Interaction of glassy fertilizers and Cd<sup>2+</sup> ions in terms of soil pollution neutralization

doi:10.4236/ns.2011.38092

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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: [email protected]

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



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

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

PO ],

mg/l

Ca2+,

mg/l

Mg2+,

mg/l

0 1403 872 686 430

3.5

577 468 298 585

0 1403 872 686 434

6 60 249 284 437

5.0

8.5 264 184 254

0 1403 872 686 439

22 137 370 291 340

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

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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

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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