Matter Cycles

Overview: Matter Cycles in the Environment

In environmental chemistry, “matter cycles” (also called biogeochemical cycles) describe how chemical elements and simple compounds move and transform between different environmental compartments: atmosphere, hydrosphere (water), lithosphere (rocks and soil), and biosphere (living organisms).

In this chapter, the focus is on:

• The chemical forms in which key elements circulate,
• The transformations between these forms,
• The main natural and human-influenced pathways.

We concentrate on a few central cycles that are especially important for environmental chemistry: the water, carbon, nitrogen, sulfur, and phosphorus cycles.

Throughout, keep in mind:

• Total mass of each element is conserved.
• What changes are the chemical form, location, and availability (e.g. for organisms).

The Water Cycle (Hydrologic Cycle)

Main Reservoirs and Fluxes

The water cycle describes the movement of $H_2O$ between:

• Atmosphere (water vapor, clouds),
• Hydrosphere (oceans, lakes, rivers, groundwater, ice),
• Biosphere (water in organisms, in soil as soil water).

Key processes:

• Evaporation: liquid water $\rightarrow$ water vapor.
• Transpiration: water vapor loss from plants.
• Condensation: water vapor $\rightarrow$ liquid droplets (clouds, fog).
• Precipitation: rain, snow, hail.
• Infiltration and percolation: water entering and moving through soil.
• Runoff: surface flow into rivers, lakes, and oceans.

Chemical Aspects Relevant to Environmental Chemistry

Water is also a transport medium for:

• Dissolved ions: e.g. $Na^+$, $Ca^{2+}$, $Cl^-$, $NO_3^-$, $SO_4^{2-}$, $PO_4^{3-}$.
• Dissolved gases: $O_2$, $CO_2$, $CH_4$.
• Suspended particles and pollutants.

Important chemical processes in the water cycle:

• Dissolution and precipitation of salts (e.g. $CaCO_3$ dissolving and re-precipitating).
• Acid–base reactions in rain and natural waters (e.g. $CO_2$ + water).
• Redox processes in surface water vs. oxygen-poor sediments.
• Transport and dilution of pollutants.

Human influences:

• Emissions that produce acid rain modify the chemistry of precipitation.
• Changes in land use (deforestation, urbanization) alter evaporation–runoff balance and pollutant transport.

The Carbon Cycle

Carbon circulates mostly as:

• Inorganic carbon: $CO_2$, $HCO_3^-$, $CO_3^{2-}$.
• Organic carbon: in biomass (carbohydrates, fats, proteins), dissolved and particulate organic matter, fossil fuels.

Main Reservoirs

• Atmosphere: mainly $CO_2$ and trace $CH_4$.
• Hydrosphere: dissolved $CO_2$, $HCO_3^-$, $CO_3^{2-}$, organic carbon.
• Lithosphere: carbonates (e.g. $CaCO_3$), fossil fuels (coal, oil, natural gas).
• Biosphere: living organisms and dead organic matter (soil humus, detritus).

Key Processes

Photosynthesis and Respiration

• Photosynthesis (highly simplified overall stoichiometry):

$$
6\,CO_2 + 6\,H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6\,O_2
$$

• Aerobic respiration (in organisms, and microbial decomposition):

$$
C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O
$$

These two processes link atmospheric $CO_2$ with organic carbon in the biosphere.

Carbonate System in Water

$CO_2$ dissolves in water and participates in acid–base equilibria:

1. Dissolution and hydration:
   $$
   CO_2(\text{g}) \rightleftharpoons CO_2(\text{aq}) + H_2O \rightleftharpoons H_2CO_3
   $$
2. First dissociation:
   $$
   H_2CO_3 \rightleftharpoons H^+ + HCO_3^-
   $$
3. Second dissociation:
   $$
   HCO_3^- \rightleftharpoons H^+ + CO_3^{2-}
   $$

This system buffers the pH of natural waters and is central to ocean chemistry and carbonate rock formation/dissolution.

Sedimentation and Rock Formation

• Formation of carbonates (biogenic or inorganic):

$$
Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3 \downarrow
$$

• Long-term burial of organic matter (e.g. in sediments) leads to fossil fuel formation over geological times.

Human Impacts

• Burning of fossil fuels:

$$
C_{\text{(fossil fuel)}} + O_2 \rightarrow CO_2
$$

• Land-use changes (deforestation, drainage of peatlands).

