Vol.3, No.8, 689-693 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.38092
Interaction of glassy fertilizers and Cd2+ ions in terms of
soil pollution neutralization
Irena Wacławska, Magdalena Szumera*
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Cracow, Poland;
*Corresponding Author:
Received 23 November 2010; revised 15 January 2011; accepted 25 January 2011.
ABSTRACT
Immobilization of cadmium contamination in
soils by precipitation of nonassimilable for
plants Cd-phosphates was considered. Glassy
fertilizer of controlled release rate of the nutri-
ents for plants as a source of phosphate anions
was applied. The negative role of Cd complex-
ing citric acid solution simulating the natural
soil conditions, which inhibits the Cd-phos-
phates formation, was stated.
Keywords: Soil Environment Protection; Glassy
Fertilizer; Cd Immobilization
1. INTRODUCTION
The symptom of soils chemical degradation is, among
others, the accumulation of toxic elements in its top lay-
ers emitted mainly by industry, pesticides, also by min-
eral fertilizers and liquid wastes used to fertilize soils.
Ecological risk connected with the toxic elements con-
centration in upper soils level not only results from their
easy assimilation by plants; it involves also the assimila-
tion of these elements by soil microorganisms and
mesofauna, being important link of elements migration
in feeding chain. Cadmium is element especially mobile
in soil environment and activity if different biological
processes inhibiting. Physiological effect of cadmium
excess in plants results from the disorder of photosyn-
thesis, transpiration and nitrogen compounds transfor-
mation as well as with the changes of membrane cellular
permeability and DNA structure. Easily accumulation of
cadmium by plants makes a risk to place it in the human
body, where it undergoes long time accumulation in or-
gans functioning important roles (liver, kidneys, bones)
[1,2].
One method of neutralizing such a type of soil envi-
ronment contamination is bonding of toxic elements
contained in it into compounds difficult to dissolve,
which makes them nonassimilable for plants. Phospho-
rus reacts with many heavy metals to form secondary
phosphate precipitates that are stable over a wide range
of environmental conditions. While it is true that toxic
elements content in the soil does not undergo any change
in this way, their mobility and toxic influence on living
organisms are reduced.
Experimental studies, in which well soluble phos-
phates and phosphate fertilizers were used for cadmium
removal were conducted on contaminated soils [3,4].
Synthetic fertilizers used as a source of phosphorus
are an additional source of toxic elements (Cd, Cu, Ni,
Pb, Zn), located in the soil [5]. They come from the raw
materials used to production this type of fertilizers, their
source is also the process of their production.
This study refers to the possibilities of using chemi-
cally active silicate-phosphate glasses acting as vitreous
fertilizers of controlled release rate of the nutrients (P,
Ca, Mg, K, microelements) for plants [6,7] for simulta-
neous bonding of cadmium, constituting particularly
harmful soils contamination, into the form of insoluble
compounds. Glassy fertilizers because of the production
does not constitute an additional source of toxic ele-
ments.
Characterization of processes and products of reaction
between glassy fertilizer VitroFosMaK of 42 SiO2·12
P2O5·10 K2O·22 MgO·14 CaO [wt%] composition and
cadmium chloride solutions (“ex situ” reactions) under
soil environment simulating conditions, is the subject of
the present study.
2. EXPERIMENTAL
2 wt% citric acid solution was used as an extractor re-
leasing 3
4
PO

ions from the glassy fertilizer structure.
The glass to solution weight ratio was 1:100. Such con-
ditions simulate physico-chemical state similar to the
natural environment of plant roots and the surrounding
soil [8].
Additionally, the inorganic acid (HCl) was used an

I. Wacławska et al. / Natural Science 3 (2011) 689-693 690
extractor releasing 3
4
PO  ions from the glassy fertilizer
structure.
Experiments were conducted applying the following
procedure.
Dissolution of 1 g of VitroFosMaK (0.1 - 0.3 mm) in 100 ml of
H3Cit/HCl by shaken for 1/2 h
↓
Filtration of the reacted solution
↓
Adding 50 ml 0.0125 M CdCl2 to 100 ml of filtrate
↓
Filtration of the precipitate after different time intervals
↓
Content of ions in solution determination by ICP-AES method,
precipitate TG/DTG/DTA, XRD, FTIR, SEM-EDS analysis
Thermal analysis was carried out with Derivatograph-C
(Hungarian Optical Works). Experiments conditions
were: samples mass 80 mg, heating rate 10˚C·min–1, air
atmosphere. To identify the solid products of reactions
diffractometer Philips X’ Pert Pro with Cu (Kα) source
was applied. The FTIR and SEM-EDS studies of pre-
cipitates were carried out on the Digilab FTS 60v Spec-
trometer with samples prepared in the form of KBr pel-
lets and JSM 5400 Jeol scanning electron microscope
equipped with an energy dispersive X-ray analysis re-
spectively.
3. RESULTS AND DISCUSSION
The course of the cadmium ions reaction with the
phosphate ions extracted from the glassy fertilizer under
the citric acid action was presented in Table 1.
It has been found that cadmium removal process was
influenced by pH conditions. The most effective (~96%)
process of cadmium ions immobilization from the solu-
tion at pH = 5, took place after 6 days. Together with the
time elongation the amount of cadmium ions was gradu-
ally decreasing, achieving after 29 days the amount of
8.5 mg/l resulting in 99.5% immobilization of this
chemical element in the precipitate. At the same time the
reduction of phosphate concentration was less effective
(~70%). Simultaneously, together with the reaction time
elongation, the amount of calcium ions in the examined
solution was gradually decreasing.
SEM image (Figure 1(a)) showed the precipitated re-
action products of cadmium ions and phosphate ions
after 29 days of the reaction, with a morphology of
amorphous compound.
According to EDS analysis (Figure 1(b)) the precipi-
tate contains not only the O, Cd, Ca and Cl atoms but
Table 1. Evolution of ions concentrations in the chloride solu-
tion in the presence of citric acid with the reaction time.
Time,
days pH [Cd2+], mg/l
[3
4
PO ],
mg/l
Ca2+,
mg/l
Mg2+,
mg/l
0 1403 872 686 430
22
3.5
577 468 298 585
0 1403 872 686 434
6 60 249 284 437
29
5.0
8.5 264 184 254
0 1403 872 686 439
22 137 370 291 340
43
7.0
179 229 156 240
(a)
(b)
Figure 1. SEM/EDS analysis of Cd-precipitate after 29 days of
reaction in citric acid solution.
also large amounts of C atoms (~52 at%), suggesting the
cadmium and calcium citrates formation.
Because of the impossibility of identification of the

