Palygorskite characterizes by many characters; some of them, it cannot swell because the structure consists of three dimensional chains and it has an unusual needle-like shape is formed by cleavage parallel to the 110 plane along the Si- O-Si bonds holding the strips together [1]. Palygorskite is a crystalline hydrated magnesium aluminium silicate with a unique chain structure that gives it unusual colloidal and sorptive properties. It consists of a double chain of tetrahedrons of silicon and oxygen (Si4O11) running parallel to the long axis. Palygorskite is clay can adsorb metal cations from solution [2]. The palygorskite was found to be rather receptive to the adsorption of heavy metal ions, and fairly high amounts of calcium, potassium and magnesium ions were sorbed from the Palygorskite into the solution [1]. Palygorskite is very sorpative clay due to the extremely large surface area which is 167m2/g approximately. Both external and internal structures contribute to this surface area. The internal bundles, or haystacks, aid the external surface area in achieving great amounts of adsorption. Palygorskite can take up water to 200% of its own weight [1]. The presence of heavy metals in aquatic environments because of their toxic nature and other adverse effects on receiving ecosystems has been of great concern for scientific community [3,4]. The treatment of rich heavy metal aqueous solutions is realizable by mean of many methods including: ion exchange, chemical precipitation, ultra filtration, electrochemical deposition, among which, adsorption on appropriate adsorbent (low cost and abundance of the latter) is one of the most efficient in terms of simplicity and feasibility of operation and low consumption of energy [5].

The purpose of this research is to provide the adsorption capacity of a palygorskite collected from palygorskite-rich sediments from Iraq to remove Pb from aqueous in a single batch system under and to determine the solid to liquid ratio, adsorption equilibrium time, pH and metal concentration.

Adsorption experiments were conducted using palygorskite clay mineral which was collected from Digma Formation from the Western Desert of Iraq to assess the ability of palygorskite on adsorbing Pb from solution. The collected samples were palygorskite-rich sediments, therefore, and for preparing a pure palygorskite, one sample of the palygorskite-rich sediment was chosen for separating palygorskite. It was prepared in the Laboratory of Geochemistry, Department of Geology, College of Science, and University of Baghdad. A sample of pure palygorskite must be prepared. To meet this issue, two major processes were performed. First is that, clay minerals are separated from the whole samples of palygorskite-rich sediments. Second, palygorskite is separated from clay mineral suits to obtain a sample of as pure palygorskite as possible as. The detailed procedures of Paul et al. [6] and Robert and Dennis [7] were done on whole sample (crude sample). A preparation processes done on pure palygorskite include drying and gentle grinding. Palygorskite was dried in an electric oven at 80°C for several hours before use. The stock solution was prepared by using Pb(NO₃)₂. Ten Pb initial solutions of concentration (10, 25, 50, 75, 100, 125, 150, 175, 200 and 225 ppm) were prepared. All experiments were conducted at room temperature to determine the solid to liquid ratio, conditioning time, effect of solution pH, and the effect of Cd concentration on sorption efficiency.

A patch style of set of laboratory experiment was conducted to determine the appropriate conditions. The effect of solid to liquid ratio, Adsorption equilibrium reaction time, pH and Pb ion concentration are studied as follows:

Sorbant mass (solid to liquid ratio)

The sorption (% (of Pb on palygorskite was studied at different palygorskite masses (0.25, 0.5, 1.0, 1.5, 2.0, 2.5, and 3 g/100 ml) keeping initial Pb concentration (100 and 200 ppm), temperature (25°C) and contact time (1 h) constant. The results of the lab experiments are listed in Table 1. Such approximate conditions have been used by Hefne et al. [8]. The results showed that the adsorption (%) of Pb was increased with increase the palygorskite mass, and the maximum removal of Pb was observed with 1 gm dose (Figure 1). With increasing palygorskite mass, the corresponding increase in adsorption (%) is less, because the cations find difficult to approach the adsorption sites due to overcrowding of clay mineral particles (Palygorskite) termed as a kind of solid concentration effect [9]. Accordingly, the solid to liquid ratio was determined to be 1gm palygorskite to 100 ml metal solution.

Table 1: Results of Pb sorption (%) on different masses of palygorskite.

Figure 1: Effect of palygorskite mass on adsorption % of Pb at different concentration (100 and 200 ppm) and at 25°C.

Equilibrium reaction time

A set of the laboratory experiments was performed in order to quantify the time required for completing the adsorption and absorption of Pb onto palygorskite. In order to perform accurate sorption (%) experiments, the effect of time on sorption (%) of metals onto palygorskite must be determined. The results of time versus 100 ppm Pb sorption (%) are listed in Table 2. The sorption increases proportionally with increasing time. The maximum sorption (89.9%) achieved after a time of 1h (Figure 2). Whilst, the sorption efficiency decreased when the time reaction continued to 70 and 80 min (Table 2) and (Figure 2). On this basis, the equilibrium reaction time is determined to be 1h.

