1-Introduction of Abiotic factors and Sorption2-Sorption Reactions3-Sorption Processes in soils4-Properties of Sorptives and sorbents5-Factors governing Sorption in soil6- Effects of sorption of heavy metals with Abiotic factors as soil and water.7- Conclusion

Abiotic Sorption

By Maryum AtiqueM.Phill ChemistryUniversity of Agriculture, FSD.

Abiotic sorption means absorption or adsorption of minerals by non living species such as Soil, Water, Air etc.

Introduction

Abiotic components (also called abiotic factors) are non-living chemical and physical factors in the environment, which affect ecosystems. Abiotic phenomena underlie all of biology. The abiotic factors of an ecosystem can be listed through SWATS: Soil, Water, Air,

Temperature, and Sunlight.

Abiotic factors

Sorption is a physical and chemical process by which one substance becomes attached to another. 

What is sorption?

Absorption - the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by a liquid). Adsorption - the physical adherence or bonding of ions and molecules onto the surface of another phase (e.g., reagents adsorbed to a solid catalyst surface).Ion exchange - an exchange of ions between two electrolytes or between an electrolyte solution and a complex.

Sorption include

Sorption Reactions

Sorption processes

Depend on: Surface area & amount of sorption “sites” Relative attraction of sorptive to sorbents

Mineral surfaces can have: Permanent structural charge Variable charge

Mineral Surfaces

Minerals which are precipitated can also interact with other molecules and ions at the surface

Attraction between a particular mineral surface and an ion or molecule due to: Electrostatic interaction (unlike charges attract) Hydrophobic/hydrophilic interactions Specific bonding reactions at the surface

Charged Surfaces

Mineral surface has exposed ions that have an unsatisfied bond in water, they bond to H2O, many of which rearrange and shed a H+

≡S- + H2O ≡S—H2O ≡S-OH + H+

H+

OH

OH

OH

OH

H+OH2

OH

OH

Sorption processes in soil

General term referring to the retention of material on solid surfaces - can include adsorption, surface precipitation, and polymerization

Sorption processes in soil

chemicals can be grouped into three sorptive categories:

(a) anionic sorptives that are negatively charged (e.g., the nutrient orthophosphate, PO4

3-, is an anion).(b) cationic sorptives that are positively charged (e.g., the divalent cations Ca2+ and Pb2+).(c) uncharged organic sorptives that exhibit a range of polarities (non-polar to polar)  (e.g., the aromatic ring compound, benzene etc).

Sorptives and Sorbents in Soils

The major solid phase materials (sorbents) in soils are :

1-layer silicate clays ( are primarily negatively charged because their stacks of aluminum-oxygen and silicon-oxygen sheets)

2-metal-(oxyhydr)oxides (Metal-(oxyhydr)oxides are variably charged because their surfaces become hydroxylated when exposed to water and assume anionic, neutral, or cationic forms based on the degree of protonation) hus,

these variably charged minerals adopt a net positive surface charge at low pH and a net negative surface charge at high pH. 3-oil organic matter (SOM) SOM contains a multitude of reactive sites ranging from potentially anionic hydroxyls (R-OH) and carboxylic groups (R-COOH) to cationic sulfhydryl (R-SH) and amino groups (R-NH2). Principally unpolar region of soil solid surface.

Factors governing Sorption in Soil1-Sorptive concentrationSorptive concentration has perhaps the most significant influence on the accumulation of a sorbate on or within a sorbent By increasing (or decreasing) sorptive concentration will result in an increase (or decrease) in sorbate concentration.

Anions will be attracted to positively charged sorbents and cations will be attracted to negatively charged sorbentsOn basis of charge two types of complexes are formed. Inner-sphere adsorption complex: Sorption of an ion or molecule to a solid surface where waters of hydration are distorted during the sorption process and no water molecules remain interposed between the sorbate and sorbent.

2-Sorptive and Sorbent Charge

Outer-sphere adsorption complex: Sorption of an ion or molecule to a solid surface where waters of hydration are interposed between the sorbate and sorbent.

As the solution pH increases, sorbent hydroxyl and carboxyl functional groups are deprontonate. In turn, this increases negative charge density on the sorbent thus facilitating cation adsorption  while decreasing anion adsorption and vice versa.

3-Solution pH

With increase in ionic radius sorption increases. As ionic radius increases the electron cloud extends further away from the nucleus and can more easily distort and engage in covalent (electron sharing) bonds with the sorbent.

4-Sorptive size

sorbent

sorbate

sorptive

General overview of sorption processes occurring in soil

Effects of soil sorption

Transportation of minerals to Plants

Plant growth and development largely depend on the combination and concentration of mineral nutrients available in the soil as a result of sorption. Plants often face significant challenges in obtaining an adequate supply of these nutrients to meet the demands of basic cellular processes due to their relative immobility.

For instance, adding fertilizer to a soil will increase the solution potassium (K+) concentration and subsequently increase the amount of K sorbed by the solid phase. Conversely, as growing plants uptake K+ from the soil solution, this will drive desorption of the sorbate K+ from the soil. In this way, the soil serves as a nutrient reserve for plants and soil organisms.

Soil sorption is very important for biogeochemical cycles

Factors affecting the fate of soil-applied

herbicides

Mining, manufacturing, and the use of synthetic products (e.g. pesticides, paints, batteries, industrial waste, and land application of industrial or domestic sludge) can result in heavy metal contamination of urban and agricultural soils. Heavy metals also occur naturally, but rarely at toxic levels.

Lead – mental lapse.Cadmium – affects kidney, liver, and GI tract.Arsenic – skin poisoning, affects kidneys and central nervous system.

Excess heavy metal accumulation in soils is toxic to humans and other animals. Exposure to heavy metals is normally chronic due to food chain transfer. Acute poisoning from heavy metals is rare through ingestion or dermal contact, but is possible. Chronic problems associated with long-term heavy metal exposures are:

As sorption of heavy metals also occur with water or soil sediments or soil bed of rivers, sea etc, it adversely effect on the aquatic life as can be seen below.

Conclusion

Sorption is arguably the most important chemical processes governing the retention of nutrients, pollutants and other chemicals in the soilWe have touched on the fundamental aspects of sorption in soils, but this review is by no means a comprehensive treatise on the subject.

Finally, we emphasize that in soils, sorption occurs concurrently with biotic and abiotic transformation reactions in an open hydrodynamic system Though sorption results in contamination of soil beds and water but it also play an important role in biogeochemical cycles and replenish mineral requirements of plants.