Ion Exchange and Solvent Extraction

A Series of Advances, Volume 16

Arup K. SenGupta, Yitzhak Marcus

• Publisher: CRC Press
• Year: 2004
• ISBN:
  9781135531454
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Reflecting the sustained and diverse experimental momentum in the field of ion exchange, Volume 16 summarizes revolutionary advances on par with the consistently high-level research related by this series. This text discusses the kinetics, theoretical models, experimental results/supporting data, and applications for isothermal supersaturation, met

1
Adsorption and Ion-Exchange Properties of Engineered Activated Carbons and Carbonaceous Materials

Michael Streat, Danish J. Malik, and Basudeb Saha

Loughborough University, Loughborough, Leicestershire, United Kingdom

I. INTRODUCTION

The importance of environmental pollution control has increased significantly in recent years. Environmentalists are primarily concerned with the presence of heavy metals, pesticides, herbicides, chlorinated hydrocarbons, and radionuclides in groundwater, surface water, drinking water, and aqueous effluents due to their high toxicity and impact on human and aquatic life.

Several techniques have been developed and used to remove and/or recover a wide range of micropollutants from water and a variety of industrial effluents. Adsorption using activated carbon is well established for the removal of organic molecules from aqueous solution but to a much lesser extent for the removal of toxic heavy metals. Of course, polymeric ion-exchange resins are applied for water treatment and for trace metal removal under extreme conditions, ranging from highly acidic to highly alkaline solutions. The high sorption capacity of these materials, usually greater than that of carbonaceous adsorbents, and good selectivity toward metal ions render them attractive candidates for a wide range of applications. However, the use of ion-exchange resins involves significant capital and operating costs. Activated carbons are generally a cheaper alternative because they are derived from renewable natural materials although the operating costs remain a significant factor.

Table 1 Maximum Admissible Concentrations of Undesirable and Toxic Metals in Water

The removal of toxic metal ions from dilute or concentrated solutions has received considerable attention in the last few decades. In recent years, stringent statutory regulations were introduced to reduce the discharge of toxic metals to low levels at the source, particularly from plating shops and other metal processing industries. The Environmental Protection Agency (EPA) and the European Community (Directive 98/78/EC [1]) have highlighted the most common heavy metals arising in residual water, and the maximum admissible concentrations are given in Table 1. Cadmium and mercury are two of the most toxic metals present in the aqueous environment; hence, their maximum permissible concentrations in drinking water have been set at 5 and 1 µg/L (ppb), respectively, and this presents a particularly challenging problem. The threshold limits given in Table 1 are achieved partially by minimization and recycling of existing resources. With the increasing demand for cleaner water, attention has been focused on improvements to existing treatments and the development of new techniques and materials.

The removal of metal contaminants from effluent streams has the advantage of reducing the cost of waste disposal. In most cases, the treatment of wastewater gives rise to secondary effluents. Efficiency of such processes can be improved by recycling treated water and/or by metal recovery. General methods applied to the removal of metals include ion exchange, precipitation, coagulation, flocculation, evaporation, and membrane processes. By using ion exchange or adsorption, most of the water can be recycled without the need for further treatment. In some cases, the metal can also be recovered in a useful form.

A variety of materials have been investigated for the removal of metals from metallurgical effluents. Conventional activated carbons are used extensively in water treatment for removal of color, odor, and organic contaminants [2,3]. These carbonaceous materials possess the potential for removal of inorganic species from effluent streams. Activated carbons have high porosities and high surface areas and are prepared from readily available carbonaceous precursors such as coal, wood, coconut shells, and agricultural wastes. These precursors are normally exposed to a number of different activation methods in an effort to achieve an activated carbon with the most favorable properties for a particular application. The texture of activated carbons can be adapted to suit the situation by adequate choice of the activation procedure.

Removal of metals by conventional activated carbons has been studied by a number of authors (4–6). In general, ordinary activated carbons possess a large surface area but have a relatively low capacity for metal ions. Modified activated carbons have been examined as alternatives to conventional polymeric ion-exchange resins. By far the most widely developed large-scale application of activated carbon in hydrometallurgy is the recovery of gold from dilute cyanide leach solutions. An extensive review of this process is given by Bailey [7]. Tai and Streat [8] reported the ion-exchange reactivity of oxidized carbon for the removal of copper, zinc, and nickel from solution.

In this chapter, we discuss the preparation, properties, and metal sorption performance of a range of as-received and oxidized samples of granular and fibrous activated carbon that were either prepared or modified in our laboratory. Samples were evaluated for the removal of trace toxic metal ions from aqueous solutions. Batch and column experiments were performed to elucidate the relationship between sorptive performance and the physical and chemical structure of these materials.