Table 2: Results of sorption (%) of 100 ppm Pb (100 ml) onto 1 gm palygorskite with time.

Figure 2: Equilibrium time of sorption (%) of 100 ppm Pb onto palygorskite.

Sorption experiments

The experimental results of Pb treated with 1 gm of palygorskite for ten concentrations are listed in Table 3. The Pb concentrations in the initial solutions were reduced after the end of the experiment. The adsorption (%) at the initial concentration (10 ppm) was 99.5% (Table 3). It began to decrease with increasing the Pb concentration. Pb adsorption (%) for the initial solutions (25, 50, 75, 100, 125, 150, 175, 200, 225 ppm) was 99.8, 96.72, 91.1, 81.9, 76.48, 67.2, 31.43, 23.5, 22.2 %) respectively (Table 3). Figure 3a clearly shows the status of decrease adsorption efficiency with increase concentration.

Figure 3: The effect of Pb adsorption on palygorskite; a: adsorbance (%) for ten initial solutions of different concentration, b: the effect of concentration on adsorption, c: TDS pattern for initial solutions (blue line) before experiment and final solutions (red line) after experiment, d: changes of pH before the experiment (blue line) and after the experiment (red line).

Table 3: Results of laboratory experiments of lead (Pb) sorption on palygorskite in ten solutions of various concentrations.

A considerable amount of Pb adsorbed onto palygorskite with increasing concentration of the initial solution has been detected. In case of the initial solution (10 ppm), the amount of Pb tends to be almost totally adsorbed (9.95 ppm); but in case of the initial solution of 150 ppm, the amount of pb adsorbed on palygorskite is 100.8 ppm (Figure 3b); this is the maximal sorption (%) of Pb on palygorskite. Amount of adsorption decreased sharply for solutions of high concentrations (175, 200 and 225 ppm) (Figure 3b). Basically, as of our knowledge, the Total Dissolved Solid (TDS) is directly proportional with increasing concentration; this means a positive relationship between TDS and concentration (Figure 3c). The decrease of TDS value was an evidence for the happening adsorption process (Figure 3c). TDS of initial solutions ranges between 850 ppm and 19120 ppm (Table 3) due to the Pb concentration in each solution. After the end of the experiment, the decrease of TDS from 482 ppm to 11900 ppm indicates for adsorption. For this set of experiments, pH (2.9-1.36) of the initial solutions increased with increasing of adsorption until it reached between 7.0 and 2.37 (Table 3 and Figure 3d). A significant amount of Pb adsorption in the pH 2.97 was found.

The following conclusions are drawn from the present study:

• A strong competition between cations (Heavy metals) on the negative sites on palygorskite occurs at a high concentrated solution causing decrease in sorption efficiency.

• The decrease of TDS and EC values in the final solution is a function of the sorption efficiency of palygorskite. TDS and EC decrease with the trend of alkaline pH due to adsorb the positive charged metal ion (Pb) from the solution onto the negative palygorskite surface.

• The mass of adsorbents and reaction equilibrium time are controlling factors for sorping metals onto the palygorskite. The mass of adsorbent (palygorskite) affects on the sorption (%). The sorption (%) of Pb increased and the maximum removal was observed with 1 gm dose. With the increasing of palygorskite mass, the adsorption efficiency least, because difficulty of the Pb to approach the adsorption sites due to overcrowding of clay mineral particles (palygorskite) termed as a kind of solid concentration effect. Accordingly, the solidliquid ratio was determined to be 1 gm to 100 ml. The sorption (%) increases proportionally with the equilibrium reaction time where the maximum sorption (%) is determined at 60 minutes.

• The basic mechanism governs the heavy metals adsorption characteristics of palygorskite at different pH is adsorption and ionic exchange. The hydrogen number (pH) changes in the initial solutions toward alkaline referring to the occupation some negative sites in palygorskite structure by protons (H⁺). Hydrogen also competes with heavy metals (Pb) on the negative sites in palygorskite.

• The tendency of solution toward the alkaline occurs with the solutions of low concentration rather than the solutions of high concentration because no further cations competes with the H⁺ on the negative site of palygorskite [10].

This study has found that the palygorskite is good absorbent material promising for liquid purification of pollutants, in particular heavy metals. Study of the reaction using palygorskite for the removal of heavy metals in waste-water is a fruitful scope of the present investigation. These investigations could be extended to try the removal of organic compounds from liquids. In addition, the adding of catalytic organic matter will be beneficial.