The removal of Cr(VI) from aqueous solution by almond green hull waste material: kinetic and equilibrium studies

The discharge of industrial ef ﬂ uents containing hexavalent chromium into the environment can be very harmful to living things. Therefore, prior to ef ﬂ uent discharge into the environment, hexavalent chromium should be removed from contaminated water and especially from wastewaters. In the present work, almond green hull powder (AGHP) was investigated for the removal of hexavalent chromium from wastewater. The effects of pH (2 – 10), adsorbent dose (2 – 24 g L (cid:1) 1 ), Cr(VI) concentration (10 – 100 mg L (cid:1) 1 ), contact time (1 – 60 min), and temperature (5 – 50 W C) were studied. All the experiments were performed in triplicate and average results were reported. The surface morphology, pore volume and size, pH of zero point charge (pH ZPC ) and surface functional groups of AGHP were characterized. Isotherm and kinetic evaluations were also conducted in the present study. The results revealed that the adsorption of Cr(VI) by AGHP was an adsorbate, adsorbent, and temperature dependent process that was favorable under acidic conditions. Furthermore, AGHP absorbed over 99% of chromium from the solutions containing 10 – 100 mg L (cid:1) 1 of Cr(VI) based on the Freundlich model. In summary, hexavalent chromium was not found in almond kernel. Biosorption onto AGHP is an affordable and economical adsorption process for treating Cr(VI)-laden industrial wastewater.


INTRODUCTION
The discharge of heavy metals into aquatic ecosystems has become a matter of concern over the last few decades (Mane & Bhosle ). They are extremely toxic elements which can seriously affect plants and animals and cause a large number of afflictions (John Babu et al. ). Increased knowledge about toxicological effects of heavy metals on the environment is well recognized and therefore, the search for different methods to reduce water pollution is inevitable (Vinodhini et al. ). The major sources of heavy metal contaminant ions are the industrial effluents.
Due to their persistence in nature, it becomes essential to remove them from wastewaters (Klimmek et al. ). Inorganic micro-pollutants are of considerable concern because they are non-biodegradable, highly toxic, and have possible carcinogenic effects (Cimino & Caristi ; Pehlivan & Altun ).

Cr(VI) is a common pollutant introduced into natural
waters from a variety of industrial wastewaters including those from textile dyeing, leather tanning, electroplating, and metal finishing industries (Aadli et al. ). Chromium was discovered in 1797 by the French chemist Louis Vauquelin. It was named chromium (Greek Chroma, 'color') because of many different colors found in its compounds. Chromium is the Earth's 21st most abundant element (about 122 ppm) and the sixth most abundant transition metal. The principal chromium is ferric chromite, FeCr 2 O 4 , found mainly in South Africa (with 96% of the world's reserves), Russia, and the Philippines. Less common sources include crocoite, PbCrO 4 and chrome ochre, Cr 2 O 3 . The gem stones emerald and ruby owe their colors to traces of chromium. Chromium occurs in 2 þ , 3 þ and 6 þ oxidation states but Cr 2þ is unstable and very little is known about its hydrolysis (Mohan & Pittman ). Chromium usually exists in both trivalent and hexavalent forms in aqueous systems. The other two oxidation states of chromium have different chemical, biological, and environmental characteristics (Hongqin et al. ). Cr(III) is relatively insoluble and is required by microorganisms in small quantities as an essential nutrient trace metal, while Cr(VI) is of great concern because of its toxicity. Cr(VI) has been reported to be a primary contaminant to humans, animals, plants, and microorganisms and it is known to be carcinogenic (Rengaraj et al. ).  (Demirbas et al. ), and so on.
In the present study, an agricultural waste material, almond green hull, was examined as an economical sorbent for the removal of Cr(VI) ion from aqueous solutions.
Almond trees are abundant in the world, especially in Iran.
Almond hull is an agricultural crop residue that cannot be used by animals and is usually burnt. The annual production of almond (with hard shell) is about 108,000 tonnes in Iran.
Almond green hull is estimated to be about 0.25-0.6 wt% of whole almond fruits depending upon their various types.
Therefore, about 36,000-160,000 tonnes of this waste material, generated in the agricultural sections of the country, can be used for Cr(VI)-laden industrial waste water treatments annually. To the best of the authors' knowledge, this material has not been used before for this kind of application.
Utilization of almond green hull not only provides a low cost and easily available sorbent for the removal of heavy metals such as Cr(VI), but it could also help in environmental pollution control (Ahmadpour & Doosti ).

