Solution concentrates, pesticides

Sorption Modeling. Pesticide sorption is characterized by describing sorption isotherms using the Freundhch equation, S = Kj, where S is the pesticide sorbed concentration, C is the pesticide solution concentration after equdibration, and fy and N are constants. Although other equations have been used, the Freundhch has satisfactodly described experimental sorption results for a wide range of pesticides in a variety of sods. The value of N is usually <1 and between 0.75 and 0.95, which indicates that pesticides are proportionally more sorbed at low solution concentration than at high solution concentration. 

Because many studies have shown a direct relationship between pesticide sorption and organic carbon content of sod, attempts have been made to develop a universal sorption coefficient based on sorption of the pesticide to sod organic carbon (44). Sorption based on sod organic carbon is expressed as C, where is pesticide sorbed per unit mass sod organic carbon, and C is pesticide solution concentration after equdibration. If. is the fraction of organic carbon, can be obtained from i in the equation. Assumptions in the use of this approach include... 

Pesticide Solution Concentration MM Hydrolysis Rate/ Chemical Rate... 

A 250.0-mL aqueous solution contains 45.1 tg of a pesticide. Express the pesticide s concentration in weight percent, parts per million, and parts per billion. 

For many modeling purposes, Nhas been assumed to be 1 (42), resulting in a simplified equation, S = C, where is the linear distribution coefficient. This assumption usually works for hydrophobic polycycHc aromatic compounds sorbed on sediments, if the equdibrium solution concentration is <10 M (43). For many pesticides, the error introduced by the assumption of linearity depends on the deviation from linearity. 

If oiganophosphorus pesticides are involved, then it may be necessary to wash the remains with a 2% sodium hypochlorite bleach solution (i.e., 2 gallons of water for every gallon of household bleach). This concentration of bleach will not affect remains but will neutralize organophosphorus pesticides. Higher concentrations of bleach can harm remains. The bleach solution should remain on the cadaver for a minimum of 5 minutes before rinsing with water. 

Adsorption of metobromuron onto ACC was monitored spectrophotometrically at 244 nm by the procedure described above. Absorbance data, obtained in 1-min intervals until equilibrium, were converted into concentration data using the corresponding calibration relation and then plotted as a function of time in Fig. 22.2. Initial concentration of pesticide in the solution was 7.64 x 10 M and adsorption equilibrium time using 18 mg ACC, determined as the time after which the concentration of the pesticide solution remained unchanged during the course of adsorption process, was 1,156 min. The decrease in concentration of pesticide is fast at the early stages, while it slows down toward the end of adsorption period. Concentration... 

Chemicals processed. Waste pesticide solutions were collected after spray operations during late May, June, and July of 1983, and consisted primarily of three compounds 2,4-D [(2,4-dichloro-phenoxy)acetic acid], atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)- -triazine) and paraquat (1,1 -dimethyl-4,4 -bipyridinium dichloride). Our efforts were primarily directed at these pesticides, which are shown in Table I together with their formulations and concentrations. 

Pesticide solutions were prepared in tap water (pH 8.1 0.1) as reported previously (18). Initial pesticide concentrations were 180 ppm for mevlnphos, 14 ppm for diazlnon and methyl parathlon,... 

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

The detection of many pesticides at extremely low levels can be best achieved not by direct detection of the pesticide itself but rather by detection of its inhibitory effects on enzyme reactions. An enzyme-electrode is first constructed and its response when exposed to a suitable concentration of its substrate determined. When an electrode is then exposed to a dilute pesticide solution, the pesticide interacts with the enzyme and diminishes (or completely destroys) its activity. This inhibition can then be easily quantified by further exposure to the initial substrate concentration and comparison with the response prior to pesticide exposure. 

