NANOPOLYMERS *
CHAPTER 6 CHAPTER 6
6.3 ENVIRONMENTAL APPLICATIONS
6.3.2 HEAVY METAL ION REMOVAL .1 Introduction
The removal of toxic metals from wastewater is of great concern due to its high impact on the envi- ronment and public health. This problem can be solved by utilizing adsorption technology, such as ion-exchange resins, activated charcoal, and ion-chelating agents. This has been an improvement over conventional precipitation methods which can re-pollute the water due to difficulties in recovery.
The removal and recovery of heavy metals from wastewater has been categorized by various chemical processes. The four major classes of chemical separation technologies are chemical precipitation, elec- trolytic recovery, adsorption/ion exchange, and solvent extraction/liquid membrane separation. Fig. 6.5 illustrates these chemical separation techniques and categorizes them based on specific types of treatment.
FIGURE 6.4
Designated bilayer membranes.
From Shabani, I., Hasani-Sadrabadi, M. M., Haddadi-Asl, V., & Soleimani, M. (2010). Nanofiber-based polyelectrolytes as novel membranes for fuel cell applications. Journal of Membrane Science, 233–240.
126 CHAPTER 6 NANOPOLYMERS
6.3.2.2 Processes of removal 6.3.2.2.1 Chemical precipitation
Chemical precipitation is the most common technology used in removing dissolved (ionic) metals from solutions, such as process wastewaters containing toxic metals. The ionic metals are converted to an insoluble form (particle) by the chemical reaction between the soluble metal compounds and the pre- cipitating reagent. The particles formed by this reaction are removed from solution by settling and/or filtration. The unit operations typically required in this technology include neutralization, precipitation, coagulation/flocculation, solids/liquid separation, and dewatering.
The effectiveness of a chemical precipitation process is dependent on several factors, including the type and concentration of ionic metals present in solution, the precipitant used, the reaction conditions (especially the pH of the solution), and the presence of other constituents that may inhibit the precipita- tion reaction.
The most widely used chemical precipitation process is hydroxide precipitation (also referred to as precipitation by pH), in which metal hydroxides are formed by using calcium hydroxide (lime) or sodium hydroxide (caustic) as the precipitant. Each dissolved metal has a distinct pH value at which the optimum hydroxide precipitation occurs—from 7.5 for chromium to 11.0 for cadmium. Metal hydroxides are amphoteric, which means that they are increasingly soluble at both low and high pH values. Therefore, the optimum pH for precipitation of one metal may cause another metal to solubilize or start to go back into solution. Most process wastewaters contain mixed metals and so precipitating these different metals as hydroxides can be a tricky process (Lewinsky, 2007).
Heavy metal ion in wastewater
Chemical precipitation
Ferrite treatment Membrane
Adsorption
Sulfide precipitation Carbonate precipitation
Insoluble metal oxide salt sludges
Concentrated metal ions in solvent stream
Metal ions immobilized on
solid support Solid metal
Solvent
extraction Ion exchange Electrolytic recovery
FIGURE 6.5
various chemical treatment methods for heavy metal removal from wastewater.
From Lewinsky, A. A. (2007). Hazardous materials and wastewater: Treatment, removal and analysis. New York: Nova Science Publishers Inc.
127 6.3 ENviRONMENtAL APPLicAtiONS
6.3.2.2.2 Solvent extraction
Solvent extraction is a common form of chemical extraction using organic solvent. It is commonly used in combination with other technologies, such as solidification/stabilization, incineration, or soil washing, depending upon site-specific conditions. Solvent extraction also can be used as a stand-alone technology in some instances. Organically bound metals can be extracted along with the target organic contaminants, thereby creating residuals with special handling requirements. Traces of solvent may remain within the treated soil matrix, so the toxicity of the solvent is an important consideration. The treated media are usually returned to the site after having met best demonstrated available technology and other standards.
