Contaminants uptake rate

For the estimation of the contaminants uptake rate and consequently the time required for the phytoremediation of a contaminated site, single mathematical models may be applied. In the following paragraphs, analysis as well as the examples given in Groundwater Remediation Technologies Analysis Center Technology Evaluation Report on Phytoremediation, by Schnoor, is presented without modification.8... 

Figure 7.1 Comparison of the patterns of organic contaminant uptake rates (as related to log Kows) by SPMDs and across fish gills (McKim et ah, 1985). Reprinted with permission from the American Petroleum Institute (Huckins et al., 2002).

Uptake rates may also differ with age. Young children, for example, absorb lead via the gastro-intestinal tract four to five times more efficiently than adults, which makes them more susceptible to the effects of lead. Certainly much work remains to be done on soil contaminant uptake rates but current research illustrates that uptake rates are highly variable and dependent on a range of factors such as the contaminant type, physical and chemical properties of the soil, residence time in the gut etc. 

From equation 14.24 it is possible to estimate the uptake rate of the contaminants). First-order kinetics can be assumed as an approximation for the time duration needed to achieve remediation goals. The uptake rate should be divided by the mass of contaminant remaining in the soil ... 

In order to properly interpret differences in sampling rates among compounds and among exposure conditions, it is important to discriminate between membrane-controlled uptake and WBL-controlled uptake. From Eq. 3.8 (in the absence of biofouling), contaminant uptake is rate-limited by the membrane when... 

Booij et al. (2003b) made an effort to model contaminant uptake by buried passive samplers. The major assumptions underlying this model are that the sampler can be regarded as an infinite sink for target contaminants, that the depletion of the bulk sediment phase is insignificant, and that the contaminant desorption kinetics are not rate-limiting. 

Meadows et al. (1998) conducted a 28 d exposure of brown trout (Salmo trutta), standard SPMDs and hexane fllled dialysis bags (Sodergren, 1987) to spring water (total organic carbon < 1 mg L ) contaminated with PCBs. Trout were not fed during the exposure, and temperature and flow conditions remained constant throughout the exposure. A good correlation (r = 0.89) was found between the uptake rate constants ( u,fs) for whole body trout and the uptake rate constants... 

Table 4 presents the results of a batch test of a soil contaminated by 6.5g/kg dry matter of mineral oil, established to evaluate the feasibility of biosparging as a remediation technology (J. Gemoets, personal communication). Apart from a sterile control, air was added to two other microcosms. In one, additional nutrients were added. Only in the aerated situation, together with the addition of nitrogen and phosphorous, was any degradation observed (32%), and a peak O2 uptake rate of 1.2g/kg/d was... 

Since AAS is a ratio method, many instrumental errors (e.g. long-term source drift, small monochromator drifts) should cancel out, as 7 is ratioed to I . However, a stable uptake rate, or aspiration rate, is required. This falls as the viscosity of the solution sprayed is increased. Nebulizer uptake interferences can be minimized if the dissolved salts content of samples and standards is approximately matched. For example, when determining pg cm sodium levels in 2 M phosphoric acid, ensure that the standards are also dissolved in 2 M phosphoric acid, using a blank to check for contamination. 

The solution to be nebulized is usually pumped to the nebulizer using a peristaltic pump, unlike for FAAS, where the solution uptake is by free aspiration. The solution is pumped through polymeric tubing [usually poly(vinyl chloride)] and also connecting tubing (usually Teflon) to the nebulizer. Both of these materials can be manufactured to a high degree of purity, hence contamination is minimized. The solution is pumped at a rate of 1 -2 ml min, which is much slower than the 5-10 ml min uptake rate for FAAS. This tends to favour the formation of fewer but smaller droplets, which results in less noise but a lower overall sample transport efficiency. 

Of crucial importance for the potential risk associated with this contamination of the diet is the question how much of the ingested PCDD/Fs is actually absorbed and retained in the body There is, however, remarkably little information on this subject. There is one report of absorption in an adult man where 87% of a single dose of radioactively labelled 2,3,7,8-Cl4DD dissolved in corn oil was taken up.77 In another study, in which only faeces samples from two individuals on an unmodified diet were analysed, it was found that the excretion rate was very high compared to the theoretical uptake rate,78 which would suggest that absorption is low. No work beyond these two somewhat contradictory studies was found. 

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