Consequences:

• Increased atmospheric $CO_2$ and other greenhouse gases ($CH_4$),
• Ocean acidification (shift in carbonate equilibria),
• Changes in carbon storage in soils and vegetation.

The Nitrogen Cycle

Nitrogen $N$ occurs in many oxidation states, from $-3$ in $NH_3$/$NH_4^+$ to $+5$ in $NO_3^-$. Transformations among these states are largely mediated by microorganisms and are crucial for plant nutrient availability and environmental pollution.

Main Nitrogen Species

• Atmospheric: $N_2$ (dinitrogen, very stable), $N_2O$ (nitrous oxide), $NO$, $NO_2$.
• In soils and waters:
• Reduced forms: ammonium $NH_4^+$, ammonia $NH_3$.
• Oxidized forms: nitrite $NO_2^-$, nitrate $NO_3^-$.
• Organic nitrogen: amino acids, proteins, nucleic acids.

Key Processes

Nitrogen Fixation

Conversion of inert $N_2$ into bioavailable forms (mainly $NH_3$/$NH_4^+$).

• Biological nitrogen fixation (by certain bacteria and archaea):

$$
N_2 + 8\,H^+ + 8\,e^- \rightarrow 2\,NH_3 + H_2
$$

• Industrial fixation (Haber–Bosch process, discussed in another chapter):

$$
N_2 + 3\,H_2 \rightleftharpoons 2\,NH_3
$$

• Abiotic fixation: lightning forms nitrogen oxides that are converted to nitrate in rain.

Nitrification

Microbial oxidation of ammonium to nitrate via nitrite:

1. 
   $$
   NH_4^+ + 1.5\,O_2 \rightarrow NO_2^- + 2\,H^+ + H_2O
   $$
2. 
   $$
   NO_2^- + 0.5\,O_2 \rightarrow NO_3^-
   $$

This process acidifies soils (production of $H^+$) and creates mobile nitrate.

Denitrification

Under oxygen-poor conditions, bacteria use $NO_3^-$ as an oxidizing agent, forming gaseous nitrogen species:

$$
2\,NO_3^- + 10\,e^- + 12\,H^+ \rightarrow N_2 + 6\,H_2O
$$

Intermediate gases: $NO_2$, $NO$, $N_2O$.

Ammonification (Mineralization)

Decomposition of organic nitrogen (proteins, etc.) to ammonium:

$$
\text{R-NH}_2 + H_2O \rightarrow NH_3 + \text{other products}
$$

$NH_3$ in water largely exists as $NH_4^+$ depending on pH:

$$
NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-
$$

Human Influences

• Industrial fertilizer production (large inputs of reactive nitrogen).
• Combustion processes (vehicles, power plants) forming $NO_x$.
• Agricultural practices: manure and fertilizer use → nitrate leaching and $N_2O$ emissions.

Environmental effects:

• Eutrophication of waters by nitrate.
• Acidification via nitrification and $NO_x$-derived acids.
• Contribution of $N_2O$ to greenhouse effect and stratospheric chemistry.

The Sulfur Cycle

Sulfur occurs in oxidation states from $-2$ (sulfide) to $+6$ (sulfate). Its cycle involves gases, aqueous ions, and solid minerals.

Main Sulfur Species

• Reduced: hydrogen sulfide $H_2S$, metal sulfides $MS$ (e.g. $FeS_2$), organic sulfur compounds.
• Intermediate: sulfur dioxide $SO_2$, elemental sulfur $S_8$.
• Oxidized: sulfate $SO_4^{2-}$ (often as salts like $CaSO_4$, $Na_2SO_4$).

Key Processes

Oxidation of Sulfur Compounds

• Atmospheric oxidation of $SO_2$:
1. 
   $$
   SO_2 + OH\cdot \rightarrow HOSO_2\cdot
   $$
   followed by reactions leading to $H_2SO_4$.
2. In aqueous phase (cloud droplets):
   $$
   SO_2 \cdot H_2O + H_2O_2 \rightarrow H_2SO_4 + H_2O
   $$
• Oxidation of sulfide minerals:

$$
2\,FeS_2 + 7\,O_2 + 2\,H_2O \rightarrow 2\,Fe^{2+} + 4\,SO_4^{2-} + 4\,H^+
$$

This can cause acid mine drainage.

Reduction of Sulfate

Under anoxic conditions, sulfate-reducing bacteria reduce sulfate to sulfide:

$$
SO_4^{2-} + 8\,e^- + 10\,H^+ \rightarrow H_2S + 4\,H_2O
$$

$H_2S$ can escape as a gas or react with metal ions to form metal sulfides.

