Heavy metals sorption onto alluvial soil under various operational conditions

C_i

Initial ion concentration (mg/l).

C_e

equilibrium ion concentration (mg/l).

V

Volume of solution (L).

m

mass of alluvial soil (g).

q_eq

adsorption capacity at equilibrium (mg/g).

q_max

max adsorption capacity (mg/g).

K_L

Langmuir constant (l/mg).

K_f

Freundlich constant (mg/g).

n

Adsorption intensity of adsorbent.

q_t

Amount of metal ions removal at time t(mg/g).

K₁

pseudo-first order rate constant (min⁻¹).

t

Contact time (min).

K₂

Pseudo-scond order rate constant of adsorption (g/mg/min).

Many countries in the world are facing water scarcity (Kotsilkova et al. 2018). The use of treated wastewater and the recycling of industrial wastewater are available approaches for solving this problem. The vast majority of industries discharge effluents containing heavy metals that are in most cases toxic (Mesli & Belkhouche 2018). Heavy metals such as lead, copper and cadmium are severe pollutants which have many negative impacts on the environment, and harmful effects on human physiology and other biological systems, even at very low concentrations (Pettinato et al. 2015a, 2015b). These toxic metals can cause accumulative poisoning, cancer and brain damage. Therefore the removal of heavy metal ions from their leaching solutions before being discharge to water streams is of a great importance to protect the environment (Mousa et al. 2016).

Industries such as manufacturers of vehicle batteries, printing and photographic materials lead to the release of enormous amounts of lead in wastewater. Lead can have a serious impact on the nervous system and disrupt the normal functioning of internal organs (Bou Kheir et al. 2019). Copper is discharged from many industries such as electronics manufacturing, metal plating, automotive industry, battery products, mining industries, chemical manufacturing and steel industries. The presence of copper in water streams may cause serious health problems for human beings, affecting their internal organs such as the heart, kidneys, liver, pancreas and brain, leading to intestinal distress and anemia. Cadmium has a prime role in many manufacturing industries such as plating, cadmium-nickel battery, phosphate fertilizer, stabilizers and alloys. It is regarded as one of the most poisonous heavy metals present in industrial wastes. Despite its appearance in low concentration, cadmium compounds are massively dangerous and are found at significant levels in the ecosystem. Additionally, they also cause hepatic toxicity and lung cancer. The permissible levels for lead, copper and cadmium in the environment are 0.05, 0.01 and 0.043 (mg/L), respectively (WHO 1996). Therefore, heavy metals concentrations must be below the permissible limits according to environmental regulations before being discharged to water streams.

Many techniques for metals ion removal from industrial wastewater have been developed and used, such as chemical precipitation (Oncel et al. 2013), filtration (Al-Qahtani 2016), adsorption electrochemical processes (Ghiloufi et al. 2016), membrane processes (Yurekli 2016), oxidation and chemical reduction (Yirsaw et al. 2016), evaporation (Lima et al. 2014), ion exchange and reverse osmosis (Ricci et al. 2015). Most of these techniques may be ineffective for heavy metal removal due to technical economic and environmental reasons (Mesli & Belkhouche 2018). Adsorption process is one of the most effective methods for heavy metal removal from industrial effluents (Kamar et al. 2016). The adsorption process offers flexibility in design and operation and in many cases will produce high-quality treated effluent (Kosa et al. 2012). Consequently, developing new economic adsorbents with high specific surface area and sorption capacity becomes a hot spot research subject (Sun et al. 2016). Researchers have utilized various adsorbent materials in order to remove heavy metals found in water and wastewater streams. Adsorbents like binary compounds, hazelnut hush, steel slag, maghnite caly, eggshell, olive stone activated carbon, rubber wood saw dust and alluvial soil (AL) of Indian origin have been used for heavy metals removal (Pettinato et al. 2015a, 2015b; Bohli et al. 2017). AL seems to be a promising low cost adsorbent; therefore more research is required to clarify the effect of operational conditions on the sorption capacity of AL and its selectivity for different ions.

