pH Affect on Ion Sorption (part 1)
The Technical Definition of pH

pH is a logarithmic measure of hydrogen ion concentration, initially defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. The Oxford English Dictionary states that “p” stands for potenz, which, in German, translates to “power,” and “H” represents hydrogen. Therefore, the abbreviation pH translates to “power of hydrogen.” In a paper written by Sørensen, pH is formerly written as PH. However, the modern notation “pH” was adopted by W. Clark for typographical convenience in 1920.

Hydrogen ions, or protons, are crucial suppliers to various chemical and biological processes, indicated by the abbreviation “pH.” A “low pH” resembles a very high concentration of protons, and a “high pH” signifies a low concentration of protons. The concept of pH goes further as water contains polar bonds that can ionize, forming a hydroxide ion and a proton. An individual water molecule–water contains a few hydroxide ions and a few protons– does not ionize very quickly; however, factoring in a large number of water molecules, a few take on an ionized state.

Regular water generally has a pH near 7. The range may vary slightly higher or lower depending on the presence of dissolved minerals. A low pH of 1-3 is deemed very acidic, while a high pH of 12 to 14 is very basic. Water contains lots of protons and very few hydroxide ions at low pH. Mostly due to the equilibrium dysregulation between ionized and non-ionized water caused by the extra protons pushing to the non-ionized side. This equilibrium dysregulation falls under le Chatlelier’s principle in two ways: (1) There is overconcentration of hydroxide ions and no free protons at high pH. (2) the remaining free protons will interact with the hydroxide ions to re-form water.

How pH Works in the Soil

Whether the soil is acid or alkaline depends mainly on rainfall and temperature. All rain and snow are acidic due to CO2 gas dissolved in the soil. CO2 gas dissolved in water, H2O forms carbonic acid, H2CO3, quickly dissociating to HCO3- and H+. If soil is alkaline, i.e., it has an excess of OH- ions, these ions become neutralized as free H+ ions interact with carbonic acid associated with rainfall. The rainfall on alkaline soil releases OH- ions which combine with H+ forming stable H2O, causing a drop in pH. In soils with more rainfall than evaporation, most alkaline rocks will eventually leach base cations into the soil as the cations react with the carbonic acid in the soil. Many of the base cations will be held on negative exchange sites, preventing leaching and maintaining nutrient availability to the plant and organisms in the soil. Soils with low CEC contain too little charge to prevent soluble cations from leaching away. 

As long as precipitation exceeds evaporation, the excess H+ ions in the soil will continue to multiply and begin displacing the base cations present (Ca, Mg, K, and Na) at negative exchange sites. First and foremost, the Calcium and Sodium leach away, leaving behind magnesium and potassium-rich soil. Many communities of well-adapted plants thrive in low Ca, high Mg, and K acidic soils, including rhododendrons, azaleas, holly, blueberries, coffee, cacao, and other members of Ericaceae.

Soils grow more acidic as negative exchange sites fill with H+ ions due to continuous leaching and acidification. Eventually, the leaching and acidification lead to the loss of more base cations, eventually Aluminum+++ ions, as Silicon 4+ is dissolved from the aluminosilicate clay matrix, leaving Al+++ in the solution. Lastly, the soil loses most of its exchange capacity and holds on to nutrient ions, becoming more degraded. The degradation leads to a nutrient-poor and very acidic soil containing high concentrations of soluble aluminum and other toxic metals.

Rainfall contains acidic molecules that seek a balance with alkaline elements. If precipitation exceeds evaporation, the acidity eventually oxidizes and disbands alkaline minerals. In most soils, alkaline compounds will no longer readily dissolve below pH 7. Furthermore, the remaining base cations will be held on exchange sites used by soil biology or remain in solution within the soil. Many agricultural soils are from pH 6.0 to 7.5, containing abundant nutrient cations Ca, Mg, and K. The soil pH is neither too acidic nor alkaline, so base cation nutrients and elements like P, Fe, Mn, Cu, and Zn are readily available.

Source: istockphoto.com

Figure 1. Measuring Soil pH A quality pH meter is the best way to get an accurate pH reading of the soil, and soil pH can also be measured using pH testing paper. Ideally, you want a dry soil-to-water ratio of 1:1 by weight. Stir or shake them together, and let the sample rest for approximately 60 minutes with occasional agitation. Insert the pH meter probe or testing strip into the sample mixture and record the pH reading.

Ion Movement in Soil and Root Absorption

Ion sorption by plant roots is decided by root physiology and ion mobility. This ionic movement in soil affects plant growth, crop yield, and product quality. 

Mass flow, diffusion, and root interception are the three main precursors for ion movement in the soil.

