Role of loosely bound humic substances and humin in the bioavailability of phenanthrene aged in soil.

A study was conducted to determine a possible role of loosely bound humic substances (i.e., humic and fulvic acids) in bioavailability of aged phenanthrene with time. In this study, long-term residence of phenanthrene in soil is defined as aging or sequestration, and the effect was determined by the declined bioavailability to bacteria of the polycyclic aromatic hydrocarbon with increased residence time. After 1, 7, and 100 days of aging of phenanthrene in Lima loam, about 90-93% of initial phenanthrene was recovered from the humin-mineral fraction of Lima loam whereas less than 12% was found in humic and fulvic acids of the same soil. Mineralization rates of phenanthrene aged in the humin-mineral fraction significantly decreased with time by the test bacterium P5-2. In terms of extents of mineralization, the difference with time was not appreciable, but still significant at P<0.05. Additional decreases in the rates and extents of mineralization were observed with the whole soil (i.e. Lima loam) to which phenanthrene had been aged. Data suggest that major sequestration sites for phenanthrene may reside in the humin-mineral fraction, and probably humic and fulvic acids may act as a physico-chemical barrier to bacterial degradation so that the compound's bioavailability may be limited.

Phosphorus and Phosphorus Chemicals

Elemental phosphorus (P4) is the starting material for phosphorus-containing chemicals used in a wide range of industrial markets. Thermal (furnace) phosphoric acid, the principal first-order P4 derivative and its derivative phosphate (P2O5) salts are covered in detail in the CEH Industrial Phosphates report. Therefore, this report provides only summary information on P4 consumption via thermal acid. The focus of this report is on the relatively small-volume but important high-unit-value “nonacid” P4 market segments, particularly the production and use of inorganic phosphorus chemicals derived directly from P4.

China accounts for the largest share of elemental phosphorus capacity, at about 84%. China has greatly increased its capacity during the past decade, primarily with a large number of small plants, many of which are believed to be idle at present. Kazakhstan accounts for about 6%, followed by the United States and Western Europe, each with about 5%.
The most important chemicals derived directly from P4 are phosphorus trichloride (PCl3), phosphorus pentasulfide, phosphorus pentoxide and sodium hypophosphite, a relatively small-volume chemical used primarily in electroless nickel plating solutions. Phosphorus trichloride, phosphorus pentasulfide and phosphorus pentoxide are the building blocks for a large number of derivative inorganic and organic chemicals, which in turn are used in a wide variety of high-value specialized applications.

Complete world data for the elemental phosphorus industry are not available. The following pie chart shows consumption of elemental phosphorus by major region:

Red phosphorus is consumed mainly for flame retardants for plastics. Other uses include safety matches, pharmaceuticals, pesticides and bronze, as well as the production of ultrapure red phosphorus for electronic applications. Currently, Western Europe is the primary market for red phosphorus, followed by China and India.
Thermal phosphoric acid is the primary end-use market for elemental phosphorus and accounts for about 56% of world phosphorus consumption. Demand for elemental phosphorus has continued to decline sharply over the past decade because of environmental restrictions on the use of phosphates, particularly as a builder in laundry detergents, as well as competition from the less costly purified wet-process phosphoric acid in thermal acid markets. The largest market for yellow phosphorus in China is thermal phosphoric acid. Since China was lacking in wet-process phosphoric acid technology, almost all of downstream industrial products of phosphoric acid are produced from thermal acid.
In contrast to declining thermal acid demand, demand for the major phosphorus chemicals (phosphorus trichloride, phosphorus pentasulfide, phosphorus pentoxide and sodium hypophosphite) has increased over the past five years. Phosphorus trichloride is the leading inorganic phosphorus chemical produced from elemental phosphorus, with about two-thirds of PCl3 used to manufacture glyphosate-based herbicides.

Further growth is expected for the phosphorus chemicals covered in this report—principally phosphorus chlorides—over the next five years. However, demand for thermal phosphoric acid, currently the largest-volume derivative of elemental phosphorus, is expected to grow only slowly in all regions. In fact, the impact of slowing phosphoric acid demand is expected to result in an overall growth of about 1% for world elemental phosphorus, despite the positive demand growth for phosphorus chemicals. Demand growth will be most pronounced in China for the PCl3 derivatives—organophosphate herbicides (specifically glyphosate), plastics and elastomer additives, and sequestrants and surfactants.

