Learning goals Know the carbon atom Where acid rain comes from What is pH and how to calculate...

Learning goals Know the carbon atom Where acid rain comes from

What is pH and how to calculate Carbonate equilibrium reactions

Why important Alkalinity Chemical weathering

Learning goals Climate controls on atmospheric CO2 Ocean acidification

What causes it Why important What does the future hold

CARBON Shells: 2,4 Minimum oxidation

number is –4 Maximum oxidation

number is +4

Carbon Isotopes C-12 C-13 C-14

Carbon forms Graphite Diamond Buckmisterfullerene Organic Matter DOC Particulate C

Types of carbon compounds Gas phase

CO2, methane, volitale organic compounds (VOCs)

Organic Amino acids, DNA, etc

Water Dissolved inorganic carbon (DIC) Dissolved organic carbon (DOC)

DOC in GROUNDWATER Less than 2 mg/L Microbial decomposition Adsorption Precipitation as solid > 100 mg/L in polluted ag systems Increases geochemical weathering

ORGANICS in WATER Solid phases (peat, anthracite, kerogen Liquid fuels (LNAPL), solvents (DNAPL) Gas phases Dissolved organics (polar and non-polar)

CARBONATE SYSTEM Carbonate species are necessary for all

biological systems Aquatic photosynthesis is affected by the

presence of dissolved carbonate species. Neutralization of strong acids and bases Effects chemistry of many reactions Effects global carbon dioxide content

DIPROTIC ACID SYSTEM Carbonic Acid (H2CO3)

Can donate two protons (a weak acid)

Bicarbonate (HCO3-)

Can donate or accept one proton (can be either an acid or a base

Carbonate (CO32-) Can accept two protons (a base)

OPEN SYSTEM Water is in equilibrium with the partial

pressure of CO2 in the atmosphere

Useful for chemistry of lakes, etc Carbonate equilibrium reactions are thus

appropriate

PCO2 = 10–3.5 yields pH = 5.66

»What is 10–3.5? 316 ppm CO2

What is today’s PCO2? ~368 ppm = 10-3.43

»pH = 5.63

Ocean pH and atmospheric CO2

NATURAL ACIDS Produced from C, N, and S gases in the

atmosphere H2CO3 Carbonic Acid

HNO3 Nitric Acid

H2SO4 Sulfuric Acid

HCl Hydrochloric Acid

pH of Global Precipitation

http://www.motherjones.com/tom-philpott/2015/01/noaa-globes-coral-reefs-face-massive-bleaching-event-2015

OPEN SYSTEM• Water is in equilibrium with the partial

pressure of CO2 in the atmosphere

• Useful for chemistry of lakes, etc

• Carbonate equilibrium reactions are thus appropriate

Carbonic acid forms when CO2 dissolves in and reacts with water:

CO2(g) + H2O = H2CO3

»Most dissolved CO2 occurs as “aqueous CO2” rather than H2CO3, but we write it as carbonic acid for convenience»The equilibrium constant for the reaction is:

»Note we have a gas in the reaction and use partial» pressure rather than activity

»First dissociation:

H2CO3 = HCO3– + H+

FIRST REACTION

»Second dissociation:

HCO3– = CO3

2– + H+

SECOND REACTION

Variables and Reactions Involved in Understanding the Carbonate System

Activity of Carbonate Species versus pH

CARBONATE SPECIES and pH

pH controls carbonate species Increased CO2 (aq) increases H+ and

decreases carbonate ion Thus increasing atmospheric CO2

increases CO2 (aq) and causes the water system to become more acidic

However, natural waters have protecting, buffering or alkalinity

ALKALINITY refers to water's ability, or inability, to neutralize acids.

The terms alkalinity and total alkalinity are often used to define the same thing.

Alkalinity is routinely measured in natural water samples. By measuring only two parameters, such as alkalinity and pH, the remaining parameters that define the carbonate chemistry of the solution (PCO2, [HCO3

–], [CO32–], [H2CO3]) can be

determined.

Total alkalinity - sum of the bases in equivalents that are titratable with strong acid (the ability of a solution to neutralize strong acids)

Bases which can neutralize acids in natural waters: HCO3

–, CO32–,

B(OH)4–, H3SiO4

–, HS–, organic acids (e.g., acetate CH3COO–, formate HCOO–)

Carbonate alkalinity Alkalinity ≈ (HCO3

–) + 2(CO32–)

Reason is that in most natural waters, ionized silicic acid and organic acids are present in only small concentrations

If pH around 7, then Alkalinity ≈ HCO3

–

CLOSED CARBONATE SYSTEM

• Carbon dioxide is not lost or gained to the atmosphere

• Total carbonate species (CT) is constant regardless of the pH of the system

• Occurs when acid-base reactions much faster than gas dissolution reactions

• Equilibrium with atmosphere ignored

TOTAL CARBONATE SPECIES (CT)

How does [CO3–2] respond to changes in Alk or DIC?

 CT = [H2CO3*] + [ HCO3

–] + [CO3–2]

 ~ [ HCO3

–] + [CO3–2] (an approximation)

 Alk = [OH–] + [HCO3

–] + 2[CO3–2] + [B(OH)4

-] – [H+]

 ~ [HCO3

–] + 2[CO3–2] (a.k.a. “carbonate alkalinity”)

 So (roughly): 

[CO3–2] ~ Alk – CT

  CT ↑ , [CO3

–2] ↓ Alk ↑ , [CO3–2] ↑

Diurnal changes in DO and pHWhat’s up?