The inhibition and the subsequent signal detection were performed in two different solutions. First the pesticide solution was added and then after 10 min (incubation time) the sensor was moved into a new buffer solution where the substrate (5mmoll 1 acetylthiocholine) was injected and the signal measured. This procedure is particularly suitable when a complex matrix, which could pose problems for the direct measurement of thiocholine oxidation, is used. The analytical characteristics of pesticide determination in standard solutions were then evaluated. Detection limits, defined in this work as the concentrations giving an inhibition of 20%, were 30 and 10 ppb for aldicarb and paraoxon, respectively. By increasing the incubation time up to 30 min, an increase in the degree of inhibition could be observed and lower detection limits both for Aldicarb (5 ppb) and Paraoxon (3 ppb) were achieved. 

Garbin et al. (2007) investigated the direct and indirect photolysis of pesticide residues atrazine, imazaquin, and iprodione (3-(3,5-dichlorophenyl)-/V-(l-methylethyl)2,4-dioxo-l-imidazoline-carboxamide) in aqueous solutions in the presence and absence of HS and under ultraviolet and visible radiation (280-480 nm) (Figure 16.29). All pesticides showed a fast direct photolysis following a first-order kinetics. HS were added to the pesticide solutions in concentrations from 1 to 100 mg liter1 by means of HS mixture and pesticide stock solutions. HS only exhibited photocatalytic effect within specific concentration ranges—that is, about 30 mg liter-1 for atrazine and below 10 mg liter-1 for iprodione. For imazaquin, only a decrease was observed in the photolysis rate with HS addition. 

PCNs) and organochlorine pesticides. K a can be described as the ratio of the chemical (solute) concentration in octanol to the concentration in air at equilibrium, represented as... 

Resin adsorption. The resin adsorption is a good option for the selective removal of waste. This technique is normally used for the removal of ther-molabile organic solutes from aqueous waste streams. The solute concentration of solution ranges fiwm 1 to 8 percent. Moreover, synthetic cationic and anionic resins may be used to remove a hydrophobic, hydrophihc, or neutral solute, which can also be recovered by chemical methods. These resins are also used with a high concentration of dissolved inorganic salts in the waste stream. Their appUcations include phenol, fat, organics, and color removal from wastewater. They can be apphed for the removal of pesticides, carcinogens, and chlorofluoro compounds. 

What is the simplest adsorption model that can adequately define sorption in the system of interest for purposes of our study The simpler the model, the less information is needed to parameterize it. The distribution coefficient model requires only entry of the mass of sorbent in contact with a volume of water and a value for K,. Pesticide adsorption can often be modeled adequately using a simple K ) approach (cf. Lyman et al. 1982). For smectite and ver-miculite clays and zeolites that have dominantly pH-independent surface charge, ion-exchange or power-exchange models may accurately reproduce adsorption of the alkaline earths and alkali metals. If the system of interest experiences a wide range of pH and solution concentrations, and adsorption is of multivalent species by metal oxyhydroxides, then an electrostatic model may be most appropriate. 

Kunia. To determine initial conditions for the Kunia spill it was necessary to estimate the area of the spill and calculate the approximate depth of pesticide penetration. A simple mass balance analysis indicated that the soil solution was saturated with DBCP to a depth of 5 cm soon after the spill. We estimated volatilization loss of DBCP in a three-day period prior to the first rainfall with a first-order decay coefficient of 1.26 day l (19). The calculated initial solution concentration in the soil solution at the first rainfall was 3480 ppb. Clearly, this quantity could be in error, so that subsequent calculated concentrations are suspect. 

The adsorption isotherms of metobromuron onto ACC were determined on the basis of batch analysis. ACC pieces of varying weights were allowed to equilibrate with metobromuron solutions each having the same initial concentration of 1.05 x 10 M at 25°C for 48 h. Preliminary tests showed that the concentration of pesticide solution remained unchanged after about 19 h contact with ACC. So, the allowed contact time of 48 h ensures the equilibration for metobromuron. The equilibrium concentrations of pesticide solutions were measured spectrophotometri-cally. The amount of pesticide adsorbed per unit mass of ACC at equilibrium, q, was calculated by Eq. 22.1,... 

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