Particles in aqueous environments

Size: px

Start display at page:

Download "Particles in aqueous environments"

Gertrude Oliver
5 years ago
Views:

1 Lecture 11 Particle-Aqueous Solute Interactions Today 1. Particle types and sizes 2. Particle charges 3. Particle-solute Interactions Next time Please continue to read Manahan Chapter Fresh-salt water mixing interactions (e.g., in estuaries) 2. Particle-solute interactions in the Estuarine water column 3. Heavy Metals in Estuaries 4. Related processes in Estuarine Sediments Particles in aqueous environments Sparingly soluble and insoluble elements and compounds in aqueous environments are largely affected by interactions with particles. The transport and fate of particles is thus a key parameter for describing such environments Chemical reactions on particle surfaces cause numerous predictable effects, such as: charge transfer chemical exchanges Note that colloids are extremely small particles that do not settle effectively, and are very important agents of transport in the environment. 1

Size relationship of aqueous dissolved and suspended materials (Remember, 1µm = 10-6 m) Particles and micro-fine particles (colloids) cause many unusual chemical effects, not the least

2 Size relationship of aqueous dissolved and suspended materials (Remember, 1µm = 10-6 m) Particles and micro-fine particles (colloids) cause many unusual chemical effects, not the least of which is non conservative mixing between bodies of waters (i.e., mixing results in gains or loss from solution rather than simple dilution) Colloids are so small and dispersed that they behave almost as if they were part of the dissolved load (TDS), even though they are technically part of the particulate load. Spectroscopic Image of Spherical FeO Colloids FeO colloids are commercially available in a range of sizes for water treatment applications. The bar at the bottom of represents 10 micrometers. 2

3 Chemical interactions between colloids and dissolved solutes play a major role in the overall chemistry of natural waters. Chemical elements associated (e.g., chelated, sorbed) with colloids can be removed from the water when the latter are removed from solution. Colloids form from agglomerations of fine particles kept in solution by electronic forces acting on their surfaces. Colloids are composed of H 2 O solvated particles called sols that can organize into structures known as micelles. Removing non-bound H 2 O from a colloidforms a gel (a reversible process). Drying a gel further remakes a solid. only stable at certain conditions of o ph o pe o TDS When these conditions change they become unstable. Destabilized colloids are flocculated (removed) from solution and added to the sediment (true particulate) component of a natural or manmade aqueous system. 3

4 Properties of sub µm particles in water: Colloids form because electronic forces (also known as solvation forces) acting upon their surfaces are stronger than gravitational forces that cause particles to settle. In general, it takes a particle smaller than 1µm (= mm) for solvation be "stronger" than gravity. The role of charge: Particles become charged by interaction with water. Just like true solutes, small particles that carry some sort of chargable or polar component are solubilized by water to form colloids. The more charge a particle acquires from interactions with water, the more easily it forms colloids. Particle Surface Charge is acquired by: acid-base (primary mechanism) ion exchange reactions. charge transfer (e.g., ligand/donor, sorption) Colloids can be made from non-dissolved organic or inorganic particulates. Each compound has specific properties that dictate how and when a colloid will form, and at what conditions it can stay in "solution". 4

5 Reactions on solid surfaces giving rise to charge in H 2 O. Traditional acid-base : (same reaction as for solutes) Gain or loss of a proton by an acidic or basic functional group of the solid's surface. Each functional group has an equilibrium constant for loss of a proton (K a ) and gain of a proton (K b ) in water. Like any acid or base, these are ph dependent. Traditional acid-base : For instance, the R-OH (hydroxyl group) of an organic or inorganic compound can lose a proton to be come R-O - or gain one to become R-OH 2+. And whether or not it is attached to a solid is immaterial. Both are ph dependent reactions. 5

chemical compounds), both negative and positive charges can be found on their surfaces.

