Precipitation and Coprecipitation

Initially, we need to know the thermodynamic properties or particularly the constant of solubility, besides the energy involved and verify if the process is thermodynamically possible. It is also very important to know about the kinetics of precipitation for determining the precipitation rates. 

There are two types of materials resulting from the precipitation, i.e., amorphous and crystalline, or mixed, which have different properties. 

To obtain crystalline materials, the precipitation occurs in two steps [1-8]  

Nucleation—Germs are formed in solution in homogeneous phase. There is interaction between ions and molecules in solution that form clusters, groups, or curls, and the crystallization begins. If precipitation occurs in the presence of a support, it occurs in a heterogeneous phase germs are formed due to the interaction of ions with the surface to form crystal nucleus. 

Growth—This is a physicochemical process at the sohd-solution interface, growing crystal nuclei, but the growth rate depends on the kinetics. 

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The coefficient actually measures multiple processes (reversible and irreversible adsorption, precipitation, and coprecipitation). Consequently, it is a purely empirical number with no theoretical basis on which to predict adsorption under differing environmental conditions or to give information on the types of bonding mechanisms involved. 

Precipitation can remove soluble nickel Ifom water. In aerobic waters, nickel ferrite is the most stable compound (Rai and Zachara 1984). Nickel may also be removed by coprecipitation with hydrous iron and manganese oxides. Nickel removed by precipitation and coprecipitation settles into the sediment. 

Byrne, R. H. Kim, K.-H. 1993. Rare earth precipitation and coprecipitation behavior The limiting role of PC>4 on dissolved rare earth concentrations in seawater. Geochimica et Cosmochimica Acta, 57, 519-526. 

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

Sorption, ion exchange, precipitation, and coprecipitation of arsenic in water 2.7.6.1 Introduction... 

Once arsenic dissolves in natural water, it may remain in solution for an extended period of time or participate sooner in abiotic or biotic reactions that remove it from solution. Depending upon the pH, redox conditions, temperature, and other properties of an aqueous solution and its associated solids, dissolved arsenic may precipitate or coprecipitate. Arsenic may also sorb onto solid materials, usually through ion exchange. Due to their importance in understanding the behavior of arsenic in natural environments (Chapter 3) and their applications in water treatment (Chapter 7), the sorption, ion exchange, precipitation, and coprecipitation of arsenic have been the subjects of numerous investigations. 

Schwertmannite may also be associated with the biological precipitation and coprecipitation of As(V). Through field and laboratory investigations, Ohnuki et al. (2004) studied the coprecipitafion of As(V) from pH 4.7 mine water by iron-rich bacterial colonies of Gallionella sp. They found that As(V) was associated with iron and sulfur in schwertmannite precipitates within the colonies. 

The processes of adsorption, precipitation and coprecipitation are difficult to distinguish on that basis from the analysis of the diminution of the ions from the solution, changes of pH and kinetics. Only the spectroscopic investigations of the molecular interactions between adsorbent and adsorbate may help to distinguish a type of the process [146,147]. As an adsorption of the ions, is assumed process of the two-dimensional structure formation, whereas for three-dimensional structures precipitation or surface precipitation takes place. From this reason an AFM method may be useful at investigations of the morphology changes of the adsorbate surface [147]. 

Four main routes exist for preparing the primary solid deposition, precipitation and coprecipitation, gel formation, selective removal. 

Precipitation and coprecipitation Synthesis of defect-rich materials, easy to perform Homogeneity difficult to achieve... 

Schiith F, Hesse M, Unger KK. Precipitation and coprecipitation. Handbook of Heterogeneous Catalysis. Weinheim Wiley-VCH Verlag GmbH Co. KGaA 2008. p. 100-119. 

Khoudiakov, M., Gupta, M. C. and Deevi, S. (2005). Au/Fe203 nanocatalysts for CO oxidation A comparative study of deposition - precipitation and coprecipitation techniques. Appl. Catal. A-Gen. 291(1-2), 151-161. 

For some case studies on surface precipitation and coprecipitation, see Dzombak and Morel (1990), Sposito (1986), and Wersin et al. (1989). 

In the analysis of high-purity substances, general matrix removal is often very important so as to pre-concentrate the elements to be determined. To this aim all separation techniques such as ion exchange, liquid-liquid extraction of a metal complex with organic solvents, fractionated crystallization, precipitation and coprecipitation as well as electrochemical methods may be used (for a systematic treatment, see Ref. [300]). These principles can also be applied in on-line systems, as is now possible with solid phase extraction. Here matrix elements or the analytes can be adsorbed as complexes onto the column and eluted for direct determination by AAS. 

The diagram in Figure 4.15 emphasizes the continuity of precipitation and coprecipitation processes with chemisorption, both in time and space. Low levels of adsorbate (whether metal cations or anions) are usually bound by chemisorption, higher levels by the formation of sohd solutions or by the nucleation of small adsorbate clusters at surfaces. The highest levels of adsorbate lead to precipitation of separate mineral phases, a process that can be viewed as an extension of cluster growth that allows a new solid phase to become detectable. 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

Schiith, F. and Unger, K. (1997) in Precipitation and Coprecipitation, Handbook of Heterogeneous Catalysis (eds G. ErtI, H. Knozinger and... 

This method is closely related to precipitation, but makes use of the fact that when / ertain substances are precipitated other substances are coprecipitated. In radiochemistry in particular, coprecipitation with isotopic or non-isotopic carriers is still commonly practised today. In classical water analysis coprecipitation is often used to concentrate trace elements. In this way, for example, arsenic can be more or less completely coprecipitated and isolated with iron (III) hydroxide. With manganese in a manganese (III) or manganese (IV) compound, thallium can be coprecipitated and concentrated. Sections 3.2 to 3.5 and Section 3.7 deal with cases where precipitation and coprecipitation are still in general use in water... 

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