It has also been shown to be effective in treating sediments, sludge, and soils containing primarily organic contaminants such as PCBs, VOCs, halogenated solvents, and petroleum wastes. The process has been shown to be applicable for the separation of the organic contaminants in paint wastes, syn- thetic rubber process wastes, coal tar wastes, drilling muds, wood-treating wastes, separation sludge, pesticide/insecticide wastes, and petroleum refinery oily wastes (Lewinsky, 2007).
6.3.2.2.3 Ion exchange
Ion exchange is a reversible chemical reaction wherein an ion (an atom or a molecule that has lost or gained an electron and thus acquired an electrical charge) from a wastewater solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion-exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to spe- cific applications (Lewinsky, 2007).
An organic ion-exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of a similar charge from the wastewater. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin. Table 6.1 summarizes some of the various components of the ion-exchange process.
6.3.2.2.4 Electrolytic recovery
This process utilizes what is called the electrolytic cell to recover heavy metal ions from wastewater.
The cell is composed of an anode and a cathode submerged in an electrolyte. When a current is applied, dissolved metals in the electrolyte are reduced and deposited on the cathode. One advantage of this process is that it can target specific contaminants in the wastewater without the addition of chemicals that can produce a large amount of sludge. Through this process the metal is often reusable, defining this as a “recovery” process as opposed to an end-of-pipe process.
Electrolytic recovery is not a useful method for all contaminants. It is most effective in removing the noble metals, such as gold and silver because of their high electrode potential and ease of being reduced and deposited onto the cathode. Metals such as aluminum and magnesium which favor oxidation and have lower electrode potentials cannot be removed by this process. For metals such as copper and tin, a higher current must be applied for this method to be utilized. The following chart is an illustration of the electrolytic recovery process (Lewinsky, 2007) (Fig. 6.6).
The application of nanofibers to the removal of heavy ions from wastewater was explored by Teng, Wang, Li, and Zhang (2010) using a thioether-functionalized organic–inorganic composite membrane with mesostructure. The aforementioned thioether was developed using a combination of a sol–gel process and electrospinning. The film that resulted was fabricated using polyvinylpyrrol-iodone
128 CHAPTER 6 NANOPOLYMERS
Table 6.1 Selectivity of Ion-Exchange Resins in Order of Decreasing Preference
Strong Acid Cation Exchanger Strong Base Anion Exchanger
Barium Iodide
Lead Nitrate
Calcium Bisulfite
Nickel Chloride
Cadmium Cyanide
Copper Bicarbonate
Zinc Hydroxide
Magnesium Fluoride
Potassium Sulfate
Ammonia Sodium Hydrogen
Reproduced from Table 2 in Cheremisinoff, N.P. (2002). Handbook of water and wastewater treatment technologies (pp. 372–445).
Copyright © 2002 Elsevier Inc.
Metal parts
Additonal site specific wastewater treatment
Chemical precipitation
POTW F006
F009 F009
F007 F008
Stripped rack or part
F006 - Wastewater treatment sludges F007 - Spent cyanide plating bath solutions F008 - Plating bath sludges
F009 - Spent stripping and cleaning bath solutions Alkaline chlorination
Cyanide- bearing rack
and part stripping bath
Periodic rinse water discharge Surface
preparation
Cyanide- bearing
plating bath Rinse Electro-plated part
FIGURE 6.6
Hypothetical bearing electroplating process.
From Lewinsky, A. A. (2007). Hazardous materials and wastewater: Treatment, removal and analysis. New York: Nova Science Publishers Inc.
129 6.3 ENviRONMENtAL APPLicAtiONS
(PVP)/SiO2 nanofibers to make the final product. These mesoporous fiber membranes have a very high selective adsorption of Hg2+ and are very easy to recover from a large sample of, say for example, wastewater. This makes them the perfect candidate for this specific heavy ion removal. Fig. 6.7 shows a magnified view of the thioether membranes.
The Hg2+ adsorption capacity of the thioether-functionalized (PVP)/SiO2 membranes was main- tained after being recycled three times. It is this combined with the ease of recovery of the membranes that makes them very promising for heavy metal ion removal (Teng et al., 2010).