3.2.2. Concentration of Arsenic in Plants
The amount of arsenic in plants varied according to the species. The As concentration in plants increased as a function of As concentration in the soil, especially in the aerial parts (Figure 1
), where the As content ranged from 2.20 to 535 mg·kg−1
, with the maximum in the shoots of H. annuus
in soil sample E. However, the plants contained a relatively low As amount and, as also reported in other studies [39
], they accumulated the As primarily in their roots, suggesting a metal storage in the radical cells and a low mobility of the metal within the plants. For L. albus
and B. juncea
, the amount of As in roots was found on average up to 22 fold more than in the shoots, in the soil sample with the highest As extractable percentage (sample C), reaching up to 2600 mg·kg−1
In all selected species, the As concentrations in vegetal tissues increased with the application of phosphate. This highlights the potential effect of phosphate in increasing As accumulation in plants grown in As contaminated soil, due to the enhanced mobility and bioavailability of the metal in soil. Since arsenate is a phosphate analogue, in the presence of phosphorus, the arsenic adsorbed on soil surfaces is replaced [33
], promoting a possible increase of metal uptake by plants. In fact, both ions in the radical cells of tolerant and non-tolerant plant species compete in the same transport system [5
]. In this experiment, the phosphate effect was more pronounced in L. albus
, particularly in samples A and B, in which the As content increased up to nine times, both in the aerial parts and in the root system. Also in B. juncea
and H. annuus
, after the treatments, the amount of arsenic increased in all the vegetal tissues, by about four and three times, respectively.
Regardless of the concentration present in the polygons, the plants absorbed a greater amount of As after the phosphate treatment. However, the pattern of As concentration in the aerial parts of plants compared to the total concentration in the soil was similar. In the polygons with the highest concentrations, the plants absorbed a greater amount of As, but the trend was not linear.
The transfer of inorganic ions from soil solutions to plants has been frequently interpreted as a biosorption process, and a Freundlich-like equation has been used to describe the uptake of contaminants by plants [15
The Freundlich-like Equation (1) used is the same of that used for adsorption processes:
However in this case, q is the contaminant concentration in plants (mg·kg−1) and C is the concentration of contaminants in the soil (mg·kg−1). In the Freundlich-like equation, K can be considered as the sorption capacity (a larger K indicates a larger capacity), whereas the value of 1/n is indicative of the strength of sorption.
Even if a Freundlich-like equation can be usefully used to study absorption of metals by plants, we have to consider that plant uptake cannot be considered a biosorption process. Biosorption is the sorption process of a contaminant by non-living biomass due to the presence of adsorbing surfaces characterized by functional groups able to interact with the contaminant. Biosorption is characterized by different processes such as adsorption on the surfaces, precipitation, ion exchange and complexation.
Plant uptake involves living plants with a physiological contaminant transport mechanism, which is dependent on the plant species. Thus, the Freundlich-like equation can be used as an operational tool for planning phytoremediation, without attributing thermodynamic significance to parameters K and 1/n, but using them exclusively for an indication of the applicability of phytoremediation under the specific conditions of the contaminated site under examination.
A Freundlich-like equation can be successfully used to describe the absorption trend in relation to the concentration in the polygons (Figure 2
). The Langmuir equation [43
] was also tested in terms of its ability to describe the uptake from different polygons, however the results were much lower than those of the Freundlich-like model (data not shown).
Freundlich-like equation data are reported in Table 2
A Freundlich equation efficiently describes the uptake of plants in the site under study, considering the polygons with different As concentrations, with values of R2
generally greater than 0.90. The results are in agreement with previous findings obtained, with different plant species, by Freundlich or similar models [42
By operationally using the Freundlich model parameters, it can be hypothesized that the uptake capacity increased with an increasing value of K
. The results show that for all the plant species the uptake always increased after phosphate treatment. Adding phosphate to the soil influenced desorption of arsenate from soil surfaces, and their release in soil solution. According to the data, the parameter 1/n
is less than 1. This coefficient has been interpreted as an index of a plant’s ability to control metal accumulation [15
]. For this reason, it is reasonable to assume, for the tests with the same type of plants, the value 1/n
as a shared parameter in the estimation process, since that parameter is closely related to the specific species under consideration, whereas it is not particularly affected by the type of treatment adopted.
Therefore considering tests conducted with the same species, with treated and untreated soils, there is a strong correlation precisely through that parameter.
When the relationship between plant concentration and total soil concentration was examined considering the root portion (Figure 3
), the R2
values of the Freundlich-like equation decreased as reported in Table 3
Also for As concentration in the roots is confirmed the same trend that sees the value of K grow for each species downstream of phosphate treatment.
The same Freundlich-like model was also applied by correlating the amount absorbed by the plants with the potentially bioavailable metal concentration in the polygons. The results are reported in Table 4
and Table 5
for shoots and roots, respectively.
Also in this case, a Freundlich-like equation can be used to describe the pattern of plant uptake with respect to the potentially bioavailable concentrations in the various polygons. The K and 1/n coefficients changed however the trend was similar to the uptake versus total concentration, with the highest K values in the untreated and treated soils, for H. annuus.