User:Jierui J/sandbox

Soil Permanent Charges and Associated Soil Colloids Types
Soils generally have net negative charge due to the presence of permanent changes and pH dependent charges on soil colloids. Soil permanent charges are associated with soil colloids, crystalline silicate clay, which are dominated by phylosilicates with tetrahedral and octahedral crystal sheets. The associated clay groups for permanent charges are majorly associated with 2:1 type dioctahedral minerals including Montmorillonite, Beidellite, Notronite, Vermiculite, Illite, Muscovite, as well as 2:1 type trioctahedral minerals including Vermiculite and Chlorite. Clay minerals are the smallest soil particles smaller than 2 um. Due to the smaller size of soil particle or colloids, they have very high surface area or surface per unit mass, and the net negative charges make the surface extremely reactive.

Soil Permanent Charge Formation Processes
Soil permanent charges are formed through a process called isomorphic substitution during initial crystal forming processes. In this process, one atom is replaced by another atom with similar morphology (size) but different charge. As the size is similar, the crystal structure is not disrupted, but results in the creation of a charge imbalance, which is generally net negative. For example, if Al3+ in an octahedral layer is replaced by Fe2+ or Mg 2+, or  Si4+ in a tetrahedral layer are replaced by Al3+, a net negative charge is created at the clay surface. However, isomorphic substitution may also take place when one atom is replaced by another with similar size but the same charge or a more positive charge, creating neutral or positive charge balance on the clay surface. Generally, most of the isomorphic substitution leads to the development of negative charges, thus negative charges are the dominant charges on the clay surfaces.

'''Table 1. Clay Types, Associated Tetrahedral and Octahedral Layer Formula and Charge per Unit Formula''' Source:

Net Negative Charges in relation to Diffuse Double Layer
Isomorphic substitution is neutralized or balanced via the electrostatic attraction of cations and repulsion of anions. In detail, the net negative charge on the clay surface repel to anions adjacent to them through negative adsorption process. However, the net negative charges are balanced or neutralized by adsorption of cations on the clay surfaces. Therefore, closer towards the clay surface, the greater the concentration of cations. However, increase in distance away from the soil particle surface show presence of greater concentration of anions. As further increase in the distance from the clay surfaces, the concentration of cations and anions will reach the same as the soil bulk solution. The negative charge on the clay surfaces, and unequal distribution of cations and anions adjacent to clay particles together form electron double layer (EDL) or diffuse double layer (DDL).

Soil pH Dependent Charges and Associated Soil Colloids Types
Soil pH dependent charge can be described from the words it self that the charges can be altered into net positive charges or net negative charges based on soil pH condition. The pH dependent charge is named because it increases in magnitude as the pH of the soil solution increases. pH dependent charges are associated with soil colloids including non-crystalized silicate clays dominated by amorphous clays, Iron and Aluminum oxides dominated by goethite and gibbsite and organic colloids dominated by long carbon chain molecules.

Soil pH Dependent Charge Formation Processes
The formation of pH dependent charges is through protonating and deprotonation processes where H+ or OH- in the soil solution reacts with large complex organic humus molecules consist of wide range of functional groups such as carboxyl group, phenolic hydroxyl group, alcoholic hydroxyl group and so on. Sometimes, reaction between H+, OH- and hydroxides at the edges of mineral particles occur and form soil pH dependent Charge. When soil has low pH where H+ is dominant in the soil solution, H+ reacts with OH bond on the edges of organic matters’ functional groups or hydroxides at clay surfaces. The attraction force absorbs H+ into the OH bond on the soil colloids therefore there is a formation of H2O+ on the soil colloidal surfaces and a positive charge is formed. While, when soil pH is greater than 7, soil solution is active in OH-, it reacts with OH bonds of soil colloids. The strong attraction force of OH- will break OH bonds on the functional groups or hydroxides, therefore, there is formation of water molecule H20 while leaving O- on the soil collides, and a negative charged is formed.

Example of negative charges formation

Organic matter:  CO-OH + OH- CO–O-+ H2O

Mineral edge: AlO-OH + OH- AlO–O-+ H2O

Example of positive charges formation

Organic matter: CO-OH + H+ CO-OH2+

Mineral Edge: AlO-OH + H+ AlO-OH2+

'''Table 2. Comparison of Charges on Different Types of Soil Colloids at pH 7.''' Source

Overview
Soil charges are distributed around the soil colloidal surfaces. They have ability to adsorb or repel cations and anions from the soil solution via attraction and repellent forces. Therefore, soil particle surfaces or exchangeable phases have ability to store, release and exchange cations and anions. From a micro-perspective, the amount of charges distributed around the soil surface charges are directly related to the cation exchange capacity, anion exchange capacity and cation exchange processes. From the macro-perspective, soil surface charges are not only directly related to storage and release of available micro and macro nutrients for plant uptake; but also related to adsorption and absorption of organic and inorganic contaminants at the soil particle surfaces.

Cation Exchange Capacity and Anion Exchange Capacity
Cation Exchange Capacity is a measure of how many cations can be retained on soil colloidal surfaces. These cations neutralize the amount of negative charges on the soil colloids since the positive cations or molecules bond to negatively charged via electrostatic attraction. While, anion exchange capacity is related to the amount of anion that retain on the soil exchange phase for neutralizing positive charges on soil colloidal surfaces. Generally, surface of Kaolinite, Allophane and Iron and Aluminum oxides carries certain amount of positive charges. The soil cation exchange capacity and soil anion exchange capacity alter soil chemical environment. They measure the soil fertility and soil pollutants. Cation exchange capacity indicates the capacity soil in holding nutrients like Mg2+, K+, NH4+, Ca2+ in plant available forms. It also indicates the soil capacity in terms of adsorbing heavy metal cations such as Pb, Se, As in free cation forms. While, soil anion exchange capacity measure the ability of soil holding nutrients like NO3-, SO42-.

Cation Exchange
Since majority of soil have net negative charges, the exchangeable phase allows mono, di and trivalent cations to be exchanged between soil solution and soil colloidal surfaces. In terms of neutralizing the particle surface charges, trivalent cations with small radia and high charges have preference over divelent and monovalent cations that have large hydrated radia and low charges. The exchange preference order is Al3+ > Ca2+ = Mg2+ > K+ = NH4+ > Na+.