Limnology (2+1)

4.2. Dissolved solids and Dissolved organic matter

Unit 4- Chemical characteristics

4.2.1. Dissolved Solids and Dissolved Organic Matter
All waters in nature contain dissolved solids .Water is the universal solvent dissolving more different materials than any other liquid. Natural waters come in contact with soluble substances in many ways such as mere contact with its own basin, erosion at shore line, wind blown materials, inflow of surface waters, inflow of seepage and other forms of subterranean waters and decay of aquatic organisms. Rain water contains 30 to 40 ppm of dissolved solids.
Solubility of solids in water
Salts are composed of ions which in the solid form are held together by ionic forces. The strong ionization of the salts leads to the formation of hydrates with water in which the water acts as a dipole to which the ions are attached. The solubility of solid substances is strongly dependent on the pH and the redox potential in the water. It usually increases with temperature and is largely independent of pressure. Most substances dissolve either in the molecular form or dissociated into ions. Some important constituents such as humic acids, salicilic acid and ferric oxyhydrate are dispersed in colloidal form.
Major ions in freshwaters
The major ion contents vary in different fresh waters due to five factors, which are climate, geography, topography, biotic activity and time. These are not completely independent and they interact.
Carbonate is the principal anion in most fresh-waters. Generally carbonate occurs as bicarbonate ion with calcium in water. Bicarbonate ion is customarily expressed as CO₃ because evaporation of a known amount of calcium bicarbonate solution leaves only the carbonate of calcium to be weighed. During evaporation, gaseous CO₂ and water are lost, from bicarbonate ions, converting them to a lesser weight of carbonate.