The current study aimed to assess the adsorption behavior of AL for the treatment of water streams containing various concentrations of lead(II), copper(II) and cadmium(II) ions. The effect of operational parameters such as initial pH, initial metal concentration, adsorbent dose and contact time on the sorption performance was investigated. The selectivity of AL was identified. Langmuir and Freundlich models were utilized to explain the sorption isotherms. Pseudo first order and second order kinetic models were utilized to define the sorption kinetics of the used adsorbent.

Lead nitrate, copper nitrate and cadmium nitrate were all purchased from Loba Chemie (India). All chemicals were not purified prior to usage. Distilled water was used to prepare the solutions.

AL was collected from the River Nile shore in Mansoura, Egypt. AL samples were initially dried by the sun for 5 days, and then heated at 500 °C for 7 hours. The dry soil was then ground to provide partial size by mesh screen of 200 μm, and afterwards put in storage to be used as an adsorbent.

The effect of adsorbent dosage on the adsorption performance was carried out by changing the dosage from 0.1 to 1 g in 100 ml of water solution. All mixtures were shaken at room temperature and pH equal to 7 ± 0.5.

The adsorption experiments were carried out at various pH values ranged from 2 to 9. The experiments were repeated twice and the average value was calculated where the experimental error had not exceeded 2%. PH of solution was adjusted using HCl and NaOH with concentration of 1 M.

The Fourier transform infrared spectroscopy (FTIR) of AL was performed using a FTIR spectrometer (Thermo Scientific Nicolet IS10) as shown in Figure 2. IR spectra were performed within the range of 4,000–500 cm⁻¹. Table 1 shows the main elemental compounds of AL. The precence of carboxyI groups, oxides and silica has been extensively confirmed, which is important for heavy metal binding (Volesky & Holan 1995). CarboxyI groups are also responsible for AL selectivity towards metals.

Scanning electron microscopy SEM represents an additional important tool concerning the examination of the surface morphology and the porous nature of an adsorbent. Figure 3 shows that the adsorbent has a porous nature and irregular surface structure. The average pore diameter was in the range between 0.975 μm to 0.310 μm. The surface of AL beads is irregular with many spaces, channels and holes, which are helpful for mass transfer of metal ion into AL soil.

The initail pH value of solution is an important controlling parameter that influences the adsorption process because it may effect both the properties of the adsorbent and the composition of the solution. The pH can affect both the adsorbent's surface, as well as the adsorbent's degree of ionization. Figure 4 shows the effect of pH on the adsorption of heavy metals ions on to AL.

It is clear that the ions removal efficiency was the lowest at pH 2. At low pH more H⁺ competed with cations for available adsorption sites, which made it harder for the cations to bind to activated sites and negatively impacted the removal efficiency of ions. Also, a large number of adsorbent active sites may become positively charged at very low pH. The highest removal efficiency was achieved at pH 9. The increase in cations removal efficiency at higher pH may also be attributed to the reduction of H⁺ ions, which compete with metal cations at lower pH (Ofomaja et al. 2010).

FTIR spectral analyses showed the presence of carboxylate (–COO–) and hydroxyl –OH functional groups on the AL surface, which may essential for the adsorption of specific metal ions (Zhang et al. 2016). At low pH, the –OH functional group is protonated and restricts the approach of cations toward the adsorbent surface, which results in lower removal efficiency of metals. While at higher pH, the –OH functional group is deprotonated, which increases the negative charge density on the adsorbent surface and results in higher metals up take.

The sorption of Pb(II), Cu(II) and Cd(II) ions onto AL depends mainly on the reaction time. Physical properties of the adsorbent, especially the amount of available sorption sites, are the main factors that affects the time required to achieve the thermodynamic equilibrium. Cations sorption onto AL has the characteristic time evolution of saturation phenomena that is shown in Figure 5. In the beginning, the experiments were carried out for 3 hours, and no change in the removal efficiency was noticed after 120 min. AL achieved removal efficiency of 96.00%, 98.23% and 92.73% for Pb⁺², Cu⁺² and Cd⁺², respectively at an equilibrium time of 120 min.