Photosynthesis is the biochemical process in which plants and algae harness the energy of sunlight to produce food. Photosynthesis of aquatic plants and algae in the water occurs when sunlight acts on the chlorophyll in the plants. Here is the general equation:

6 H20 + 6 CO2 + light energy —> C6H12O6 + 6 O2

Note that photosynthesis consumes dissolved CO2 and produces dissolved oxygen (DO). we can see that a decrease in dissolved CO2 results in a lower concentration of carbonic acid (H2CO3), according to:

CO2 + H20 <=> H2CO3 (carbonic acid)

As the concentration of H2CO3 decreases so does the concentration of H+, and thus the pH increases.

Cellular Respiration

Cellular respiration is the process in which organisms, including plants, convert the chemical bonds of energy-rich molecules such as glucose into energy usable for life processes.

The equation for the oxidation of glucose is:

C6H12O6 + 6 O2 —> 6 H20 + 6 CO2 + energy

As CO2 increases, so does H+, and pH decreases.

Cellular respiration occurs in plants and algae during the day and night, whereas photosynthesis occurs only during daylight.

LITHOSPHERE Linkage between the atmosphere and the

crust Igneous rocks + acid volatiles =

sedimentary rocks + salty oceans (eq 4.1)

IMPORTANCE OF ROCK WEATHERING[1] Bioavailability of nutrients that have no

gaseous form: P, Ca, K, Fe

Forms the basis of biological diversity, soil fertility, and agricultural productivity

The quality and quantity of lifeforms and food is dependent on these nutrients

IMPORTANCE OF ROCK WEATHERING[2] Buffering of aquatic systems

-Maintains pH levels

-regulates availability of Al, Fe, PO4

Example: human blood.-pH highly buffered-similar to oceans

IMPORTANCE OF ROCK WEATHERING[3] Forms soil

[4] Regulates Earths climate

[5] Makes beach sand!

RockCycle

Sedimentary Processes1) Weathering & erosion

2) Transport & 3) deposition

4) Lithification

Weathering: decomposition and disintegration of rock

Product of weathering is regolith or soil

Regolith or soil that is transported is called sediment

Movement of sediment is called erosion

Weathering Processes

Chemical Weathering-

Decomposition of rock as the result of chemical attack. Chemical composition changes.

Mechanical Weathering -

Disintegration of rock without change in chemical composition

Mechanical Weathering

•Frost wedging•Alternate heating and cooling

•Decompression causes jointing

Chemical Weathering Processes Hydrolysis - reaction with water (new minerals

form) Oxidation - reaction with oxygen (rock rusts) Dissolution - rock is completely dissolved

Most chemical weathering processes are promoted by carbonic acid:

H2O +CO2 = H2CO3 (carbonic acid)

CARBONIC ACID

Carbonic acid is produced in rainwater by Reaction of the water with carbon dioxide Gas in the atmosphere.

CARBONATE (DISSOLUTION)

All of the mineral is completelyDissolved by the water.Congruent weathering.

DEHYDRATION

Removal of water from a mineral.

HYDROLYSIS

H+ replaces an ion in the mineral.Generally incongruent weathering.

HYDROLYSIS Silicate rock + acid + water = base cations

+ alkalinity + clay + reactive silicate (SiO2)

Hydrolysis

Feldspar + carbonic acid +H2O= kaolinite (clay) + dissolved K (potassium) ion + dissolved bicarbonate ion+ dissolved silicaClay is a soft, platy mineral, so the rock disintegrates

HYDROLYSIS Base cations are

Ca2+, Mg2+, Na+, K+

Alkalinity = HCO3-

Clay = kaolinite (Al2Si2O5(OH)4)

Si = H4SiO4; no charge, dimer, trimer

OXIDATION

Reaction of minerals with oxidation.An ion in the mineral is oxidized.

Oxidation can affect any iron bearing mineral, for example, ferromagnesian silicates which react to form hematite and limonite

Oxidation

Oxidation of pyrite and other sulfide minerals forms sulfuric acid which acidifies surface water and rain

Pyrite + oxygen + water = sulfuric acid + goethite(iron sulfide) (iron oxide)

Products of weathering

Clay minerals further decompose to aluminum hydroxides and dissolved silica.

Removal of Atmospheric CO2 Slow chemical weathering of continental rocks balances

input of CO2 to atmosphere Chemical weathering reactions important

Hydrolysis and Dissolution

Atmospheric CO2 Balance Slow silicate rock weathering balances

long-term build-up of atmospheric CO2

On the 1-100 million-year time scale Rate of chemical hydrolysis balance rate of

volcanic emissions of CO2

Neither rate was constant with time

Earth’s long term habitably requires only that the two are reasonably well balanced

What Controls Weathering Reactions? Chemical weathering influenced by

TemperatureWeathering rates double with 10°C rise

PrecipitationH2O is required for hydrolysis

Increased rainfall increases soil saturationH2O and CO2 form carbonic acid

VegetationRespiration in soils produces CO2

CO2 in soils 100-1000x higher than atmospheric CO2

Climate Controls Chemical Weathering

Precipitation closely linked with temperature Warm air holds more water

than cold air Vegetation closely linked with

precipitation and temperature Plants need water Rates of photosynthesis

correlated with temperature

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that

reduces the intensity of climate warming

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that

reduces the intensity of climate cooling