|Abstract or Summary
- The objectives of this research were to determine the effects of
various soil parameters on the reaction of Isopropyl-N-(3-chlorophenyl)Carbamate (CIPC) with a soil and to relate enthalpy values for
the adsorption to chemical or physical bonding mechanisms.
Six Oregon soils providing a range of pH, organic matter, clay,
iron oxide, and exchangeable and extractable aluminum were equilibrated
with aqueous CIPC solutions as a function of temperature (0°C,
25°C, 50°C), time, and CIPC concentration, Aqueous samples of the
soil-CIPC system were removed through a fritted glass filter by
vacuum at time intervals of ten minutes, one-half, one, three, five,
and seven hours. The CIPC was extracted from the aqueous phase
with CS₂ and subsequently analyzed by gas chromatography.
Equilibrium between the soil and the CIPC solution was achieved
in three hours except for the Hembre 1 which required nine hours.
The rate at which the soils reached equilibrium was not the same for
all soils and a diffusion phenomena appears to control the rate at
which a soil attains its maximum CIPC adsorption.
CIPC adsorption on the soils increased as a function of the
organic matter content. Nyssa sil (1% organic matter) and Hembre 1
(29% organic matter) adsorbed 192[superscript μg CIPC] / [subscript g soil] and 636[superscript μg CIPC] / [subscript g soil]
respectively. The internal pore space of a soil component such as
pumice may also contribute to CIPC absorption as illustrated by the
Deschutes sl which contains 0.9% organic matter but adsorbed 334[superscript μg CIPC] / [subcript g soil]
In general more CIPC was adsorbed at 0°C and 25°C than at 50°C. Negative ΔH values over a temperature range of 0°C to 25°C suggest an exothermic process of adsorption while positive ΔH values over a temperature range of 25°C to 50°C suggest a more endothermic
process of adsorption. The low ΔH values (+2.5 kcal to -7.0 kcal) indicate a weak physical adsorption due to coulombic forces, Van der Waal forces, and hydrogen bonding. Adsorption of
CIPC on soil was described by the Freundlich equation, but not by
the Langmuir equation.
Higher than normal rates of CIPC application must be practiced
in soils with a high organic matter content as much of the applied
CIPC will be adsorbed onto the organic matter complex and thus less
effective in weed control. The low ΔH values observed for CIPC adsorption onto the soil indicates that CIPC from a purely chemical
standpoint is weakly adsorbed, however, relative to certain other
herbicides, CIPC is strongly retained in a soil system.