Preparation of adsorbent
Almond green hull was obtained from a local fruit field in the eastern part of Iran (Southern Khorasan). The sorbent was washed thoroughly several times with deionized water to be free of any dust or pollutant and was dried at room temperature for some days. Then, the dried sample was powdered and sieved (in two ranges of 2< to <4 mm).
In order to identify the surface characteristics of adsorbent, a LEO 1450 VP (Zeiss, Germany) was used for scanning electron micrograph (SEM) analysis and an AVATAR 370 FT-IR was used for Fourier transform infrared spectroscopy (FTIR).

Reagents and solutions
The chemicals used were made by the German company Merck. The devices used were pH meter 765 (German Climatic Company), shaker (IK Company), by which the sample, water contaminated with Cr(VI), was combined with the adsorbents, and a digital balance (KERN, Germany) to weigh all the solid chemicals. The incubator shaker, model Aerotron (Infors Company, Sweden) was also used to change the temperature. To make the samples even, Whatman 125 mm paper was used.
In this study, the stock solution of Cr(VI) was prepared by dissolving a known quantity of potassium dichromate (K 2 Cr 2 O 7 ) in deionized water. The stock solution was further diluted to obtain the required concentrations of Cr(VI) solutions. 1.0 N sodium hydroxide (NaOH) and 1.0 N hydrogen chloride (HCl) were used for pH value adjustments (APHA ).

Batch adsorption experiments
Batch adsorption studies were performed at different pH ions were calculated from the change in metal concentration in the aqueous solution before and after equilibrium sorption by using the following equation: where q e is equilibrium of metal adsorbed (mg g À1 adsorbent) on the AGHP, V is the solution volume (L), W is the amount of sorbent (g), and C in and C out (mg L À1 ) are the initial and final Cr(VI) concentrations of the solution, respectively. The Cr(VI) removal percentage (%) was calculated using the following equation: Kinetics, mechanism and isotherm analysis The adsorption isotherm studies were carried out by varying the Cr(VI) initial concentrations from 10 to 100 mg L À1 at fixed volume (100 mL), AGHP dose (4 g L À1 ), pH (2)

RESULTS AND DISCUSSION
Characteristics of AGHP before and after biosorption Since almond kernel used in this study was continuously exposed to Cr(VI) through irrigation, there was a possibility that hexavalent chromium existed in almond kernels in the area. Fortunately, Cr(VI) ion was not seen in SEM and EDX analysis.

Effect of pH
Since the pH of the aqueous solution affects the surface charge of the adsorbents as well as the degree of ionization and speciation of different pollutants, the solution pH can have a significant effect on the adsorption of chromium.
As pH was increased from 2 to 10, the removal efficiency with an acidic pH value is HCrO À 4 , which is released through hydrolysis of the dichromate ion Cr 2 O À2 7 . The effluent concentration of Cr 2 O À2 7 increases with increasing pH.
At pH values lower than pH zpc (the zero point charge of the adsorbent), the adsorbent surface is positively charged and therefore suitable for the sorption of Cr(VI) anions