The EU and UK national standard for any individual pesticide in drinking water at the point of supply is 0.1 µg/L (0.1 ppb), with a maximum of 0.5 µg/L for all detected compounds. In 1990, the most frequently detected pesticides in UK drinking water supplies were atrazine, simazine, isoproturon, diuron, chlorotoluron, and mecoprop. Atrazine is one of the most difficult herbicides to remove from potable water supplies and as a consequence was prohibited from nonagricultural uses in England and Wales in 1993. Since that time there has been an increase in the use of alternative herbicides for both agricultural and nonagricultural purposes. Imazapyr and triclopyr are two of a group of four herbicides (benazolin, bentazone, imazapyr, and triclopyr) that have been identified as alternatives. These herbicides are more soluble in water than atrazine and therefore also constitute a potential pollution hazard. The average concentration of the pesticides in source waters is generally below the legal limit of 0.1 µg/L. However, seasonal variations in the use of pesticides results in concentrations that significantly exceed the limit, so that the source cannot be used for drinking water production. Data provided by the UK Environment Agency show the maximum concentration of each herbicide under investigation during sampling in 1995, 1996, and 1997 (see Table 2).

Table 2 Maximum Concentration of Herbicides in UK Water during 1995, 1996, and 1997

The adsorption of herbicides onto activated carbon is also discussed in detail in this chapter, and the findings are compared with data for a set of novel hypercross-linked polymer phases that offer an alternative approach for the treatment of potable waters.

II. PREPARATION AND PROPERTIES OF ACTIVATED CARBON

The extraordinary ability of carbon to combine with itself and other chemical elements in different ways is the basis of organic chemistry and life itself [10]. As a consequence, there is a rich diversity of structural forms of solid carbon because it can exist as any of several allotropes. It is found abundantly in nature as coal or as natural graphite and also in much less abundant form as diamond. Engineered carbons can take many forms, e.g., coke, graphite, carbon and graphite fiber, carbon fibre–carbon composite, carbon monoliths, glassy carbon, carbon black, carbon film, and diamond-like film.

The principal reasons that engineered carbons find extensive use as adsorbents are their porous and highly developed internal surface area and the complex nature of their surface chemical structure. In this chapter, we review the synthesis, structure, and adsorption properties of engineered carbons with reference to the body of work carried out in our research laboratory. We also present techniques that we have employed to modify the surface properties of engineered carbons by introducing heteroatoms such as oxygen, sulfur, nitrogen, and phosphorus. Active carbon fibers (unfunctionalized and functionalized) as adsorbent materials are also of considerable interest because they offer a kinetic advantage over granular activated carbons (typical dimension 0.4–5 mm) owing to their smaller diameters (6–20 µm).

We do not apologize for omitting other exciting areas such as the advances in the synthesis of carbon nanotubes, nanocones, and multiwalled carbon spheres and in the production of carbon films (employing physical vapor deposition and chemical vapor deposition techniques) including amorphous as well as diamond films. These topics, although of immense interest, are beyond the scope of this chapter. In particular, the cost of the materials currently limits their potential application in the field of environmental remediation. However, we recognize the importance of these new areas although the potential application of fullerenes and carbon nanotubes has not yet been fully realized.

The majority of engineered carbons discussed in this chapter have graphitic or disordered graphitic microstructures. Also, for most engineered carbon materials the originating precursor is organic and the materials arise from heat treatment of the precursor in inert atmospheres (carbonization). A selection of technically important carbons arising from solid, liquid, and gaseous organic precursors is presented in Table 3.

Engineered carbons are the product of the carbonization process, i.e., of pyrolysis of the carbon-containing material, conducted in the absence of air (usually in a nitrogen, argon, or similar oxygen-free atmosphere*). Depending on the choice of the starting precursor material, the char obtained as a result of the carbonization process may be virtually inactive as regards adsorption, with a specific surface area of several square meters in gram. On the other hand, we have shown that carbonization of porous polyaddition/polycondensation polymers results in retention of the porosity of the original polymer structure in the final char and hence in retention of the highly developed surface area. The inactive char obtained in the previous instance requires activation to convert the char into an adsorbent of high porosity (pore volume at least 0.2 cm³/g, although values as high as 1 cm³/g have been recorded). During the activation process, the carbonaceous material is subjected to selective thermal treatment under suitable conditions, which results in gasification of loosely bound pyrolysis products and thereby in the opening up of innumerable pores, fissures, and cracks. The surface area occupied by pores per unit mass of the material increases significantly. The activation process may be carried out by impregnation with chemicals (primarily dehydrating agents such as zinc chloride and phosphoric acid) or activation by steam, air, or carbon dioxide. The content of volatile substances (dehydration products and cyclization product such as polyphenolic-type compounds) in these materials is an important parameter of their susceptibility to undergo activation. Materials containing a small proportion of such compounds will not be amenable to significant surface activation.

Table 3 Starting Precursors for Carbons

Activation ...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• Contributors to Volume 16
• Contents of Other Volumes
• 1: Adsorption and Ion-Exchange Properties of Engineered Activated Carbons and Carbonaceous Materials
• 2: Entropy-Driven Selective Ion Exchange for Hydrophobic Ionizable Organic Compounds (HIOCs)
• 3: Ion-Exchange Isothermal Supersaturation: Concept, Problems, and Applications
• 4: Metal Separation by pH-Driven Parametric Pumping
• 5: Selectivity Considerations in Modeling the Treatment of Perchlorate Using Ion-Exchange Processes
• 6: Ion-Exchange Kinetics for Ultrapure Water