Adsorption mechanisms of removing heavy metals and dyes from aqueous solution using date pits solid adsorbent

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Abstract

A potential usefulness of raw date pits as an inexpensive solid adsorbent for methylene blue (MB), copper ion (Cu2+), and cadmium ion (Cd2+) has been demonstrated in this work. This work was conducted to provide fundamental information from the study of equilibrium adsorption isotherms and to investigate the adsorption mechanisms in the adsorption of MB, Cu2+, and Cd2+ onto raw date pits. The fit of two models, namely Langmuir and Freundlich models, to experimental data obtained from the adsorption isotherms was checked. The adsorption capacities of the raw date pits towards MB and both Cu2+ and Cd2+ ions obtained from Langmuir and Freundlich models were found to be 277.8, 35.9, and 39.5 mg g−1, respectively. Surface functional groups on the raw date pits surface substantially influence the adsorption characteristics of MB, Cu2+, and Cd2+ onto the raw date pits. The Fourier transform infrared spectroscopy (FTIR) studies show clear differences in both absorbances and shapes of the bands and in their locations before and after solute adsorption. Two mechanisms were observed for MB adsorption, hydrogen bonding and electrostatic attraction, while other mechanisms were observed for Cu2+ and Cd2+. For Cu2+, binding two cellulose/lignin units together is the predominant mechanism. For Cd2+, the predominant mechanism is by binding itself using two hydroxyl groups in the cellulose/lignin unit.

Introduction

The existence of heavy metals and dyes in the aquatic system can be detrimental to a variety of living species. Many industrial processes discharge aqueous effluents containing heavy metals and dyes. Heavy metals and dyes are non-biodegradable and tend to accumulate in living organisms, causing various diseases and disorders [1].

Over 7 × 105 tons of synthetic dyes are produced annually worldwide [2]. It is estimated that 10–15% of the dyes are lost in the dye effluent during such dyeing processes. The dyeing operations are of primary environmental concern for the textile industry for a number of reasons: dyeing is a water intensive process; large amounts of salt are often needed to improve dye fixation on the textile material; many dyes contain heavy metals (e.g. chromium and copper) as either a dye component or as a contaminant and unfixed dye releases high doses of color to mill effluent, as well as salt and metals [2].

Cadmium concentrations in the range 0.1–100 mg dm−3 are typical in wastewater from several industries (chemical and metal product facilities, leather and tanning processes, electricity and gas production and sanitary industries) [3]. Cadmium and copper, the metals considered in this study, are the widely used elements, where an intake of excessively large doses by man may lead to serious kidney failure and liver disease [4].

Accordingly, improved and innovative methods for water and wastewater treatment are continuously being developed to treat water-containing metals [1]. Various treatment techniques available for heavy metals and dyes are reduction, ion exchange, adsorption, reverse osmosis, and chemical precipitation [5]. Most of these methods suffer from drawbacks like high capital and operational costs and there are problems in disposal of the residual metal sludge [5]. The use of activated carbons to remove organic and inorganic pollutants from waters is widely extended, because of their high surface area, microporous characteristic, and the chemical nature of their surfaces. However, they are expensive and their regeneration cost is also high. So there is a dire need for low-cost and readily available materials for the removal of toxic pollutants from water [6].

A potential usefulness of raw date pits (RDP) as an inexpensive solid adsorbent for a number of metal ions has been demonstrated earlier [7], [8]. However, adsorption mechanisms of these metals onto the RDP were not fully investigated. Banat et al. [9] used raw date pits and date pit activated carbons for the adsorption of Zn2+ and Cu2+ ions from water. Activated date pit carbons were also used for the adsorption of cadmium ions and other organic compounds from water [10]. They studied the effect of contact time, pH, temperature, cadmium ion concentration, adsorbent dose, salinity, as well as the activation temperature on the removal of cadmium ions by date pits. Non-activated date pits exhibited higher Zn2+ and Cu2+ ion uptake than activated date pits. The uptake of both metal ions increased on increasing the pH value of the system from 3.5 to 5.0 as well as on decreasing the temperature from 50 to 25 °C. Adsorption capacities for the non-activated date pits towards Cu2+ and Zn2+ ions as high as 0.15 and 0.09 mmol g−1, respectively, were observed.

Dates constitute part of a popular subsistence among the populace of the Middle Eastern peninsula [11]. The fruit of the date palm is composed of a fleshy pericarp and seed. Pits of date palm (seed) are a waste product of many date fruit processing plants producing pitted dates, date powders, date syrup, date juice, chocolate coated dates, and date confectionery [11]. At present, pits are used mainly for animal feeds in the cattle, sheep, camel, and poultry industries. However, value can be added in several food products [11]. Thus, potential applications include oil extraction from the pits or to use them as a dietary-fiber provider in bakery formulations. An additional function includes roasting RDP and making a caffeine-free drink which can substitute coffee when caffeine is a concern but a coffee-related flavor is desired [11].

Adsorption is undoubtedly the most important of the physicochemical processes responsible for the uptake of inorganic and organic substances in the aqueous environment. Factors such as pH, nature and concentration of substrate and adsorbing ion, ionic strength, and the presence of competing and complex ions affect the extent of adsorption. This work is also to explore the feasibility of using the RDP before and after chemical modification with microemulsions as adsorbents for the removal of heavy metals and dyes. The results of this study will be presented in the future publications.

Therefore, the present paper would report the salient features of the findings regarding the available functional groups, e.g. hydroxyl groups on the RDP and the adsorption profile of MB molecules, Cu2+, and Cd2+ ions onto the RDP as an effective and low-cost solid extractor for the removal and/or concentration minimization of MB, Cu2+, and Cd2+ ions species from aqueous and wastewater samples. This work was conducted to provide fundamental information from the study of equilibrium adsorption isotherms and to investigate the adsorption mechanisms for the adsorption of MB, Cu2+, and Cd2+ from aqueous solution onto the RDP. The results summarized herein are also part of an investigation conducted to evaluate the adsorption capacities by taking into consideration the experimental parameters such as pH, particle size, and initial solute concentration. Moreover, the most probable adsorption mechanisms will also be fully investigated.

Section snippets

Chemicals

Deionized distilled water was used to prepare all solutions and suspensions. Stock solutions of metal ions were prepared from their sulfates (CuSO4·4H2O, CdSO4, analytical reagent grade, Fluka Chemie AG, Buchs, Switzerland). Methylene blue, Basic blue 9 (C16H18N3S+Cl, C.I. 52015, Fluka Chemie) was used. All solute standards were prepared by dissolving appropriate amount of solute into demonized water.

The adsorbent

The raw date pits were collected and washed with distilled water to be completely free from

Effect of pH value

Preliminary investigations showed that the equilibrium is attained in 72 h. In these experiments, the initial pH of solute containing solution was adjusted to the desired value (pH value: 2–11) using either 1 M HCl or 1 M NaOH solution. To the pH adjusted solute solution, the RDP was added. The effect of pH on the percentage of solute removal from aqueous solution by the RDP is illustrated in Fig. 1. The figure shows the maximum percentage removal of the MB from aqueous solution taking place at pH

Conclusions

Knowledge of adsorption parameters is essential for understanding the adsorption mechanisms involved. The maximum adsorption capacities for MB, Cu2+, and Cd2+ onto the RDP were reached after 72 h. The pH of the solute solution is a very important parameter since it affects the solute adsorption capacities on the RDP. For MB the adsorption capacities increased with the increase in pH. The opposite effect is observed for Cu2+ and Cd2+. There is a slight effect of particle size of the RDP on

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