  1. Mass flow is defined as the movement of all dissolved ions present in the soil’s water profile, the downward movement created by rainfall and irrigation water, or the upward movement created by evaporation of soil water. The significant ions moved primarily by mass flow are the nitrate (NO3-) and chloride (Cl-) anions and sulfate (SO42-) anions/cations, potassium (K+), and magnesium (Mg2+). Other ions are potentially carried via moving soil water–depending on the biogeochemical characteristics of the soil, including the elemental concentration and characteristics in the soil.
  2. Diffusion is the shifting of ions within water films surrounding a soil particle. The ion concentration gradient constantly changes as ions move from high to low ionic concentration areas. These ions in solution are moved via diffusion, occurring in varying distances around and between the soil particulates.
  3. Root interception occurs as the growth of roots through the soil increase–increasing root surface area, making contact with soil particles, and increasing the opportunity for ion absorption.

Source: https://extension.uga.edu/publications/detail.html?number=C1040&title=cation-exchange-capacity-and-base-saturation

The capacity of the soil to sorb sulfate is an important factor that influences sulfate leaching and hence the availability of sulfur to plants. Sulfate sorption is more affected by an increase in pH; there is a large electrostatic component to the bond between sulfate and the surface, with little sorption occurring without a positive charge. Changes in sulfate sorption with increasing pH are similar to differences in sorption capacity observed between soil types. The absolute changes in the amounts of anion sorbed at any given concentration in solution are greater for phosphate than for sulfate. Phosphate sorption can be explained by invoking a ligand exchange mechanism in which a strong covalent bond is formed between the phosphate group and the surface. The band has a strong chemical component, so electrostatic considerations are less important, and P can be sorbed on neutral or even negative surfaces.

  Mobility and bioavailability depend on the increase of the concentration in the soil. Plants can absorb metal or be translocated through leaching into underground water sheets. It involves sorption, desorption, precipitation, complexation, oxy-reduction, and dissolution reactions.

 In soils with reversible interfaces, adsorption changes in response to variations in solution parameters, such as the pH and ionic strength (I). In general, pH is the primary factor that governs heavy metal adsorption and availability due to alterations in the metal species in solution and the variation in the intensity of deprotonation of the electrically charged surface. However, while the solubility of some metal ions (e.g., Ni and Cd) is strongly controlled by the pH, Cu retention does not seem to depend on this parameter alone.

  At pH around 5.0, where almost all the Cu was adsorbed, the competition between Ca and Cu for nonspecific sites was more intense because of the higher Cu concentration. Furthermore, pH influence on Cd adsorption aligns with the information that soil pH is the most important soil solution parameter affecting metal-ion adsorption in soils, controlling both surface net charge and the metal-ion speciation. A strong increase in metal adsorption density from zero to nearly 100% has been frequently observed within a very narrow pH range, usually less than two units. Cd sorption is highly sensitive to pH as each 0.5 unit increase in pH resulted in twice as much sorption of Cd. The proportion of Cd adsorbed increased uniformly and continuously with the increase in pH. 

According to Bolan et al. (2003), alleviation of phytotoxicity could be attributed primarily to the immobilization of Cd by enhanced pH-induced increases in negative charge. However, soil pH is considered the main factor in Cd adsorption because it is also related to the metal ions’ hydrolysis constant. Over the pH range for the adsorption edge, the concentration of the metal-hydroxy species rapidly increases. Metal-ion hydrolysis and specific adsorption have been well-established mechanisms for Cd adsorption on pure mineral systems and soils. 

The precise mechanisms for changing the net negative charge of soil and mineral surfaces with increasing pH still need to be fully understood. Still, the generation of negative charge either through the dissociation of H+ ions from surfaces or consumption of OH2 ions by soils is generally accepted. The reversibility capacity of surface charges is characteristic of soil rich in oxyhydroxides because the loss or incorporation of H+ ions can charge the surface negatively or positively. In soil solution, electropositive metals form aquocomplexes from a simple hydrolysis reaction. Transition metals, such as Cd, may undergo a series of electrolytic reactions releasing H+.

  At pH values greater than 6.0, Cd adsorption decreased with an increase in ionic strength, and this effect distinguished specific and electrostatic Cd adsorption mechanisms. The increase of ionic strength decreased Cd adsorption, which is attributed to competition among ions for exchange sites, especially Ca. Effects of pH and ionic strength indicate the Cd concentration in soil solution is predominantly controlled by cation exchange reactions.

Of the many factors affecting the solubility of heavy metals in soil, pH is likely to be the most easily managed. While Pb and Cu retention appears not to be strongly correlated to soil pH of a variety of soils with widely divergent characteristics, solubility and plant availability of most heavy metals in any given soil are known to be inversely related to pH. Other adsorption studies have indicated a direct correlation between soil pH and metal retention. Conversely, a pH below about 5.5 may retain slightly more Ni than Cu.

Further Reading

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