The removal efficiency was high at the beginning and then gradually decreased with time until it reached equilibrium after an operation time of 120 min. This may be due to the availability of large specific areas of adsorbents and more active sites for metal sorption during the beginning of the experiment. The higher adsorption rate is a function of the concentration gradient between the liquid layer of the adsorbate and the surrounding active sites of adsorbent available for adsorption of cations. After the equilibrium time the adsorption rate becomes slower, which is probably due to diffusion of cations into the pores of the adsorbent. When these sites are exhausted, the uptake rate is controlled by the rate at which the adsorbate is transported from the exterior to the interior sites of the adsorbent particles (Verma et al. 2008).

Due to the difference in the ionic size of metals and the nature and distribution of active groups on the adsorbent, the removal efficiency of cations varies from one metal to the other, as well as the mode of interaction between the cations and the adsorbents. Ion selectivity on the used adsorbent followed the order of Pb⁺² > Cu⁺² > Cd⁺². Ions with the higher charge and the smaller hydrate radius have a higher affinity, as do ions with greater polarization, which facilitates electrostatic ion exchange (Papageorgiou et al. 2006).

In this study, the adsorbent dosage of AL was raised from 0.1 to 1 g /100 ml, using an ions concentration of 30 mg/l at pH 7 ± 0.5. The obtained results are presented in Figure 6. The removal efficiency increased with increasing the adsorbent dosage from 0.1 to 0.5 and no further significant increase was achieved after words. The highest removal efficacy of Pb⁺², Cu⁺² and Cd⁺² achieved was 96%, 98% and 96%, respectively. This adsorption trend is commonly due to the increase of exchangeable sites available for the ions' sorption. The maximum adsorption of Pb⁺² was achieved using an adsorbent dosage of 0.5 g, while for Cu⁺² and Cd⁺² this was achieved using an adsorbent dosage of 1 g. This may be due to the non-availability of active sites on the adsorbent and establishment of equilibrium between the cations on the adsorbent and in the solution. The equilibrium adsorption capacity of adsorbents is shown in Figure 7. It was clear that as the adsorption dosage increased the adsorption capacity decreased. This may be due to the decrease in total adsorption surface area and increase in diffusion path length resulting from overlapping or aggregation of adsorption sites (Akar et al. 2009). Thus, the amount of cations adsorbed onto the adsorbent get reduced with increasing adsorbent mass, which decreased the sorption capacity of the adsorbent.

The rate of adsorption is a function of initial concentration of the adsorbate. The effect of different initial cations concentrations on the removal efficiency of AL at a dose of 0.5 g/100 mL is presented in Figure 8. For all metal ions the removal efficiency decreased with increasing the initial ions concentration.

By increasing the ions' initial concentration from 30 to 200 mg/l, the removal efficiency of Pb⁺² decreased from 96.00% to 87.40%, from 97.6% to 72.50% for Cu⁺² and from 82.80% to 62.20% for Cd⁺².

Increasing the cations' concentration increased the number of ions competing for the accessible adsorption sites, since the number active sites on the AL is constant. Pb⁺² and Cu⁺² can be classified as intermediate acids while Cd⁺² is regarded as a ‘soft’ acid (Papageorgiou et al. 2006). According to the hard-soft-acid-base (HSAB) theory, ‘hard’ acids would prefer to bind with ‘hard’ bases rather than with ‘soft’ bases. AL with the ‘hard’ carboxyl base would achieve higher sorption capacity towards Pb⁺² and Cu⁺² compared to Cd⁺².

Isotherm models can be used in order to determine the equilibrium modeling of sorption systems. Freundlich and Langmuir models are the most widely used models to observe the sorption process.

Langmuir and Freundlich adsorption constants and the correlation coefficients are shown in Table 2.

Figure 9 shows Langmuir and Freundlich isotherm models for the experimental results for various conditions. The results show that the sorption of ions onto the used adsorbent is well correlated with both Freundlich and Langmuir equations within the studied concentration range.

