Chromium and nickel metal ions removal from contaminated water using Nigerian bentonite clay

In this work bentonite clay was characterized and investigated for the adsorption of chromium and nickel metal ions from aqueous solution. The clay calcined at 650 °C was characterized for physical, chemical and textural properties. Nickel sulphate hexahydrate (NiSO 4 .6H 2 O) and chromium tri oxide (Cr 2 O 3 ) solutions were used as metal model compounds to evaluate the adsorption ef ﬁ ciency of the clay in a batch mode. The initial metal ions concentration range from 10 to 50 mg/L and the maximum removal ef ﬁ ciency was 99.40% for Cr (III) and 71.50% for Ni (II) metal ions. Langmuir and Freundlich models were utilized for the analysis of adsorption equilibrium isotherm. The experimental data ﬁ tted well into Freundlich model for Cr (III) with regression coef ﬁ cient (R 2 ) of 0.996 and the Langmuir model for Ni (II) having R 2 value 0.994. The Pseudo second order kinetic model ﬁ tted well for both chromium and nickel and their adsorption from single metal solutions followed the order Cr . Ni. scholarly journal publication. Theremoval of nickel and chromium using this clay has been found effective. The dataobtained will form a baseline for references and learning purposes.


INTRODUCTION
Contamination of aquatic systems is a serious environmental issue and therefore the development of an efficient and suitable technology to remove heavy metals from aqueous solutions is necessary. Different toxic organic and inorganic pollutants such as solvents, spilled oil, polyaromatic hydrocarbons (PAHs), dyes, metal and metalloid species, etc have been found at basic levels in waste water, ground and surface waters (Pandey & Ramontja 2016a).
Metal ions such as mercury, cadmium, chromium, nickel, palladium, etc are introduced in the environment through natural sources such as weathering and erosion of rocks; and anthropogenic sources such as chemical manufacturing plants, electroplating, battery, pesticides and fertilizers industries. Even at very low concentration of these metal ions tend to accumulate in living organisms, causing different health disorders in human and animals depending on the exposure rate and This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). and followed by modification using calcinations. Beneficiation is the treatment of raw material to improve its physical or chemical properties by removing impurities which results in higher grade product.

Beneficiation of bentonite
The beneficiation of bentonite was carried out according to the methods adopted by Ahmed et al. (2012) and Nwafulugo et al. (2014). 10 kg of raw bentonite clay was weighed and crushed down from lumps to coarse powder form using wooden mortar and pestle. The coarse powder was soaked in 90 litres of water in a plastic container and the mixture was allowed to stay for 24 hours. It was then plunged (stirred) for 3 hours at room temperature. The stirred mixture was then allowed to remain in the container for 24 hours and then plunged for another 3 hours. The stirred mixture was then allowed to remain in the container for 24 hours for the coarse quartz impurities to sediment to the bottom leaving colloidal solution of bentonite at the top. The colloid bentonite was collected and separated from the quartz sediments and sieved through a 230 mesh Tyler sieve (63 μm sieve opening) to further remove coarse impurities. The clay obtained was allowed to settle and thickened for 5 days in a plastic container. The thicken clay was put in a filter cloth and pressed under a hydraulic press to squeeze out the water. The resulting cake was dried in an oven at 120°C to constant weight, broken down to powder and stored in a large polyethylene bag for the subsequent experiments.
Calcination of bentonite clay 20 g of the beneficiated clay was fired gradually in an electric furnace to 650°C and soaked at that temperature for 3 hours. The calcination temperature was selected based on the fact that the hydroxyl group (chemically combined water or water of hydration) in montmorillonites structure gets destroyed at temperatures between 600°C and 750°C (Ahmed et al. 2012). The calcined clay was allowed to cool down in desiccators and characterized prior to its application as an adsorbent.

