River water silicic acid

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

In the case of terrestrial effusions, when acid volcanic waters mixed with surface river waters and especially with sea waters in the near-shore zones, the pH value gradually increased due to dilution and neutralization and approached the equilibrium value, controlled by carbonate-silicate buffer reactions (pH within 6-8 depending on the type of buffer). As a result, colloidal silica already had begun to coagulate in the near-shore zones, and clots were deposited along with clastic components or were carried on by currents. However, maximum chemogenic deposition began some distance... 

Despite the high silicic acid load associated with rivers (discharge-weighted average silicic acid concentrations have been estimated at 150 pM Treguer et al, 1995), there is ample evidence for biological removal in estuaries and coastal waters. Nelson and Dortch (1996) demonstrated biological removal of 80—99% of the silicic acid in the Mississippi River plume. MiUiman and Boyle (1975) reported a 25% drawdown of silicic acid within the Amazon River estuary, with further depletion in the nearshore ocean. Kimmerer (2005) demonstrated ca. 85% drawdown of silicic acid within San Francisco Bay, directly related to diatom production. In contrast, silicic acid concentrations are rarely less than 0.5 pM in much of the world s oceans ( Nelson and Dortch, 1996), despite the prevalence of diatoms and the dominance of diatom productivity in export flux. 

Bicarbonate is the main anion in river water because of the reaction of C02-rich soil water with both calcium carbonate and silicate rocks (see Chapter 2). Thus, neutralization of acid in reactions with more basic rocks during weathering creates cations that are balanced by anions of carbonic acid. In this sense the composition of rocks and the atmosphere determine the overall alkalinity of the ocean. 

Distribution and seasonal variations of the silicate content in the Aral Sea waters were subject to the effect of two key factors a river flow that imported dissolved silicic acid and a biological silicate cycle in the sea proper. In general, the content of silicic acid in the Aral Sea is not high compared to other seas. 

In river water silica is present in the form of barely ionized silicic acid, H2Si03, in concentrations of 5 to 30 mg-l depending on the soil. The amount can be determined by flame emission spectrophotometry or by ammonium molybdate colorimetry. 

If ground water whose pH is controlled by the dissolutions of silicates and carbonates, inputs of ground water to river water does not decrease pH of river water. However, if acid surface water inputs to river water, pH of river water reduces and cation concentrations change. 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

In Section 4.4 we saw that most soilwaters that feed rivers and groundwater have near-neutral pH, with I ICO, as the major anion. This results from the dissolution of C02 in water (see eqn. 4.7) and from the acid hydrolysis of silicates and carbonates. The total concentration of weak acid anions like HCOf in water is referred to as alkalinity. These anions are available to neutralize acidity (H+) in natural waters, consequently it is important to understand their chemical behaviour. 

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