6 Here is an example from the text of the development of particle charge on hydrated MnO 2. Because particles may be composed of more than one type of acidic/basic functional group (or even composed of a mixture of chemical compounds), both negative and positive charges can be found on their surfaces. ph has strong influence of on the electrostatic character of particle surfaces. Low ph MOH + H + --> MOH 2 2+ High ph MOH + OH - --> MO - + H 2 O 6

Isoelectric point or the Zero Point of Charge (ZPC) the ph at which the

neither protonated (excess + charge) or deprotonate (excess - charge)

When ph < ZPC, particle surface carries a + charge.

their charge polarity are given in these tables.

7 Isoelectric point or the Zero Point of Charge (ZPC) the ph at which the sum of all surface charges are balanced (neutral) the particle is neither protonated (excess + charge) or deprotonate (excess - charge) in a net sense. When ph > ZPC, particle surface carries a - charge. When ph < ZPC, particle surface carries a + charge. ZPC for various minerals in pure water (no other ions present) and their charge polarity are given in these tables. The ph that corresponds to the ZPC is referred to as the isoelectric point. electrostatic charge repulsion is minimized here 7

8 ion exchange reactions : Some solid compounds can physically trade ions with aqueous solutions, affecting the ionic composition of both. POC and Clay minerals (phyllosilicates) are both in this category. Ion exchange: Moving a particle from one aqueous ionic solution to another can cause the population of attached ions and dissolved ions to change. Clays are a class of minerals with a particularly high number of exchangeable ions commonly attached to them. This is because lattice charge imbalances due to substitutions for structural Si, Al and Mg ions in sheets of polymerized [SiO 4 ] tetrahedra, and sheets of polymerized [AlO 6 ] or [MgO 6 ] units. These charge imbalances are balanced by exchangeable interlayer ions sitting between the sheets The composition of exchangeable interlayer ions depends on the composition of the clay and the solution. There is an equilibrium constant for each ion exchanged. 8

An example Ion Exchange for a montmorillonite clay in river (fresh) water and sea (salty) water is given below. This clay has: [0.16 mole Ca 2+, 0.07 mole Na + and 0.

9 An example Ion Exchange for a montmorillonite clay in river (fresh) water and sea (salty) water is given below. This clay has: [0.16 mole Ca 2+, 0.07 mole Na + and 0.04 mole Mg 2+ of exchangeable ions] [mole clay mineral] Ion exchange capacities: Different materials (clays and otherwise) have different abilities to exchange ions with solution). The cation exchange capacity ("CEC") of some clays are (in units of meq/[100g of solid]) are listed below: CECs are measured by repeatedly equilibrating the material with solutions of pure NaCl in water until no other ions come off the mineral and then analyzing for the amount of exchangeable Na + or Cl - it has acquired. Another ion exchange term you sometimes see is Exchangeable Cation Status (ECS). This is the CEC of a particular ion on a solid or colloid in a mixed-ion solution. Remember, an equivalent (eq) is a mole of charge, so 1 mole of NaCl in water produces 1 equivalent of Na + ions where as 1 mole of MgSO 4 produces 2 equivalents of Mg 2+. a milliequivalent (meq) is 10-3 eq. 9

10 charge transfer (e.g., ligand/donor sorption) : Sorption involves a number of related processes that all result in dissolved aqueous solutes being "stuck" to particles by Lewis acid-lewis base interactions similar to those in complexes. Let's examine sorption of negatively-charged organic ions and ortho P ions by colloid particles. [See Manahan Section 4.7 for examples of the sorption of positively-charged metal ions onto colloid surfaces.] The sorption of organics is particularly important for: changing ionic equilibria in aqueous solutions removing organic matter from aqueous solutions (such as in soils or waste water treatment) helping microorganisms to nucleate on inorganic substrates The sorption of ortho P is particularly important for: the distribution of this photosynthetic nutrient between natural waters and sediments. The sorption-desorption of materials is highly dependent on 1. the acid-base behavior of solutes (their speciation ) 2. the particle surface speciation of the sorbing species. Examples are shown as a function of ph for: a. various organic compounds bound to Al 2 O 3 b. ortho P bound to FeOOH 10

