Adsorption of Thiamin (Vitamin B1) on Soils and Clays

Recent research suggests that thiamin applied to soils or coated onto seeds may stimulate plant growth. The behavior of thiamin in soils has not been investigated. Therefore, studies were carried out to determine how thiamin hydrochloride (3‐[(4‐amino‐2‐methyl‐pyrimidinyl)methyl]‐5‐(2‐hydroxyethyl)‐4‐methylthiazolium chloride hydrochloride) is adsorbed by 17 soils and three clays. The dominate mechanism in binding of thiamin is thought to be cation exchange with clay minerals and organic matter. In soils with low to medium organic matter content, thiamin adsorption occurred primarily on clay minerals and depended particularly on the amount and composition of the clay. Adsorption is species dependent (p K a1 [negative logarithm of the first dissociation constant] = 4.85) and takes place principally in the acidic pH range, probably with position 1′ of the pyrimidine ring. Adsorption equilibrium is attained in <30 min. Adsorption in all soils, and in kaolinite and illite clays, could be described by a one‐surface Langmuir isotherm at initial concentrations ranging from 16.3 to 995 µmol L −1 . Adsorption by smectite in the same range was log‐linearly related to the equilibrium concentration. Extending the initial concentration range from 16.3 to 9890 µmol L −1 showed that a two‐surface Langmuir equation more adequately described adsorption in hydroxy‐interlayered vermiculitic and chloritic‐illitic soils, whereas a one‐surface Langmuir equation was found to be adequate in mixed layer‐smectitic soils. Only in the case of smectite clays is thiamin allowed to lie in a monolayer configuration parallel to the basal plane.