Sample characterization
The bentonite clay was characterized for chemical composition, loss on ignition (LOI), bulk density, micro porosity, SEM and TGA. The micro porosity was evaluated from the BET Thermo Scientific (Surfer machine), USA and scanning electron microscopy (SEM) was determined using a field emission scanning electron microscope model SU8020, Hitachi, Japan. The thermogravimetric analysis (TGA) curve was obtained using a Q500 equipment TA Instruments, USA. Some physicochemical characteristics of the bentonite clay were reported elsewhere (Jock et al. 2016).

Preparation of stock solution
The solutions of Cr (III) and Ni (II) metal ions were prepared from analytical grade of nickel sulphate hexahydrate (NiSO 4 .6H 2 O) and chromium tri oxide (Cr 2 O 3 ). Stock solutions of 1,000 mg/L concentrations for Cr (III) and Ni (II) metal ions were prepared by dissolving CrO 3 and NiSO 4 .6H 2 O in 1 L of distilled water respectively. Thereafter, the stock solutions were further diluted to obtain working solutions of the desired initial concentrations.

Adsorption experiments
The adsorption process was carried out in a batch reactor by varying metal ion concentration and contact time for isotherm and kinetic studies respectively. For the effect of initial concentration, standard metal solutions ranging from 10 to 40 mg/L were prepared by diluting the stock solutions. 0.5 g of the Water Practice & Technology Vol 00 No 0 calcined clay was mixed with 25 mL of the metal solution in 150 mL Erlenmeyer flasks. The mixture was shaken at 200 rpm stirring rate and 25°C for 30 minutes. The suspension was filtered and the residual concentrations of heavy metals were analyzed using atomic absorption spectrometer (AAS), Shimadzu, AA-6880 Series, Japan.
For the effect of time, 50 mg/L solutions of nickel and chromium metal ions were used. 25 mL of each solution was added in a 150 mL flask containing 0.5 g of the clay sample. Flasks were kept in a shaker at various contact times (15-60 min) and constant speed of 200 rpm and 25°C. After each time, the mixture was filter and the metal concentration of the remaining in the supernatant was analyzed by an atomic absorption spectrometer (AAS).

Characteristics of bentonite adsorbent
The chemical analysis of the clay in Table 1 shows that the clay consists mainly of silica (51.7%) and alumina (18.0%) as well high iron (14.47%) content. The elemental composition shows the presence of K and Ag in minor quantity while Ti and Mn are in traced amount. The most abundant component of Si and Al in the sample confirmed the basic elements of smectite clay group (Araujo et al. 2013). The high iron content explained the brownish and reddish appearance of bentonite clay. The clay can be designated as polycationic bentonite due to the presence of Ca 2þ and K þ cations which are normally exchangeable with positively charged pollutants (Bertagnolli et al. 2011). The pH value suggests that the bentonite clay is slightly acidic. The low LOI obtained in the clay indicates greater loss on ignition took place during calcination at 650°C. This is largely due to the expulsion of structural hydroxyl water and volatile organic components such as CO 2 and SO 2 in the clay (Saika & Parthasarathy 2010). The density value of the bentonite sample is within the range of 1.1-1.4 g/cm 3 for pure clay and other fine texture soil minerals (Kukwa et al. 2014). Microporosity of the calcined clay which might have been increased on calcination due to the liberation of hydroxyl group at elevated temperature and creating path for the pore volume is expected to enhance adsorption process (Zhu et al. 2016).
The micrograph of the raw bentonite reported elsewhere ( Jock et al. 2016) showed that the bentonite grains are aggregated mass of irregularly shape particles that appeared to have been formed by several flaky particles stacked together in the form of agglomerates while the micrograph of the calcined clay shown in Figure 1 is of leaf-like type with no defined particle format. The irregular shape particles depict the adsorptive characteristic of the clay. Generally, bentonite clay (montmorillonite) exhibits ultrafine, thin, leaf-like crystals forming a dense aggregates, or open honeycomb texture (Zuzana et al. 2012). Figure 2 shows the thermograph of the clay sample. The first weight loss related to removal of water molecules and associated cations from the clay gallery during thermal treatment. The second thermal transition of the crystal water removal collapsed with the thermal degradation of the clay which was already calcined at 650°C. This weight loss (1.5%) could also be attributed to the decomposition of Uncorrected Proof the clay structure. The low weight loss in the temperature range of dehydration step implied the changed in thermal stability of the modified bentonite (Hassan 2005).

