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The production of waste with high heavy metal ions content is a serious problem due to the disposal of heavy metals. Heavy metals discharge into water by industries such as mining, metallurgy, electronics, textiles, oil refineries and pulp (1-3). These metals increase health risks in animals, plants and human (4). Examples of heavy metals are (Cd), (Hg), (Pb), (Cu), (Cr), (Ni), (Zn), (As), (Co), (Ag), (Au), (Se), (V), (Sb), (Bi), (Mn), (Ce), (Ga), (Pt), and (Fe) that may be caused to many diseases such as central nervous system irritation, depression, affect the skin, lungs etc (5). There is a permissible limit of heavy metals in water. For example, guideline values for Cu, Ni, Pb and Cd are 2, 0.07, 0.01 and 0.003 mg/L, respectively (6,7). In order to remove these metals different methods are examined, such as chemical and electrochemical precipitation, coagulation and electrochemical deposition, ion exchange resins etc. These methods do not seem to be economical in large scales (8). So, it is necessary to investigate a low-cost method which is effective and economic. Adsorption is a simple process. It is high efficiency, easy and cost-effectiveness (5). There are a lot of cheap adsorbents for removal of containments from waters such as agricultural waste (9-10), powder of leaves and branches (11), rice bran (12), sawdust (13), peels (14), green algae (15), and so on(16). Adsorption capacity depends on the nature of adsorbent, their porosity and large surface area with more specific adsorption sites. There are three types of adsorption: chemical, physical and electrostatic. The popular adsorption is physical type (17).
Aims of the study:
The aim of this research is to study the efficiency of PHS as low-cost adsorbent in the removal of Cu, Ni, Pb and Cd ions from aqueous solutions. The effects of some key parameters on adsorption such as contact time, pH, initial concentration, adsorbent size and ionic strength were studied.
 

Materials & Methods

This research is an experimental research that was carried out in Environmental Science Research Lab. of Zanjan University.
Preperation of Adsorbent
Peganum Harmala Seeds (PHS) were purchased from Zanjan, Iran. They were washed three times by drinking water and then by distilled water. Then they were dried in an oven for 24 h in 70 °C. The dried PHS was used as an adsorbent in adsorption of heavy metals from water.
Adsorption experiments
Adsorption experiments were performed by a continuous method. A stock solution of Pb(II), Co(II), Cu (II) and Ni (II) was prepared by dissolving Pb(NO₃)₂, Co(NO₃)₃, Ni(NO₃), Cu(NO₃)₂ in a distilled water. Experiments were down in a column (15*3 cm) as a reactor. Samples were collected from the end of the column at different time. Samples were separated by centrifugation at 4000 rpm for 10 min. The concentration was determined by an atomic adsorption spectroscopy. The initial pH of the solution was adjustment either NaOH or HCl in the range of 1-8. Samples were taken each 5 minutes and analyzed by an atomic absorption spectroscopy. The amount of adsorbed heavy metals on adsorbent (q, mg/g), adsorption percent (%A) and desorption percent were calculated, using general equation (1), (2) and (3).
                                       (1)
               (2)
                        (3)
Here, q_e is the adsorption amount (mg/g), C₀ and C_t are the initial and final concentrations (mg/L), V the volume of solution (L), W is the mass of adsorbent (g), md desorbed heavy metals (mg) and ma adsorbed heavy metals (mg).
Analysis
The concentrations of heavy metals were determined by a flame atomic absorption spectrometer (Shimadzu, Japan). The pH of solution was measured by a pH meter (Metrohm 620 pH lab). The functional groups on the surface site of Peganum Harmala Seeds were characterized, using a Fourier transform infrared spectroscometer (FT-IR Nicolet is 10/Thermo) in the range of 500-4000 cm⁻¹.The surface morphology of Peganum Harmala Seeds was taken by scanning electronic microscopy ( Mira tescan).
Adsorption isotherms and kinetics
The adsorption isotherms for the Pb(II), Co(II), Cu (II) and Ni (II) removal were studied, using various initial concentrations. Equilibrium adsorption isotherm data were analyzed according to the linear forms of Langmuir, Freundlich and Temkin adsorption isotherm equations (4-6), respectively:
              (4)
        (5)
            (6)
Where q_m is the maximum adsorption (mg/g) and K_L is the Langmuir constant including the affinity of binding sites (L/mg). K_F and n are the Freundlich constants indicating adsorption capacity ((mg/g) (L/mg)1/n) and intensity, respectively. K_T and B₁ are the Temkin constants. K_T is the equilibrium binding constant (L/g) and B₁ is related to the heat of adsorption.
For studying the kinetic sorption, pseudo-first order (7), pseudo-second order (8), Elovich (9), and power function (10), models were studied (18).
                               (7)
                                            (8)
 
                                        (9)

                                   (10)

Where, qe and qt are the amount of adsorbed  (mg/g) at equilibrium and at time t (min). k₁ (min^-1) and k₂ (g/mg.min) are the pseudo-first and pseudo-second order rate constants, respectively. a_E is the initial adsorption rate (mg/g.min), β_E is the desorption constant (g.mg) during any one experiment, V is the rate constant of power function(min^-1) and k is constant of power function model (mg/g).
 
