low temp geochemistry

why is water important?

almost always involved in geochem processes, large # of H bonds for its size, good solvent, dissolves cell components

dipolarity of water

high heat capacity

why are hy shells formed?

polar H2O structures itself around the ions to neutralize (delocalize charge)

electronegativity

tendency to lose electrons

ionic potential

measures charge density at “surface” of ion

ionic potential equation

charge of ion (valence) / ionic radius 

before becoming part of a mineral structure, ions must

decrease their size by losing any hy shells

ions w/ very high ionic potential hydrated

will retain oxygens from hy shells

ion pairs

ions w/ hy shells in combination w/ those of opposite charge

how do ion pairs affect geochemical reactions?

limits the availability/activity of ions

how do we determine activity of ions?

activity coefficient * molarity

ionic strength

concentration of each specific ion and their charge

very dilute solutions

γ = 1

concentrated aq solutions

a_i < m_i, γ < 1, a_H2O > 0.99

highly concentrated solutions

not enough H₂O molecules to produce complete hy shells a_i > m_i, γ > 1, a_H2O < 0.9

how to study multiple ions in solution

quantify interaction of each ion w/ all other potential counterions

speciation diagrams

quantitative info about activities of all ions in the aq solutions

speciation

of a given element in an aq system, refers to form in which it exists as a simple dissolved ion, polyatomic ion, particulate etc

how does the abundance of species change?

with pH, redox conditions, P & T

speciation diagrams are the functions of

pH or Eh, P or T

solubility product

example: max amount of sugar you can dissolve in pure H₂O at std conditions

solubility example

max amount of sugar you can dissolve in aq solution

definition of solubility

ability for given substance to dissolve in a solvent at specific conditions

requirement to determine solubility or solubility product

minerals need to reach max dissolution stage so that equil reached

solubility product (K_sp)

equil constant for solid substance dissolving in aq solution at ambient conditions, log or exp

ambient conditions

21 degrees C, 1 atm

what does the solubility product represent?

max lvl at which solid dissolves in solution & activities = conc of ions @ equil

saturation index

parameter that tells the likelihood of precipitation of s phase

saturation index equation

SI = log(IAP/K_sp)

supersaturation

precipitation should happen if SI > 0

saturation 

precipitation rate = dissolution rate equil, SI = 0

subsaturation

precipitation should NOT happen SI<0

nitrates NO₃^- solubility

all soluble

chlorides Cl^- solubility

all soluble except AgCl, PbCl₂

sulfates solubility

SO₄^2- all soluble except PbSO₄, CaSO₄, BaSO₄

True solution

ions dissolved in H₂O, no solids at all

Colloidal solution 

solid nanoparticles, never settle, surface charge can allow repulsion

suspension

solid particles, big enough to sediment

Tyndell effect

scattering of light passing thru medium w/ dispersed particles

how to make small mineral particles

size reduction, crystallization, otswald-ripening

otswald-ripening

process where bigger particles grow ‘fueled’ by smaller ones, smaller dissolve & reprecipitate as part of larger

why does otswald-ripening occur?

particles are attempting to minimize surface E by ↑ avg particle size

site density

numerical density of sites per nm² available for ion adsorption, particle can have multiple sites

characteristics of clay (phyllosilicates)

colloidal transport, plasticity, negative surface

clay structure

silicon-oxygen tetrahedrons and O/OH^- & Mg⁺², Fe⁺², Al⁺³ octahedrons

delamination of clays/exfoliation

separation of diff clay layers, increases surface area

how are iron oxides & hydroxides useful for the environment?

absorbing/remediating arsenates, selenates, & phosphates from water, easy to obtain

what are phosphates in

fertilizer, bone tissue, ionic/semiconductors, storage of radionuclides

phosphate pollution

eutrophication from fertilizer 

how does eutrophication work?

excess of nutrients, plant growth, algal blooms, decomposition, oxygen depletion & death

acetaldehyde (CH₃CH=0)

released in combustion, by-product of PET, metabolite of ethanol, induces DNA interstrand crosslinks, carcinogenic & irritant

sorption

physiochemical process where a substance becomes attached to another

aBsorption

ions incorporated into mineral structure entering from the surface

aDsorption

ions/molecules held at the mineral structure as a hydrated species

ion exchange

ion becomes aDsorbed to surface by switching w/ similarly charged ion on surface

sorption removes

solutes from solution onto surfaces

sorbate

species removed from solution

sorbent

solid onto which solution species are sorbed

electric double layer (EDL)

hydrated structure that forms onto surface of mineral when in contact w/ a liquid