Effect of initial concentration
The effect of the initial concentration on percentage removal of Cr (III) and Ni (II) ions is calculated by Equation (1).
where; %Ads is the amount of metal ions removed, C i and C f are the initial and final concentrations (mg/L) of the metal ions respectively. The initial concentration for Cr (III) and Ni (II) were varied from 10, 20, 30 to 40 mg/L using adsorbent amount of 0.5 g and contact time 30 min. The removal efficiency slightly increases for Cr (III) and somehow decreased with the gradual increase in initial concentration of Ni (II) as shown in Figure 3. The removal efficiency of Cr (III) is between 72.32 and 99.3% initial concentrations at10 and 40 mg/L respectively. The percentage removal of Ni (II) was 80.73% at 10 mg/L and further increased in initial concentration led to gradual decrease in the removal efficiency of Ni (II). The

Effect of contact time
The time-dependent behavior of chromium and nickel adsorption was measured by varying the contact time between the adsorbate and adsorbent in the range of 15-60 min. The initial concentration of metal ions was 50 mg/L and the amount of adsorbent added was 0.5 g. The specific adsorption is determined using Equation (2).
where C i is the initial metal ions concentration, C e is the equilibrium concentration, V, volume of solution and m mass of the adsorbent. Figure 4 shows the effect of contact time on the adsorption Cr (III) and Ni (II) metal ions. There is slightly increased in specific adsorption with the increased in contact time from 15 to 45 min for both metal ions followed by a constant uptake on further increased in contact time. This indicates that equilibrium is attained at 45 min, showing that the adsorption sites are well exposed and the 60 min  Uncorrected Proof stirring time is enough for each batch to attain equilibrium (Heba & Sara 2014). However, the adsorption of Cr (III) is higher than Ni (II) and the difference in adsorption capacity of the metal ions may be due to a number of factors, including hydration radii, hydration enthalpies, and solubility of cations (Jock et al. 2018).

Adsorption isotherms
The isotherm models employed to the experimental data were Langmuir and Freundlich. The absorption parameters evaluated are given in Table 2. Langmuir isotherm provides information on uptake capabilities and is capable of showing the equilibrium adsorption behaviour. The sorption isotherm is based on the homogeneous surface by identical active sites and restricted to a monolayer (Akpomie & Dawodu 2015). The Langmuir isotherm model is represented in Equation (3) The parameter q e (mg/g) is the quantity of metal ions adsorbed on the clay; q m (mg/g) and b (L/mg) are the single-layer adsorption capacity and the Langmuir equilibrium constant respectively. The constants q m and b were evaluated from the slope and intercept of the linear plot of C e /q e versus C e in Figures 5 and 6. The Langmuir parameters are displayed in Table 2.

Uncorrected Proof
The Freundlich isotherm is an empirical expression based on multilayer adsorption on heterogeneous surface and the linear form of this model is represented by Equation (4) log where, q e is the equilibrium concentration of adsorbate in solid phase (mg/g) and C e is the equilibrium concentration of adsorbate in liquid phase (mg/L). K F is the Freundlich constant related to the sorption capacity (mg/g) (mg/L) 1/n and n is a dimensionless constant related to the adsorption intensity of the adsorbent. Figures 7 and 8 shows the linear plot of log q e versus log C e with slope 1/n and intercepts log K F . The Freundlich model parameters K F and n are summarized in Table 2. The K F value of 15.03 (mg/g)/ (mg/L) 1/n for Cr (III) compared with 0.577 of Ni(II) indicates higher adsorption capacity of Cr (III) and this is in agreement with the higher uptake, q m (99.91 mg/g) obtained using Langmuir model. The significance of n value as follows: n , 1 (chemical process); n ¼ 1 (linear) and n . 1 (physical process). Therefore, the value of n (0.93 L/mg) for Cr (III) represents chemical adsorption while n-value (1.712) for Ni (II) shows physical adsorption (Marrakchi et al. 2010). The experimental data fitted well into Freundlich model for Cr (III) having regression coefficient (R 2 ) 0.996 and Langmuir isotherm for Ni (II) with R 2 ¼ 0.994. It demonstrated that adsorption for all metal ions fit the Langmuir isotherm as well as the Freundlich model. This might suggest that the surface of calcined bentonite is heterogeneous for chromium adsorption and homogenous for nickel ion. The uptake of chromium ion (90.91 mg/g) is higher than nickel ion (7.94 mg/g) and the order of adsorption is Cr (III) . Ni (II).

