Chemical Reaction in Earth Surface

7/29/2019 Chemical Reaction in Earth Surface

1/23

Reactions at the earthssurface


2/23

Geochemistry of the earths surface is

dominated by aqueous solution and their

interactions with rock.

In this chapter, we focus on water and its

interaction with solids at the Earths surface


3/23

We can broadly distinguish two kinds of

aqueous solutions:

1) Continental water: ground water, fresh

surface waters (river, stream and lake waters),

and saline lake waters.

2) Seawater

Hydrothermal fluids are third class of water produced when water

is heated and undergoes accelerated interactions with rock and

often carry a much higher concentration of dissolved

constituents than fresh water.


4/23

Redox in Natural Waters

The surface of the Earth represents a boundary between

regions of very different redox state.

The atmosphere contains free oxygen and therefore is highly

oxidizing

In the Earths interior, however, there is no free oxygen

Natural waters exist in this boundary region and their redox

state, perhaps not surprisingly, is highly variable.

Biological activity is the principal cause of the different redox

state.

The redox state of solutions and solids at the Earths surface is

largely governed by the balance between photosynthesis and

respiration.


5/23

Water in equilibrium with atmospheric

oxygen has a pe of +13.6 (at pH = 7).

At this pe, thermodynamics tells us that all

carbon should be present as CO2 (or

related carbonate species), all nitrogen as

NO3-, all S as SO4

2-, all Fe as Fe3+, and

all Mn as Mn4+.


6/23


7/23


8/23

Biogeochemical Redox Reactions

photosynthesis and atmospheric exchangemaintains a high p in surface waters.

Once oxygen is consumed, a variety of

specialized bacteria continue to consumeorganic matter and respire utilizing oxidants

other than oxygen. Thus p will continue to

decrease.


9/23

Figure 13.1. Important biogeochemical redox

couples in natural waters.


10/23

Eutrophication

The extent to which the redox sequence

depends on a several factor:

1. Temperature structure2. Nutrient levels


11/23

In lakes with high nutrient levels:

Regions where dissolved oxygen is present aretermed oxic, those where sulfide or methaneare present are called anoxic.

Regions of intermediate p are called suboxic. Lakes are where suboxic or anoxic conditions

exist as a result of high biological producitivy

are said to beeutrophic

. The process is called eutrophication


12/23

Redox Buffers and Transition Metal

Chemistry

The behavior of transition metals in aqueous

solutions and solids in equilibrium with them

is particularly dependent on redox state. Many transition metals have more than one

valence state within the range of p of water.

Redox conditions influence a strong control onthe concentrations of these elements in natural

waters.


13/23

For the example: Fe and Mn

There two common circumstances wherehigher Fe and Mn concentrations in water

occur:

1) The first is when sulfide ores are exposedby mining and oxidized to sulfate

2) The second circumstance where higher Fe

and Mn concentrations occur is under suboxicor anoxic conditions that may occur in deep

waters of lakes.


14/23


15/23

Weathering, Soils, and

Biogeochemical Cycling


16/23

Weathering is the process by which rock is

physically and chemically broken down into

relatively fine solids (soil or sediment

particles) and dissolved components.

Weathering plays a key role in the exogenic

geochemical cycle (i.e., the cycle operating at

the surface of the Earth).

Weathering related to atmospheric CO2


17/23

What are the factors that control on weathering

rates?

1) Temperature (Berner et al. (1983) and

Berner (1991))

2) Tectonic uplift and exposure of fresh rock

(Edmond et al. (1995))


18/23

Soil Profiles


19/23

Chemical Cycling in Soils


20/23


21/23

Biogeochemical Cycling

The biota plays a substantial role in the

weathering process and in controlling the

composition of streams.

The biota will influence both soil and water

chemistry within a water-shed and can be asignificant reservoir for some elements.


22/23

There are four sources of nutrients in an

ecosystem:

1) The atmosphere

2) Dead organic matter

3) Water

4) Rock


23/23

WEATHERING RATES AND REACTIONS

The Watershed Approach

Chemical Reaction in Earth Surface

Documents