Humic acid

Humic acid is a principal component of humic substances, which are the major organic constituents of soil (humus), peat and coal. It is also a major organic constituent of many upland streams,dystrophic lakes, and ocean water.[1] It is produced by biodegradation of dead organic matter. It is not a single acid; rather, it is a complex mixture of many different acids containingcarboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Humic acids can form complexes with ions that are commonly found in the environment creating humic colloids. Humic and fulvic acids (fulvic acids are humic acids of lower molecular weight and higher oxygen content than other humic acids) are commonly used as a soil supplement in agriculture, and less commonly as a human nutritional supplement. As a nutrition supplement, fulvic acid can be found in a liquid form as a component of mineral colloids. Fulvic acids are poly-electrolytes and are unique colloids that diffuse easily through membranes whereas all other colloids do not.

Physicochemical properties of amino acids

The 20 amino acids encoded directly by the genetic code can be divided into several groups based on their properties. Important factors are charge, hydrophilicity or hydrophobicity, size, and functional groups. These properties are important for protein structure and protein–protein interactions. The water-soluble proteins tend to have their hydrophobic residues (Leu, Ile, Val, Phe, and Trp) buried in the middle of the protein, whereas hydrophilic side-chains are exposed to the aqueous solvent. (Note that in biochemistry, a residue refers to a specific monomerwithin the polymeric chain of a polysaccharide, protein or nucleic acid.) The integral membrane proteins tend to have outer rings of exposedhydrophobic amino acids that anchor them into the lipid bilayer. In the case part-way between these two extremes, some peripheral membrane proteins have a patch of hydrophobic amino acids on their surface that locks onto the membrane. In similar fashion, proteins that have to bind to positively charged molecules have surfaces rich with negatively charged amino acids like glutamate and aspartate, while proteins binding to negatively charged molecules have surfaces rich with positively charged chains like lysine and arginine. There are different hydrophobicity scalesof amino acid residues.[120]

Some amino acids have special properties such as cysteine, that can form covalent disulfide bonds to other cysteine residues, proline that forms a cycle to the polypeptide backbone, and glycinethat is more flexible than other amino acids.

Many proteins undergo a range of posttranslational modifications, when additional chemical groups are attached to the amino acids in proteins. Some modifications can produce hydrophobiclipoproteins, or hydrophilic glycoproteins. These type of modification allow the reversible targeting of a protein to a membrane. For example, the addition and removal of the fatty acidpalmitic acid to cysteine residues in some signaling proteins causes the proteins to attach and then detach from cell membranes.

Ecological effects

Organic matter soil amendments have been known by farmers to be beneficial to plant growth for longer than recorded history. However, the chemistry and function of the organic matter have been a subject of controversy since humans began their postulating about it in the 18th century. Until the time of Liebig, it was supposed that humus was used directly by plants, but, after Liebig had shown that plant growth depends upon inorganic compounds, many soil scientists held the view that organic matter was useful for fertility only as it was broken down with the release of its constituent nutrient elements into inorganic forms. At the present time, soil scientists hold a more holistic view and at least recognize that humus influences soil fertility through its effect on the water-holding capacity of the soil. Also, since plants have been shown to absorb and translocate the complex organic molecules of systemic insecticides, they can no longer discredit the idea that plants may be able to absorb the soluble forms of humus;this may in fact be an essential process for the uptake of otherwise insoluble iron oxides.

A study on the effects of Humic acid on plant growth was conducted at Ohio State University which said in part “humic acids increased plant growth” and that there were “relatively large responses at low application rates”

A 1998 study by scientists at the North Carolina State University College of Agriculture and Life Sciences showed that addition of humate to soil significantly increased root mass in creeping bentgrass turf.

Chemical characteristics of humic substances

A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic andcarboxylic substituents, linked together; the illustration shows a typical structure. The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups.Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups. There is considerable overall similarity among individual humic acids.For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density. The molecules may form a supramolecular structure held together by non-covalent forces, such as Van der Waals force, π-π, and CH-π bonds.

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.