 

Results

Characterization of adsorbent
Fig. 1 shows the FTIR of prepared Peganum Harmala Seeds (PHS).
 

Figure 1) FTIR of Peganum Harmala Seeds
 
Fig. 2 shows the SEM image of PHS in different scales.

Figure 2) SEM of Peganum Harmala Seeds
 
Effect of contact time
The effect of contact time on the removal of heavy metals was studied and the result is shown in Fig. 3.

Figure 3) Effect of contact time
 (PHS)=8g/100mL, (Pb)=50 mg/L, (Cu)= 50 mg/L; (Ni)= 50 mg/L; (Co)= 50 mg/L; pH=5;T=25°C
 
Effect of pH
The pH of solution was set in the range of 1-8. The results of the effect of pH on the heavy metals removal is shown in Fig. 4.

Figure 4) Effect of pH
(PHS)=15g/100mL, (Pb)=50 mg/L, (Cu)= 50 mg/L; (Ni)= 50 mg/L; (Co)= 50 mg/L; pH=5;T=25°C; Flow=1.8 mL/min
 
Effect of adsorbent dosage
The effect of adsorbent dosage on removal efficiency of heavy metals was studied (Fig. 5).

Figure 5: Effect of adsorbent dosage
(Pb)=50 mg/L, (Cu)= 50 mg/L; (Ni)= 50 mg/L; (Co)= 50 mg/L; pH=5;T=25°C; Flow=1.8 mL/min
 
 
 
 
Effect of heavy metals initial concentration
 
The effect of heavy metals initial concentration in removal of them by PHS  is shown in Fig. 6.

 
Figure 6) Effect of heavy metals initial concentration
(PHS)=15g/100mL, pH=5;T=25°C; Flow=1.8 mL/mim
 
Effect of ionic strength
To study the effect of ionic strength on adsorption of heavy metals onto PHS, sodium nitrate, sodium chloride and potassium chloride were used.
Effect of Particle Size
To study the effect of particle size, different size of PHS was used (0.5-1 mm, 1-2 mm and 2-3 mm). Table 1 shows the effect of particle size on removal of heavy metals by PHS.
Desorption study
Desorption of heavy metals  from the surface of PHS was investigated, using water and various acids such as HCl, H₂SO₄ and HNO^¬3.
Isotherm studies
The calculated results of adsorption of Pb²⁺, Co²⁺, Ni²⁺ and Cu²⁺ on the PHS as a function of the initial concentration of Pb²⁺, Co²⁺, Ni²⁺ and Cu²⁺ are shown in Table 2.
The results of the kinetic parameters for heavy metals adsorption onto PHS are listed in Table 3.
 
 
 
 
 
 

 
Table 1) The effect of particle size
(heavy metals)=50 mg/L; (Adsoebent)=15g/100 mL; pH=5; Flow= 3 mL /min

Size (nm)	Pb	Ni	Co	Cu
0.5-1	99.27	66.92	67.65	93.10
1-2	95.32	61.53	61.54	93.45
2-3	94.88	56.53	58.31	71.1

 
Table 2) The isotherm constants along with the correlation coefficients

Isotherm
	Langmuir				Freundlich			Temkin
(Mn+)	qmax(mg/g)	b(L/mg)	RL	R2	Kf(mg/g)	n	R2	a(L/g)	b(kJ/mol)	R2
Pb2+	1.827	0.0274	0.4264	0.9413	23.286	0.9880	0.9585	2.267	7.477	0.8380
Ni2+	0.4757	0.0238	0.4469	0.9112	60.187	1.356	0.9491	5.748	19.724	0.9091
Co2+	1.037	0.0089	0.7035	0.9566	99.518	1.152	0.9219	6.669	19.369	0.9159
Cu2+	1.0675	0.035	0.3726	0.9326	22.967	1.245	0.8847	2.356	11.717	0.7886

 
Table 3) Kinetic parameters for heavy metals adsorption onto PHS

R2
power function	Elovich		pseudo-second order	pseudo-first order	Mn+
0.969	0.969	1		0.826	Pb2+
0.865	0.875	0.999		0.969	Ni2+
0.871	0.880	0.998		0.983	Co2+
0.865	0.865	1		0.892	Cu2+