11 How is charge distributed around colloids and larger? This is a complex process that can be modeled in a number of ways. These models involve different arrangements of anions and cations from the solution around the charged surface, which stabilize the charged particle. Gouy layers involve a preferential distribution of oppositely charged particles near a charge surface. "Double Layer" models are the most realistic for most situations. They involve ions in close proximity to the charged surface (Stern or Helmholz layers) which are strongly help in place and a more diffuse balancing ion charge in solution around them. Why do we care? The size and charge density of ion clouds around surfaces are the solvation forces mentioned before. These clouds expand and collapse depending on solution properties (e.g., ph, concentration and types of free ions in solution) and the interaction of one colloid's ion cloud with that of another can affect aqueous solution composition. For instance: in an open body of water, too much ion-cloud interaction could destabilize colloids and cause them to be desolvated, pulling solutes with them. in groundwater systems, close-packed charge surfaces can form "ion traps" that allow water but not ions through. 11

12 Removal of colloids from solution: Small particles with similar surface charges tend to repel one another, preventing the close physical interaction necessary to form large particles. Flocculation, agglomeration and coagulation are all terms that refer to the collapse of sols and the removal of colloids as clumps of solid particles. (We will use the terms interchangeably) Flocculation occurs when electrical and physical forces keeping colloids in solution are exceeded by other electronic and physical forces. As flocculation removes colloids of POC, clays, metal oxides etc.. from solution, ions attached to these may also be removed to the sediments of that body of water. Four common causes for flocculation are: change of flow field in the body of water. change of ionic strength of the solution (ionic strength is a measure of the total dissolved charge in solution -ionic strength corresponds to large amounts of dissolved ions.) evaporation of H2O (which changes ionic strength). introduction of a (chemical) colloid destabilizing agent. 12

Relationship of flow field, particle size/concentration and ionic strength: The water flow field controls the size and concentration of particles in suspension, and the number of collisions among

13 Relationship of flow field, particle size/concentration and ionic strength: The water flow field controls the size and concentration of particles in suspension, and the number of collisions among colloids (which can lead to agglomeration). Ionic strength and ionic composition affects the shape and size of the electric layers around colloid particles and their stability in solution. In general, when the ionic strength gets too high, colloids are destabilized and flocculate. Simple Ionic Strength experiments: The results of various laboratory simulations of the conditions that prevail when low TDS water (river) meets high TDS water (sea water) in estuaries are given below. For instance, the rate and extent of kaolinite (clay) flocculation increases as increasing ionic strength increases. Note, Ionic strength is related to the molality of charge in solution 13

14 Another laboratory experiment examined how electrolytes cause flocculation of inorganic ions and DOC of river waters in the sea mixing zone of estuaries, using Artificial electrolytes and Real colloids. Artificial electrolytes: flocculent effectiveness: NaCl <MgCl 2 <CaCl 2. NaCl-MgCl 2 difference: there are two equivalents of Mg 2+ per mole of MgCl 2 versus only one equivalent per mole of NaCl. Mole for mole, MgCl 2 increases the ionic strength more than does NaCl. CaCl 2 -MgCl 2 difference: Ca 2+ ions are larger than Mg 2+ ions, making them better at destabilizing Fe, Al and humic acid colloids. Real electrolyte (seawater) Flocculation is fast for all major river borne ions. Fe and Humic Acids (HA) flocculate more efficiently with seawater than with pure artificial electrolytes, but the other elements behave similarly with both. 14

Lecture 13 More Surface Reactions on Mineral Surfaces. & Intro to Soil Formation and Chemistry

Lecture 13 More Surface Reactions on Mineral Surfaces & Intro to Soil Formation and Chemistry 3. charge transfer (e.g., ligand/donor sorption): Sorption involves a number of related processes that all