Study of the selection mechanism of heavy metal (Pb2+, Cu2+, Ni2+, and Cd2+) adsorption on clinoptilolite

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Abstract

The study was carried out on the sorption of heavy metals (Ni2+, Cu2+, Pb2+, and Cd2+) under static conditions from single- and multicomponent aqueous solutions by raw and pretreated clinoptilolite. The sorption has an ion-exchange nature and consists of three stages, i.e., the adsorption on the surface of microcrystals, the inversion stage, and the moderate adsorption in the interior of the microcrystal. The finer clinoptilolite fractions sorb higher amounts of the metals due to relative enriching by the zeolite proper and higher cleavage. The slight difference between adsorption capacity of the clinoptilolite toward lead, copper, and cadmium from single- and multicomponent solutions may testify to individual sorption centers of the zeolite for each metal. The decrease of nickel adsorption from multicomponent solutions is probably caused by the propinquity of its sorption forms to the other metals and by competition. The maximum sorption capacity toward Cd2+ is determined as 4.22 mg/g at an initial concentration of 80 mg/L and toward Pb2+, Cu2+, and Ni2+ as 27.7, 25.76, and 13.03 mg/g at 800 mg/L. The sorption results fit well to the Langmuir and the Freundlich models. The second one is better for adsorption modeling at high metal concentrations.

Introduction

The increasing levels of heavy metals in the environment represent a serious threat to human health, living resources, and ecological systems. Mobile and soluble heavy metal species are not biodegradable and tend to accumulate in living organisms, causing various diseases and disorders. Heavy metal contamination exists in aqueous waste streams of many industries, such as metal plating facilities, mining operations, and tanneries. The soils surrounding many military bases are also polluted and pose a risk of groundwater and surface water contamination. Some metals associated with these activities are Cd2+, Cr2+, Pb2+, and Hg2+. Wastewater discharged by enterprises processing ores and concentrates of nonferrous metals are usually polluted with heavy metal ions, such as Cd2+, Pb2+, Ni2+, Cu2+, and Zn2+. Environmental contamination by metals is mainly by the emission of liquid effluents with relatively low, although harmful, metal concentrations (up to some hundreds of mg/L) and therefore the removal of heavy metals from wastewaters is required prior to discharge into receiving waters [1], [2], [3], [4].

Numerous processes exist for removing dissolved heavy metals, including ion exchange, precipitation, phytoextraction, ultrafiltration, reverse osmosis, and electrodialysis. Among various treatment methods ion exchange looks like the most attractive one when effective, low-cost ion exchangers are used. Generally ion exchange and sorption are also preferred for the removal of heavy metal ions due to easy handling [5], [6].

In this context natural zeolites gain a significant interest among scientists, mainly due to their valuable sorption characteristics provided by combination of ion-exchange and molecular-sieve properties which can be relatively easily modified. Zeolite use as ion exchangers for environmental protection and other applications is stimulated by the good results obtained, the nontoxic nature of these materials, their availability in the many parts of the world, and the low cost [7], [8], [9], [10]. Removal and recuperation processes of heavy metals from aqueous solutions by natural zeolites are commonly cyclic and take into account the recovery of the metals and the regeneration of the zeolite to be reused [11]. Additionally, the mineral stability of zeolites and their structural changes under treatment in various media play important roles in their potential utilization as ion exchangers.

Clinoptilolite being the most common natural zeolite belongs to the heulandite family or a structural variation of the zeolite mineral group and has the following total chemical formula (Na,K,Ca)4Al6Si30O72⋅24H2O [7]. This mineral occurs in andesite, rhyolite, and basalt rocks as veins and impregnations. But the large industrial deposits are connected with volcanic-sedimentary high-silica rocks. Zeolite tuffs contain often more than 70% clinoptilolite proper. Associated minerals are usually quartz, cristobalite, calcite, aragonite, thenardite, feldspars, chlorite, montmorillonite, and other zeolites. Clinoptilolite differs from heulandite mineral by a higher Si/Al ratio as well as by a higher thermal stability (4.0–5.3 and 2.5–3.7, and 600–800 and 350–450 °C, respectively). Na+ and K+ dominate mainly among exchangeable cations of clinoptilolite, but Ca2+ with Ba2+ and Sr2+ in heulandite [7], [10]. The heulandite family is usually divided into heulandite, high-silica heulandite, and low- and high-silica clinoptilolite species [12], but it is also classified by a dominant exchangeable cation as follows: Ca-heulandite, K-clinoptilolite, Ca-clinoptilolite, etc. This zeolite with void volume 34% and cation-exchange capacity 2.16 meq/g has received extensive attention due to its attractive selectivity for many organic and inorganic substances including heavy metal cations [7], [8], [9], [10], [11], [12], [13].

Many researchers [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25] have investigated various aspects of heavy metal removal from wastewater [13], [14], [15] and synthetic aqueous solutions by clinoptilolite and other zeolites. The nature of the process as well as its deciding factors is usually the most interesting. The ion-exchange process in zeolites is influenced by several factors such as concentration and nature of cations and anions, the temperature, pH level, and crystal structure of the zeolites [19], [20]. The influence of pH level on heavy metal removal by ion exchange as well as the role of hydrogen ions has been considered in different reports [11], [12], [19], [20], [21]. Many studies are also devoted to the improvement of the clinoptilolite sorption properties by chemical modification [4], [12], [16], [22]. Investigation of hydrolysis processes and precipitation of metal hydroxides during ion exchange with zeolites is also very promising [23].

Despite the great interest in ion exchange with clinoptilolite, only few reports exist on the influence of other ions on heavy metal removal from mixed solutions. There are only some contradictory results published [11], [16], [24]. The limited data are also available for metal ion exchange with the clinoptilolite in terms of equilibrium isotherms and kinetics especially with consideration of the mechanism of the process on different stages. Adsorption of heavy metals on the Transcarpathian clinoptilolite under dynamic conditions was studied and reported [21,25]. The aim of the present work is to study heavy metal (Ni2+, Cu2+, Pb2+, and Cd2+) uptake from single- and multicomponent synthetic aqueous solutions by raw and pretreated forms of the natural zeolite and Transcarpathian clinoptilolite under static conditions and to estimate the optimal operation parameters of the process.

Section snippets

Materials

Clinoptilolite rock from Sokyrnytsya deposit (the Transcarpathian region, Ukraine) containing nearly 75% of clinoptilolite was used. Quartz, calcite, biotite, muscovite, chlorite, and montmorillonite are the main associated minerals. The exchange cations are Ca2+, Mg2+, Na+, and K+ with prevalence of the last one [26]. Their total contents are determined as 2.53 meq/g. Thermostability of the sorbent is from 923 to 973 K, and static water-storage capacity and relative moisture are 9.03 and

Kinetics study

Kinetics of ion-exchange adsorption of heavy metals from aqueous solutions onto the clinoptilolite was studied up to equilibrium. Ion-exchange adsorption of the selected heavy metals onto the clinoptilolite is a heterogeneous process with at least three different stages (Fig. 2). The first one is a very fast (instantaneous) intake corresponding to near the first 30 min for all metals. This step is established for all four metals at three different concentrations. The inversion phenomenon with

Conclusions

1. Adsorption of lead, copper, cadmium, and nickel onto clinoptilolite has an ion-exchange nature. Three different stages are observed in the ion-exchange adsorption of the metals. The process begins with fast adsorption on the zeolite microcrystal surfaces during the first 30 min. Then the inversion stage has a short-time prevalence of the desorption process connected with the diffusion flow from the clinoptilolite microcrystal's interior. The third stage is the moderate adsorption in the

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