Environmental Engineering Reference
In-Depth Information
would take for the adsorption by the soil to become saturated. The peak of phosphorus
leaching also decreased with the decreasing concentration of the low molecular weight
organic acids, and the occurrence time of the peak was delayed as well. In addition, compared
with the citric acid, the activation of soil phosphorus by malic acid, oxalic acid and acetic acid
was more significantly affected by their concentrations.
2.3. Kinetic Fitting of Phosphorus Release During Soil Leaching by Low
Molecular Weight Organic Acids
As known from Table 4, when the parabolic diffusion equation was employed for the
fitting of the dynamic leaching curve of soil phosphorus release, a significant level was
achieved in all cases (
P
<0.05). The letter in the parabolic diffusion equation represents the
relative diffusion coefficient of phosphorus. Lu et al., (1998) pointed out in their study that,
citric acid and oxalic acid could obviously improve the diffusion coefficient of phosphorus in
the soil, mainly because the citric acid and oxalic acid had increased the phosphorus
concentration of the soil. Feng (2012) found that except oxalic acid, adding low molecular
weight organic acids could improve the relative diffusion coefficient of phosphorus in the soil
to a large extent; oxalic acid was found to form precipitate under the effect of calcium and
thus lose its activation, which further affected the diffusion coefficient of phosphorus, In the
experiment, the relative diffusion coefficients of phosphorus under the effect of low
molecular weight organic acids showed the following sequence: citric acid (tribasic
acid)>malic acid (dibasic acid)>acetic acid (monobasic acid). The diffusion coefficient of
phosphorus was affected in the case of oxalic acid was susceptible to forming calcium oxalate
precipitate with calcium (Table 3-4), which was consistent with the characteristics of the
phosphorus release curve of the soil treated with oxalic acid. Meanwhile, the concentration of
low molecular weight organic acids also significantly affected the relative diffusion
coefficient of phosphorus, owing to the fact that the ability of low molecular weight organic
acids to activate phosphorus was weakened in low concentration, leading to the decreased
concentration of phosphorus in the soil decreased and thereby the decreased diffusion rate of
phosphorus.
Table 4. The rate constants of diffusion (
R
) fitting by parabolic diffusion equation
Organic acids
R
Citric acid(10mM)
8.02
Citric acid(1mM)
1.32
Malic acid(10mM)
7.62
Malic acid(1mM)
0.33
Oxalic acid(10mM)
3.95
Acetic acid(10mM)
4.68