In the current study, the Freundlich isotherm model can be suitably explained by the surface precipitation model, which describes both the precipitation reaction and the adsorption occurring on the heterogeneous surface (Robertson & Leckie 1997).This model describes the multilayer adsorption mechanism, and that the adsorbed amount increases with the concentration.

The Langmuir isotherm model illustrates monolayer adsorption with homogeneous distribution of active sites on the AL. The Langmuir equation assumes that the surface is homogenous, which results in a lower concentration. This is ascribed to the surface complexion model, which has limited utility since it explains the adsorption behavior only at lower ions concentrations (Bradl 2004).

The rate of adsorption and possible adsorption mechanism of metals onto AL was carried out using pseudo-first order and pseudo-second order equations. Kinetic modeling is a suitable rate expression characteristic of possible reaction mechanisms.

It was observed that the second–order kinetic model showed linearity with high correlation coefficient at all the studied concentrations in comparison to the first-order kinetic model. On the other hand, the calculated results of qe by means of the first-order kinetic model as shown in Figure 10 agreed perfectly with the experimental results. This indicated that chemisorption controls the rate of reaction. In chemisorptions the adsorbate molecules usually occupy certain adsorption sites on the adsorbent surface by formation of a chemical bond and searches for sites that will maximize their coordination number with the surface (Kumar & Kirthika 2009). The second order equation revealed that the initial adsorption rate increased with the increase of initial cations concentration. The lower the metal ion concentration, the lower the chance of collisions between them.

The ion selectivity of AL was investigated using a solution composed of Pb⁺², Cu⁺² and Cd⁺² with concentrations of 30 mg/l, adsorbent dose 0.5 g/100 L, agitation speed 100 rpm, contact time 120 min, pH = 7 at room temperature for all metal ions as shown in Figure 11. It follows the order of Pb⁺² > Cu⁺² > Cd⁺². The HSAB concept agrees with the obtained results. Pb⁺² and Cu⁺² can be classified as intermediate acids while Cd⁺² is regarded as a ‘soft’ acid (Papageorgiou et al. 2006). The HSAB theory shows that the ‘hard’ acids would bind with ‘hard’ bases while ‘soft’ acids bind with ‘soft’ bases. AL, with the ‘hard’ carboxyl base, would exhibit higher affinity towards Pb⁺² and Cu⁺².

Ions adsorbed onto AL depend mainly on three factors: metal ion charge density, hydrated radius and electronegativity. The values of the above mentioned factors are 0.0272, 0.802 and 2.33 for Pb⁺², respectively, 0.0290, 0.838 and 1.9 for Cu⁺², respectively and for Cd⁺² 0.0270, 0.852 and 1.68,respectively. Ions with the smaller hydrate radius have higher affinity. Also ions with greater polarization facilitate electrostatic ion exchange. The electrostatic bonding strength is a measure of the strength of covalent bonding relative to ionic bonding. The electrostatic bonding strength values are equal 798, 498 and 422 for Pb⁺², Cu⁺² and Cd⁺², respectively. It can be observed that since the value of the ion charge density is similar for all metals, the strength of covalent bonding follows the sequence Pb⁺² > Cu⁺² > Cd⁺².

In this study, the adsorption of heavy metal ions (Pb⁺², Cu⁺² and Cd⁺²) onto AL was investigated. FTIR and SEM were used for the characterization of AL. The operating parameters, such as solution pH, adsorbent dose, ions concentration and contact time have a great effect on the adsorption process. It was found that the optimum ions removal can be achieved in alkaline solutions. Ions removal efficiency is directly related to contact time and adsorbant dose and inversely related to the initial ions concentration. The amount of ions removal was increased as the contact time and adsorbent dose were increased. The adsorption isotherm was well fitted using the Langmuir and Freundlich equations. The adsorption kinetics follows pseudo second-order kinetic model with a good correlation. Ion selectivity on AL followed the order of Pb⁺² > Cu⁺² > Cd⁺². The results confirm that AL can remove heavy metals from wastewater efficiently, which is a cheap and environmentally friendly adsorbent.