Volatile Sulfur Compounds

Biological activity (especially in oceans and wetlands) produces $H_2S$ and volatile organosulfur compounds (e.g. dimethyl sulfide). These influence atmospheric chemistry and cloud formation.

Human Impacts

• Burning sulfur-containing fossil fuels releases $SO_2$:

$$
S + O_2 \rightarrow SO_2
$$

• Smelting of sulfide ores also emits $SO_2$.

Environmental consequences:

• Formation of sulfuric acid in the atmosphere → acid rain.
• Soil and water acidification, mobilization of toxic metals.
• Changes in sulfate and sulfide balances in soils and sediments.

The Phosphorus Cycle

Phosphorus is essential for DNA, ATP, and cell membranes, but unlike nitrogen or carbon it has no significant gaseous atmospheric phase under normal conditions.

Main Phosphorus Forms

• Inorganic phosphate: $H_3PO_4$ and its conjugate bases $H_2PO_4^-$, $HPO_4^{2-}$, $PO_4^{3-}$ (collectively “phosphate”).
• Mineral phosphates: calcium, iron, aluminum phosphates in rocks and soils (e.g. $Ca_3(PO_4)_2$).
• Organic phosphorus: in biomolecules.

The acid–base equilibria of phosphoric acid:

1.
$$
H_3PO_4 \rightleftharpoons H^+ + H_2PO_4^-
$$
2.
$$
H_2PO_4^- \rightleftharpoons H^+ + HPO_4^{2-}
$$
3.
$$
HPO_4^{2-} \rightleftharpoons H^+ + PO_4^{3-}
$$

The predominant species depend on pH.

Key Processes

• Weathering of phosphate-containing rocks releases phosphate ions into soils and water.
• Uptake by plants: mainly as $H_2PO_4^-$ and $HPO_4^{2-}$.
• Transfer through food webs (plant → herbivore → carnivore).
• Return to soils and sediments through excretion and decomposition (mineralization of organic phosphorus).
• Sedimentation and burial in aquatic systems; over geological times, new phosphate rock may form.

Human Influences

• Mining of phosphate rock and production of phosphate fertilizers.
• Use of phosphorus-containing detergents (in some regions).
• Fertilizer and wastewater discharges → elevated phosphate concentrations in surface waters.

Environmental effects:

• Eutrophication (similar to nitrogen) → algal blooms, oxygen depletion.
• Accumulation of phosphorus in sediments can lead to internal loading in lakes (release from sediments under low-oxygen conditions).

Interactions Between Matter Cycles

Matter cycles are strongly interconnected:

• Carbon and oxygen cycles are coupled via photosynthesis and respiration.
• Nitrogen and sulfur cycles are linked to redox conditions created by organic carbon degradation.
• Nitrogen and phosphorus cycles together regulate primary production and eutrophication in aquatic systems.
• The water cycle transports and redistributes chemical species from all cycles.

Changes in one cycle often propagate to others. For example:

• Increased atmospheric $CO_2$ affects ocean carbonate chemistry, which can change nutrient availability and biological production, thereby influencing nitrogen and phosphorus cycling.
• Input of reactive nitrogen and phosphorus from agriculture alters the carbon balance in aquatic ecosystems (more organic matter production, more anaerobic zones, more $CH_4$ and $N_2O$ emissions).

Matter Cycles and Environmental Pollution

When natural cycles are strongly perturbed by human activities, several types of environmental problems can arise:

• Concentration shifts: Local or global increases of a species (e.g. $CO_2$, $NO_3^-$, $PO_4^{3-}$, $SO_4^{2-}$) beyond natural levels.
• Rate changes: Acceleration of certain fluxes (e.g. very rapid release of fossil carbon; large-scale nitrogen fixation by industry).
• Imbalance between reservoirs: Depletion of some reservoirs (soil carbon, soil phosphorus in intensively exploited systems) and over-enrichment of others (atmosphere, surface waters).

From a chemical point of view, these disturbances:

• Modify redox conditions and acid–base equilibria in ecosystems.
• Influence solubility and mobility of metals and other pollutants.
• Affect the formation and degradation pathways of organic contaminants.

Understanding matter cycles is therefore essential for:

• Assessing environmental impacts of emissions and land use,
• Designing measures to reduce pollution (e.g. reducing nutrient inputs, restoring wetlands),
• Predicting the long-term fate of chemicals in the environment.