| Volatilization | A pKa of 7.96(1) indicates hydrazine will exist partially in the cation or anion form at pH values of 5 to 9 and, therefore, volatilization from water surfaces is not expected to be an important fate process(SRC). The Henry's Law constant for the neutral species of hydrazine is estimated as 6.07X10-7 atm-cu m/mole(SRC) derived from its vapor pressure, 14.4 mm Hg(2), and assigned value for water solubility of 1.0X10+6 mg/L (miscible)(3). This Henry's Law constant indicates that hydrazine is expected to be essentially nonvolatile from water and moist soil surfaces(4). With an air velocity at the liquid surface of 63.5 cm/sec, the volatilization of hydrazine from petri dishes containing 25%, 50% and 75% hydrazine-water mixtures was 0.5 g after 6.66 hr, about 1 g after 5 hr, and about 3 g after 5 hr, respectively(5). At each concentration, the rate of volatilization decreased with time(5). Hydrazine's estimated Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). The potential for volatilization of hydrazine from dry soil surfaces may exist(SRC) based upon its vapor pressure(1). |
| Soil Adsorption | Using a structure estimation method based on molecular connectivity indices(1), the Koc of hydrazine can be estimated to be 13(SRC). According to a classification scheme(2), this estimated Koc value suggests that hydrazine is expected to have very high mobility in soil. The pKa of hydrazine is 7.92(3), indicating that this compound will exist partially in the cation form in the environment and cations generally adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts(4). The nature and extent of hydrazine adsorption by clays and soils is very dependent on suspension pH and on the types of surface functional groups present on the solid matrix. Under acidic conditions, pH 4.0, 99.9% of the hydrazine is present as the protonated species and should be able to readily replace Na+ from exchange sites(3). Under alkaline conditions, pH 8.0, approximately 50% of the hydrazine is protonated and 50% is in neutral form. The primary mechanism of hydrazine adsorption in a montmorillonite clay suspension was cation exchange, both at pH 4 and 8; adsorption of hydrazine was lower at the higher pH value(3). The main mechanism for hydrazine retention at pH 4 and at low hydrazine concentrations in the upper Arrendondo soil horizon (fine sand) was also cation exchange. At higher concentrations more than 60% of the hydrazine interacted with a different type of binding site, possibly with organic-surface functional groups such as carbonyl groups(3). Under alkaline conditions, using upper horizon Arrendondo soil, (at pH 8.0) hydrazine was adsorbed more readily than at pH 4(3). |
Home
search mVOC
Features / Tools
mVOC Literature
Help