg (II) ion concentration, initial pH, adsorbent concentration and agitation time as well as extent of modification. The adsorption experiments indicated that the CECM particles have great potential for the removal of Hg (II) from contaminated water. The maximum adsorption capacity, Qmax of the CECM towards Hg (II) ions was found to be 384.6 mg/g at 30 ?C. Similarly, the Freundlich constant, n was found to be 2.03 at 30°C.
The industrial activity is responsible to generate a large volume hazardous containing effluents [1,2]. These hazardous effluents need to be treated before being delivered into the environment.
Mercury can be found in significant amounts in wastes from chloro-alkali manufacturing plants, electrical and electronics manufacturing, and sulfide ore roasting operations. Mercury is the most harmful to humans, plants and animals. Exposure to mercury can have toxic effects on reproduction, the central nervous system, liver, and kidney, and cause sensory and psychological impairments [
There are various established methods for the removal of heavy metals. Generally, the techniques employed include reduction and precipitation [
The adsorption process [
The present work was directed at improving the capability of CM residues for removing Hg (II) ions from aqueous solutions. To achieve this goal, the following studies were undertaken: 1) preparation of three levels of CECM with various nitrogen content using acrylamide in alkaline medium; 2) establishment of the conditions under which the maximum adsorption of Hg (II) ions onto CECM occur and 3) evaluation of the kinetics for Hg (II) ion adsorption onto CECM.
Bio-dsorbent, Cornulaca monacantha, a desert plant is widely spread in Kingdom of Saudia Arabia. There is no previous report used, Cornulaca monacantha as adsorbent material for removal of heavy metals. The roots were separated from the stems and leaves, washed with distilled water several times to remove the surface adhered particles and water soluble particles and dried in an electric oven for 24 h and ground using a mixer, and sieved to pass through a 100 - 200 mm. The roots were chosen because they contain the highest percentage of the cellulose content.
Mercuric acetate, EDTA, ethanol, sodium carbonate, and acetic acid were of analytical reagent grade supplied by Merck Company, Germany.
Three levels of CECM with various nitrogen contents were prepared by keeping other reaction conditions constant and varying the amount of acrylamide according to a reported method [
A volume of 100 ml of Hg (II) solution with a concentration in the range 100 - 700 mg/l was placed in a 125 ml Erlenmeyer flask. An accurately weighed adsorbent sample 0.05 g was then added to the solution. A series of such Erlenmeyer flasks were then shaken at a constant speed of 150 rpm in a shaking water bath with temperatures 30˚C. After shaking the flasks for 2 h, the adsorbent was separated by filtration. The filtrate was analysed for the remaining metal ion concentration by titration against standard EDTA solution. The amount of adsorbed metal, qe (mg/g) on adsorbent were calculated according to the following equation:
where, Co and Ce are the initial and final concentrations of metal ion, mg/l. V is the volume of metal ion (l), W is the weight of adsorbent (g).
The nitrogen content of CM and CECM samples was determined using the micro-Kjeldahl method [
where, V is the volume of HCl consumed;
N is the normality of standard HCl;
W is the weight of the sample (g).
In the single-component isotherm studies, the optimization procedure requires an error function to be defined to evaluate the fit of the isotherm to the experimental equilibrium data. The common error functions for determining the optimum isotherm parameters were, average relative error (ARE), sum of the squares of the errors (ERRSQ), hybrid fractional error function (HYBRID), Marquardt’s percent standard deviation (MPSD) and sum of absolute errors (EABS) [
CECM was prepared by reacting CM with acrylamide in alkaline medium. On reacting CM with acrylamide in alkaline medium to obtain carbamoylethylated Cornulaca monacantha (CECM), the following reaction will occur:
CECM samples were prepared in the form of a brown powder having a particle size range of 150 - 200 mm as shown in
Earlier studies have indicated that the pH of adsorbate is an important parameter affecting bioadsorption of heavy metals.
The effect of adsorbent concentration on adsorption capacity of Hg (II) by CECM is shown in
The equilibrium adsorption isotherm is fundamental
in describing the interactive behavior between solutes and adsorbent, and is important in the design of adsorption system.
As shown in
Langmuir equation [
where, aL is Langmuir isotherm constant (l/mg), KL is the Langmuir constant (l/g) and aL/KL represents the adsorption capacity, Qmax. The linear equation of this model could be represented by:
The linear plots (
relation between KL and Qmax is given by the following equation:
where, b is related to the energy of adsorption (l/mg). The value of constant b and correlation coefficient, R2 are also listed in
The essential features of Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL, which expressed by the following equation:
where Co is the initial concentration of Hg (II) and b is mentioned before. The RL value of Hg (II) onto CECM at 30˚C is given in
In our case the value of RL for initial Hg (II) concentrations in the range of 0.213 - 0.632 of was found to be (0 < RL < 1). This vale indicates that the adsorption is favorable. The values of the correlation coefficients, R2 of Langmuir plot was over 0.98 Hg (II) on CECM indicate that the adsorption of Hg (II) onto CECM is fitted well on the Langmuir isotherm (obey the Langmuir isotherm).
The Freundlich equation [
where, qe is the equilibrium concentration mercury on CECM (mg/g), Ce the equilibrium concentration of mercury in solution (mg/l) and KF (mg/g) and n are the Freundlich constants characteristic of the system, indicators of adsorption capacity and adsorption intensity, respectively. The linear form of Freundlich equation is:
where qe is the amount of Hg (II) ions adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of Hg (II) ions, mg/l. Linear plots of log qe vs log Ce at at 30˚C (
The Temkin isotherm [
where R is the universal gas constant (8.31441 J−1∙mol−1∙K−1), T is the absolute temperature (K), AT is the Temkin isotherm constant (g/mg) and bT is Temkin constant. The sorption data were analyzed according to the linear form of the Temkin isotherm as:
Linear plots of qe vs lnCe at 30˚C (
The comparison between the experimental data and the theoretical data obtained from isotherm models of Hg (II) onto CECM are shown in
mental data better than other isotherms.
The use of R2 is limited to solve linear forms of isotherm equation, which measures the difference between experimental and theoretical data in linearized plots only, but not the errors in non-linear form of isotherm curved. For this reason we use average relative error (ARE) to determine the best fit in isotherm models. For all isotherm models, the value of ARE for adsorption of Hg (II) onto CECM are calculated and presented in