I. Wacławska et al. / Natural Science 3 (2011) 689-693
691691
amorphous precipitates phase composition using the
XRD method, they were subjected to thermal and FTIR
examinations.
According to the TG/DTG/DTA results (Figure 2),
the lost of weight up to 320˚C can be interpreted as the
dehydration of calcium and cadmium citrates. In the
temperature interval of 320˚C - 360˚C the dehydration
continues as intermolecular process with a formation of
double C = C bond i.e. with transformation of the citrate
into aconitate. In the temperature interval of 360˚C -
400˚C the deestereification and decarboxilation of
COOH groups existed or formed as a result of the de-
estereification is masked by the exothermic effects of the
burning of H in the air [9,10]. According to [11] after
this step the formation of cadmium and calcium carbon-
ates should take place. Taking into account that in the
temperature interval of 250˚C - 500˚C the partial thermal
decomposition process of cadmium carbonate takes
place, the newly formed cadmium carbonate partially
decomposes and CdO and CdCO3, besides CaCO3, as the
final products are obtained.
XRD examinations of the precipitates after heating to
500˚C have shown (Figure 3) that their thermal decom-
position solid products are cadmium carbonate and cad-
mium oxide which are in accordance with thermal de-
composition products of cadmium citrate and calcium
carbonate as a product of thermal decomposition of cal-
cium citrate.
Comparison of FTIR spectra of precipitates before
and after heating up to 500˚C (Figure 4) confirms that
the products of cadmium ions reaction with phosphate
ions in the presence of citric acid simulating soil envi-
ronment are cadmium and calcium citrates identified
with the use of thermal methods.
The FTIR spectra of precipitates are characterized by
three groups of bands related to the vibrational frequen-
cies of the COO–, H2O and OH [12,13]. The symmetric
stretching vibrations νs (COO–) are observed at 1403 cm–1.
Figure 2. TG/DTG/DTA analysis of precipitate after 29 days of
reaction at pH = 5.0.
Figure 3. XRD analysis of Cd-precipitate after 29 days of re-
action heated up to 500˚C.
Figure 4. FTIR spectra of Cd-precipitate after 29 days of reac-
tion: (a) before and (b) after heated up to 500˚C.
The asymmetric stretching vibrations νas (COO–) appear
at 1548 cm–1. The presence of water in precipitate is
confirmed by bands at 3500 - 2800 cm–1.
The removal of organic compounds from the precipi-
tate structure was significantly manifested in the FTIR
spectrum. Bands related to carboxylate groups and water
molecules disappear, while bands characteristic of
stretching vibrations of 2
3
CO  groups originated from
calcium carbonate, which is the product of calcium cit-
rate decomposition present also in the precipitate and
cadmium carbonate which is a product of cadmium cit-
rate decomposition, appear at 1429 cm–1.
Microscopic studies on the course of cadmium ions
reaction with phosphate ions extracted from the glassy
fertilizer under the inorganic acid (HCl) action showed
(Figure 5(a)) the precipitated reaction products with a

I. Wacławska et al. / Natural Science 3 (2011) 689-693 692
(a)
(b)
Figure 5. SEM/EDS analysis of Cd-precipitate after 21 days of
reaction in HCl solution.
morphology of amorphous compound. According to EDS
analysis (Figure 5(b)) the precipitate beside the cadmium
phosphate calcium phosphate contains. Competitiveness
in the formation of calcium phosphate and cadmium
phosphate results from the similar values of ΔG formation
of these compounds (∆G298 of Ca3(PO4)2 = –4207.916
kJ/mol, ∆G298 of Cd3(PO4)2 = –3905.349 kJ/mol [14].
From the researches carried out it results that the pres-
ence of citric acid solution simulating soil environment
conditions has an inhibiting effect on the process of
cadmium bonding into the form of insoluble phosphates.
Citric acid is a polycarboxylic organic acid, which in the
presence of alkaline cations and alkaline earth cations
forms salts-soluble citrates, whereas in the presence of
non-metals (P, Si) it activates the dissolution process of
their compounds, which are usually insoluble or hardly
soluble in water. So, the presence of citric acid as a
compound strongly complexing metals [15], causes the
formation of less stable cadmium and calcium citrate
complexes.
4. CONCLUSIONS
VitroFosMaK acting as glassy fertilizer of controlled
release rate of the nutrients for plants has ability to cad-
mium ions bonding under an insoluble phosphates.
Cadmium immobilization process, influenced by pH
conditions, is accompanied by calcium phosphate forma-
tion. The presence of citric acid solution simulating
natural soils environment causes the formation of less
stable cadmium and calcium citrate complexes, thus has
an inhibiting effect on the process of bonding cadmium
ions into the form of insoluble phosphates.
5. ACKNOWLEDGEMENTS
The work was supported by Grant No. N N508 38 2035 of the Min-
istry of Science and Higher Education of Poland.
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