Adsorption kinetic
Adsorption kinetics deals with the rate of reaction and is an important characteristic that define the efficiency of an adsorbent. Pseudo first order and Pseudo second order kinetic models were applied to evaluate the adsorption mechanism of the experimental data. The linearize form of pseudo first order model is represented by Equation (5) and pseudo second order expressed in Equation (6) log (q e À q t ) ¼ log q e À 1 2:303 where; K 1 (min À1 ) is the first order rate constant and K 2 (g/mg min) is second order rate constant of adsorption (min À1 ), q e and q t are the amounts of metal ions adsorbed (mg/g) at equilibrium and time t respectively. The plots of log (q e À q t ) against (t) for Cr (III) and Ni (II) though not represented here gave poor fitted curves especially for Ni (II) indicating the adsorption process was not describe by pseudo first order kinetic model. Figure 9 shows the linear plots of t/q t vs t for pseudo second order model and values of regression coefficients for both metal ions were very high (R 2 ¼ 0.999). The parameters determined from the slope and intercept of the kinetic models for the adsorption of Ni (II) and Cr (III) are summarized in Table 3. Similarly, the higher values obtained for the uptake (q e ) further suggests that the pseudo second order model is more likely to predict kinetic behavior of adsorption process. The applicability of pseudo second order kinetic model shows that the metal ion adsorption process is controlled by the chemisorption mechanism indicating that the rate limiting step is based on chemical reaction between the metal ions and active site of bentonite adsorbent (Jock et al. 2016).

Comparison of Cr (III) and Ni (II) metal ions adsorption capacity by different adsorbents
The adsorption capacity of a bentonite clay is influenced by its textural properties as well as the chemical nature of adsorbent surface. Therefore, the possible mechanism of the adsorption of Cr (III) and Ni (II) metal ions on calcined bentonite surface include surface functional groups, cation exchange, mesoporous texture and surface charge. Bentonite are polycations due to the presence of Ca 2þ , Mg 2þ , Na þ and K þ cations. These cations are exchangeable with Cr (III) and Ni (II) in the bulk solution or their amount decreased in adsorption process. The cations are exchanged with heavy metals.
Comparing the adsorption capacity of the calcined bentonite used with that of other adsorbents presented in Table 4, it is found that the present work gave a very favourable result especially in the uptake of Cr (III) metal ions. Higher adsorption capacity of Cr (III) onto Chitosan and lower uptake by natural clay adsorbent was observed in comparison to the present study.

CONCLUSIONS
The removal of Cr (III) and Ni (II) from wastewater using a low cost bentonite clay adsorbent was investigated. The clay was characterized and examined for its potential to adsorb metal ions from contaminated water in a batch mode. They study revealed that the adsorption process is influenced by initial concentration, contact time and temperature. The increase in these parameters increased the amount of metal uptake of Cr (III) and Ni (II) metal ions. The Freundlich isotherm model obeyed the adsorption process of chromium metal ions suggesting a heterogeneous nature of the bentonite adsorbent. The kinetic modelling performed showed that the pseudo second order kinetics best fit the experimental data. The order of adsorption of heavy metals from single-metal solution is Cr (III) . Ni (II). The results show that the calcined bentonite clay is a promising adsorbent for the removal of nickel and chromium ions from wastewater.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.  (2015)