Evaporation


Alkalinity is usually a measure of carbonates. There are various compounds of carbonates with calcium, such as calcite or aragonite which have the same chemical formula (CaCO3), but are crystallized differently. Aragonite precipitates from thermal waters and is contained especially in the shells of freshwater mollusks. Magnetite, the carbonates of magnesium (MgCO3) and dolomite, a double carbonate of calcium and magnesium, Ca Mg (CO3)2 are also relatively common. Carbonates of barium (BaCO3) and strontium (SrCO3) also occur. CaCO3 is insoluble except in the presence of acid. With carbonic acid, it becomes Ca (HCO3)2. Because of this, it seems reasonable to express alkalinity titrations in terms of bicarbonate ions, but on the other hand, Ca(HCO3)2 is very unstable and when water is evaporated to determine its contained dissolved salts, the bicarbonate of calcium is destroyed and only carbonate remains.
Dissolved inorganic solids
i) Nitrogen compounds
Nitrogen occurs in natural waters in the form of numerous compounds, in inorganic form as nitrate, nitrite and ammonium and in organic form as intermediate stages of microbial protein decomposition. The most important inorganic nitrogen compounds in water are nitrate and ammonia. Natural waters contain some ammonium salts. Ammonium carbonate is probably the common form.
iii) Phosphorus compounds
Free phosphorus does not occur in nature, but in the form of phosphates it is abundant. Inorganic phosphorus compounds usually occur in dissolved form only in small amounts in natural waters, often only as traces. Total phosphorus in lake water includes two components. One is soluble phosphorus which is the phosphate form and another one is organic phosphorus which is contained in plankton organisms and other organic matter in the water. As an essential nutrient for primary producers, phosphorus thus acts more often than nitrogen as the growth limiting factor. The natural inorganic phosphate content originate from atmospheric precipitation as well as from various phosphate containing rocks especially apatite, which are flushed into the lake by tributary streams. In lakes and flowing waters three phosphate fractions occur concurrently : soluble inorganic phosphate as orthophosphate (PO4) and polyphosphate, soluble organic phosphate and particulate organic phosphate (organisms or detritus). These fractions make up to total phosphate content. The losses of phosphorus occur throughout flowing water which removes both soluble and organic form. It may also occur through removals of fish, mollusks, water plants and other organisms.
iii) Sulfur compounds
The inorganic sulfur compound occurring predominantly in natural waters is sulfate. In this form sulfur can be absorbed by phytoplankters and other photo-autotrophs. Purely chemical processes involved in the sulfur budget of natural waters are the oxidation of hydrogen sulfide to sulfur by molecular oxygen and also the formation of sulfides, especially iron sulfide in the sediment. The sulfate ion, SO4 is usually second to carbonate as the principal anion is fresh waters, although chloride sometimes surpasses it. Silica often outranks sulfate, but very little is ionized. Free or elemental sulfur is inactive at ordinary temperature. This element can combine with both metals and non-metals to form many compounds. Free sulfur is an important constituent of protoplasm; it is protein and specifically within those amino acids having sulfhydryl (SH) bonding; e.g. cystine, cyseine and methionine.
When sulfur is combined with hydrogen the most reduced state is sulfide (S- -) and the most important sulfides in limnology are the gas - hydrogen sulfide (H2S) and ferrous sulfide (FeS). Sulfates combine with hydrogen to form sulfuric acid. With the alkali metals sulfur forms the most abundant form in lakes and streams.
Atmospheric sources of sulfate have increased with man’s industrial activities. Man now contributes about ten times more SO2 than that the annual contribution from volcanoes. Coal combustion produces the gas maximum and copper smelting and paper manufacturing add to it. Through precipitation and runoff water the sulfate level of some fresh water becomes unusually high showing industrial water pollution.
The most important conversion process for sulfur in lakes can be summarized as follows: sulfate is reduced by the desulphuricans to H2S and sulfides which are deposited in the sediment. Hydrogen sulfide is also formed by the microbial decomposition of proteins which is oxidized by Thiobacteria, Chromatiaceae and Chlorobiaceae via molecular sulfur to sulfate.
iv) Silicon
Silicon does not occur in nature as a free element. Natural waters commonly contain silicon dioxide in some form of soluble silicate. Silica may also exist in certain waters, particularly in rivers, in colloidal form. River waters are relatively rich in silica. Silica is the second most abundant element in the lithosphere. Its main source in fresh water and sea water is weathering of the feldspar rocks. In inland waters it ranges from 0.1 ppm. Solubility of silica increases with the rise in temperature. Dissolved silica remains as H2SiO4. Silica is an essential nutrient for diatoms as they build up their frustules with this material. The decay of silica is slower than organic compounds and thus many diatoms frustules may be buried and lost to the lake sediments.
Other Elements
Certain elements such as calcium, magnesium, manganese, iron, sodium, potassium, sulphur, copper and others constitute elements of chemical compound dissolved in the water.
i) Calcium
The predominant compound of calcium is CaCO₃, which is very less soluble in water but in the presence of carbonic acid it is represented abundantly as the soluble Ca (HCO₃)₂. Thus there is an inseparable relationship between carbonic acid, CO2, pH and the anion CO₃-in water.
The earth’s crust contains an ample store of calcium as a constituent of certain silicates. Anorthite (CaAl₂Si₂O₈) is a common member of the feldspar group of silicates. They are the most abundant of all minerals and make up 60% of the earth’s coating. A deposit of sedimentary CaCO₃ is changed to soluble bicarbonate by the action of CO₂ rich meteoric water (rain water) which enters aquatic systems. The solubility of CaCO3 depends on CO2 which follow the reaction: CaCO₃ – Ca (HCO₃)₂. This equilibrium is disturbed when the water emerged and the pressure on it suddenly released making the escape of CO₂. Also removal of CO₂ in photo-synthesis will disrupt the equilibrium. CO₂ is assimilated in photosynthesis and CaCO₃ gets precipitated in the form of calcareous incrustations on plants and other submerged objects.
Other Minerals of Calcium
After the silicate and carbonate minerals of calcium, the sulfates rank as its most abundant store. These are gypsum, (CaSO₄, 2H₂O) and anhydrite (CaSo₄).
ii) Magnesium
Magnesium is usually the second most abundant cat ion in inland waters. Its source is both silicate and non-silicate minerals of the earth’s crust. Foresterite (Mg₂SiO₉) in the following manner:

The magnesium carbonate is called magnetite and a double carbonate is dolomite, Ca Mg (CO₃)₂. Epsom salt (Mg SO₄.7H₂O) is a soft and whitish sulfate of magnesium. It is 150 times more soluble than gypsum (CaSO₄.2H₂O). Epsom salt occurs in mineral spring deposits and in salt sediments of certain lakes.
iii) Sodium
The monovalent alkali metal is very reactive and soluble. When leached from the rocks, its compounds tend to remain in solution. For this reaction, it is at least the third most abundant metal in lakes and streams and sometimes it ranks first. Among igneous rocks, the feldspars, alumino silicates of alkali and alkaline earth metals are the most abundant of all minerals. The commonest water soluble mineral is halite or simply Na Cl. In arid tracts like Rajasthan where closed basins hold concentrated waters, there are at least 3 types of sodium lakes:
i) Slatterns, having concentration of sea water with preponderance of Na Cl.
ii) Saline lakes having Na₂SO₄ in water and
iii) The soda lakes characterized by NaHCO₃ and Na₂CO₃.
They have been termed as alkali water because of their high pH. These soda lakes are characterized in having luxuriant growth of blue-green algae.
iv) Potassium
Potassium, a close relative of sodium is usually the fourth ranking cat ion in freshwater. In usual cases it may surpass sodium in certain lakes. It is weathered from various feldspars that have the formula KAISi₃O₈ but does not remain in solution. Potassium also tends to form plates of mica, which are insoluble and unavailable to aquatic ecosystems. Because of this formation potassium becomes rarer in water than sodium. In plants, both extracellular and intracellular fluids contain an excess of K+ over Na+. In animals extracellular Na+ often surpasses potassium. There is some evidence that highly concentrated water with a pronounce potassium content are lethal to many aquatic animals, the Na/K ratio being less than ten.
Potash is the name for K₂CO₃ but he word has been used to refer to KOH or potassium oxide. The so called potash lakes occupy depressions among the sand in Nebraska in USA.
v) Iron and Manganese
Although iron is one of the most widely distributed elements on the earth, it occurs in natural waters only in relatively small amounts due to its specific solubility properties. But the ground water contains large amounts of dissolved iron and manganese. The compound of trivalent (ferric) iron is almost completely insoluble in water. Thus iron remains in solution only in the bivalent form, under reducing conditions, and chiefly as the bicarbonate Fe (HCO₃)₂. The conditions under which bivalent iron (ferrous) compounds remain in solution are: an oxygen saturation value of less than 50%, the presence of degradable organic matter, a high level of free CO₂ and a pH of less than 7.5. These conditions are found primarily in groundwater and in the hypolimnion of lakes.
vi) Chloride
It is an element of the halogen group that includes also fluorine, iodine and bromine. Among these members, chloride surpasses them in polluted as well as freshwater lakes and streams. Molecular chloride (Cl₂) is a heavy yellow lethal gas, but in natural waters, it is dissociated as chloride ions, which combine with all common cat ions. It is stored in most freshwater algal cells. Contamination of water from domestic sewage can be monitored by chloride assays of the concentrated water bodies. This is because a human and animal excretion contains an average of 5g Cl- per liter.
Dissolved Organic Matter
Freshwater contains 0.1 to 50 mg dissolved organic compounds (DOC) per litre. Various free sugars, amino acids, organic acids, polypeptides and other substances have been reported.
There are probably four sources of these dissolved materials
1. Organic compounds of allochthonous origin
2. Soluble organic material from the decay of aquatic organisms
3. Extra cellular metabolites excreted by littoral macrophytes
4. Excretion from the fresh water animals.
The organic compounds not only serve directly as source of energy but also are associated with the nutrient cycle of the ecosystem. Most metals are transported down streams or exist in lake making complexes with organic materials either being absorbed or occurring as metallic coatings on detritus. Such organic substances as humic material causing yellow stain in fresh water comes from the decay of plant material in the soil.
The origin of the dissolved organic compounds in the water is manifold. The losses are due to photorespiration, secretion of the products of algal photosynthesis and those of higher plants and also due to the excretions of bacteria.
An important group of organic substances in water consists of humic substances. They are polymeric mixers derived mostly from such plant materials as lignin. Cellulose, proteins and fats humic substances enter the water due to incomplete breakdown of plant residues in the water bodies.
They affect the material budget in as much as they enter into complex-formation with heavy metals (iron and manganese) and consequently prevent their precipitation and ensure their continued availability to the primary producers. Thus a direct relationship may be observed between the concentrations of dissolved iron and water soluble humic substances in lakes. Heavy-metal ions may also become adsorptive bound to particulate humic materials. This tendency to chemical and adsorptive binding of heavy metals is of great importance for productivity in natural waters. Calcium may precipitate as calcium humate on contact with humic acids and be deposited in the sediment.

Humic acids in sediments also form iron-humus-phosphate complexes and hence, phosphate combination with iron is substantially reduced. The humic materials hold in suspension. The humic materials hold in suspension large quantities of metallic ions and through their chelating activity hold on the essential trace metals are delayed in the presence of humic compounds and thus hold the nutrients for a longer period in water.