Effect of contact time
It can be seen in Figure

Adsorption isotherm
Adsorption isotherms are useful for describing absorption capacity in order to facilitate the evaluation of the feasibility of the application of this process and to analyze and design absorption systems (Nadeem et al. ). In this study, equilibrium adsorption of chromium was modeled using Langmuir, Freundlich, and D-R isotherms at constant temperature (25 ± 1 W C) and different adsorbent doses.
The information obtained from isotherm modeling is summarized in n is close to 1, the heterogeneity of the surface is less critical and if n is close to 10 it will be more important.
The D-R isotherm is based on heterogeneous surfaces.
In this isotherm, it is assumed that the decline in adsorption energy is against the assumed logarithmic condition in the linear form of the Freundlich equation. In this model, the interactive effects between adsorbent and adsorbate are considered indirectly and, therefore, because of these interactive effects, adsorption energy of all molecules in sorption layers decreases linearly. Based on the result of the D-R model (Table 1), the amount of free energy in Cr(VI) adsorption by AGHP is 2.58 kJ mol À1 . A value of E less than 8 Kj mol À1 indicates that physical adsorption is the dominant process under the experimental conditions, thus, the adsorption of Cr(VI) by AGHP occurs by a mechanism of physical sorption. the initial concentration of chromium: 20 g L À1 ; contact time: 60 min; stirring speed: 300 rpm. Langmuir model C e =q e ¼ 1=K L q max þ C e =q max Plot -(C e =q e ) υs: C e Fitted model -C e =q e ¼ 0:0008 þ 0:0945C e q max mg g À1 10.58

Biosorption kinetics
One of the most critical factors in designing an adsorption system (for determining contact time and reactor configurations) is the prediction of absorption process rate which is controlled by the system kinetics. Biosorption kinetics is dependent on the physicochemical characteristics of the adsorbent which affects the absorption mechanism. In order to analyze kinetic mechanism, biosorption constants can be calculated using the Lagergren equation, the pseudo first order model (Lagergren ), the Weber and Morris model and pseudo second order (Ho & McKay ). The linear form of the pseudo second order equation is: where q e is the amount of Cr(VI) adsorbed at equilibrium (mg g À1 ), q t is the amount of Cr(VI) adsorbed at time t (mg g À1 ), and K 1 is the first order equilibrium rate constant min À1 . Plotting log(q e À q t ) against t in laboratory conditions, a straight line is obtained which can be used to determine the rate constant K 1 and correlation coefficient The linear form of the pseudo second order equation can also be written as: where q e is the amount of Cr(VI) adsorbed at equilibrium (mg g À1 ) and K 2 is equilibrium rate constant of the second order kinetics model (mg(g min) À1 ). t=q t against t determines rate constant K 2 and R 2 values (Ho & McKay ). K 1 , q e , and R 2 (correlation coefficient for the first order kinetic model) and K 2 , q e , and R 2 (correlation coefficient for the second order kinetic model) values are obtained and are presented in Table 2. According to the results, the data obtained through the process of biosorption of chromium using AGHP conform to and follow the pseudo first order kinetic model (R 2 > 0=99).
To quantitatively compare the applicability of each model, a normalized standard deviation (Δq) was calculated as follows: where n is the number of data points and q e is the adsorbent capacity at the equilibrium experimental (q e,exp ) and equilibrium calculated (q e , cal ) conditions, respectively. Since Δq represents agreement between the experimental and the pre- The calculated R 2 values (correlation coefficient) by linear regression for the studied kinetics are shown in Table 2.

Comparison of almond green hull with other adsorbents
The adsorption capacity of Cr(VI) onto almond green hull was compared with several low cost adsorbents and they are shown in Table 3. The adsorption capacity of Cr(VI) on AGHP is calculated as 10.123 mg g À1 at pH ¼ 2 and room temperature. Almond green hull in the present study possesses reasonable adsorption capacity in comparison with other sorbents.

CONCLUSION
In this study, the feasibility of using almond green hull as a biosorbent for the removal of Cr(VI) from industrial wastewaters was investigated. Adsorption reached equilibrium after about 60 min. Cr(VI) adsorption was severely dependent on the solution pH and the obtained results suggest that the maximum Cr(VI) removal occurred at pH ¼ 2.
According to the results, increasing the temperature from 5 to 50 W C increased adsorption noticeably which proves that it is an endothermic reaction. AGHP can remove hexavalent chromium at a concentration of 80 mg L À1 , adsorbent dose of 2 g L À1 , pH ¼ 2, contact time of 60 min, stirring speed of 300 rpm and at room temperature, with the removal efficiency of 99.94%.
The obtained results indicate that almond green hull can be used as an effective, low cost, and easily available biosorbent for hexavalent chromium removal from wastewater.
Pseudo second order and Freundlich models fit the kinetic and equilibrium experimental data, respectively.