EDL surface charge

charged ions aDsorbed on particle surface (often negative)

EDL stern layer

counterions attracted to particle surface & closely attached by electrostatic forces

diffuse layer

film of solvent/dispersion medium adjacent to particle, contains free ions w/ higher conc of counterions

diff between ad/absorption and surface (co)precipitation

ad/absorption no new crystals form

aDsorption inner ion sphere complex

bonds to a specific site on surface, ignores overall electrostatic interaction w/ bulk surface

aDsorption inner sphere structure

mononuclear bidentate or monodentate

EDL outer ion sphere complex

remains bonded to the hy shell, no direct surface bond, purely electrostatic attraction

point of zero charge (pH_zpc)

pH where aDsorption surface has no net charge, measured by titration curves or electrophoretic mobility

electrophoretic mobility

tendency of solids to migrate towards positively charged plate

pH below the pH_zpc

more sites are pronated (H⁺), net positive charge

pH above the pH_zpc

more sites are unpronated (OH^-), net negative charge

aBsorption, substitution of ions of the

same oxidation state, coordination # & ionic radius

Results of mineral-water interactions

Heterogeneous nucleation (surface growth of new secondary phases) or homogenous nucleation (colloidal nano particles

Oriented overgrowths

When certain structural planes have similar spacings between atoms

If overgrowth covers full surface of substrate

Partial equilibrium could be reached

Co-precipitation

Overgrowth combines ions from substrate & aq solution

two steps for clean water

lime neutralization and trace toxic metals adsorption on minerals

lime neutralization process

precipitation in a high-density sludge, increases water pH to ~9, most toxic metals become insoluble & precipitate, air may be introduced to oxidize Fe & Mn

what controls the pH of most natural waters?

reactions involving the carbonate system

examples of carbonates

stromalites, chalk, Italian dolomites

why does CO₂ release in water?

to reequilibrate with the atm

acid rain

NO_xSO_x from industry dissolved in rain drops

how is seawater kept ~ pH 8.2?

buffered by calcite in contact

calcite at low P_CO2

calcite precipitates at a higher pH

calcite at a lower pH

increased activity of Ca²⁺ to precipitate calcite since most C in acidic conditions is H₂CO₃

calcite at a lower P

less Ca²⁺ need to precipitate calcite

2 most abundant CaCO polymorphs

calcite and argonite

Calcite polymorph

Ca=6-fold coordination, the most thermoD stable, main mineral of limestone

aragonite polymorph

Ca=9-fold coordination, metastable at ambient conditions, more soluble than calcite in seawater, denser and stable at higher T

rhombohedral structure

calcite-type divalent anhydrous cation carbonates, 6-fold, Ni, Mg, CO, Zn, Fe, Mn, Cd

orthorhombic structure

aragonite-type divalent anhydrous cation carbonates, 9-fold, Sr, Ba, Pb

Mg²⁺ in solution on calcite

is adsorbed onto the surface

Mg²⁺ on surface at T < 60°C

hydration shell is still present

Mg²⁺ hy shell on calcite surface

inhibits calcite growth

at high Mg/Ca ratios

CaCO₃ crystallizes as aragonite, not as affected by Mg²⁺ in solution

solubility of Mg-calcite

proportional to Mg²⁺ content, Mg²⁺-bearing calcite more soluble than pure calcite

on a geological timescale what is Mg’s effect?

alternating aragonite (today) and calcite seas in icehouse and greenhouse earths

coral conditions

retrograde/inverse solubility (better in warmer water)

diatoms

single cell algae w/ SiO skeletons, direct solubility (thrive in cold water)

carbon direct sequestration

CO₂ stored as a gas underground, direct injection of CO₂ in brines, risk of pressure and escape/release

carbon conversion

converted into minerals, long-term stability, interaction of CO₂-rich water w/ basaltic rocks, carbonation reaction