THE EFFECT OF DIALDEHYDE STARCH DERIVATIVES ON HEAVY METAL UPTAKE BY MAIZE

Andrzej Para¹, Jacek Antonkiewicz^2
1 Department of Chemistry, University of Agriculture, Cracow, Poland
² Department of Agricultural Chemistry, University of Agriculture, Cracow, Poland

ABSTRACT

Disemicarbazone and dihydrazone of dialdehyde starch were efficient ligands for metal ions. They immobilized heavy metal ions in contaminated soils as assessed based on bioaccumulation of Zn, Pb(II), Cu(II), Cd, and Ni(II) in maize grown in soil polluted with those metals. Heavy metal ions in soil drastically decreased the yield of tested crop. Dialdehyde starch dihydrazone and dialdehyde starch disemicarbazone applied to polluted soil, considerably protected maize from the effect of heavy metals by their immobilization. Concentration of heavy metals in green parts and roots of maize was significantly reduced. In aerial parts of the plant, particularly, uptake of Cd and Ni(II) ions was strongly inhibited whereas in roots, the derivatives offered better uptake control for Cu(II) and Pb(II) ions than for the other metal ions. Starch dialdehyde dihydrazone was more efficient immobilizer than corresponding disemicarbazone.

Key words: heavy metal immobilization, hydrazone, oxidized starch, semicarbazone, soil remediation.

INTRODUCTION

Diversified human activity results in considerable environmental pollution. Efficient prevention of ecosystem from further degradation and detoxication of environment is indispensable. Reduction of already existing accumulation of toxic substances is one of the principal targets in the remediation of arable soils. Heavy metals affect crops and can enter food chain. Hazard from soil contamination with heavy metals depends not only on their total soil content but, first of all, on the proportion of their readily soluble forms, available to plants and liable to leaching to ground waters [1].

Various remediation techniques have been applied to clean up soils polluted with heavy metals. Chemical and/or thermal remediation is considered aggressive. It provides fast elimination of pollutants but, simultaneously, biological life in such soil vanishes. In case of low concentration of pollutants, phytoremediation may prove efficient [2-4].

Phytoremediation is a biological method of soil cleaning with some plants called hyperaccumulators [5-7], capable to uptake and accumulate pollutants by up to hundred orders higher than do other plants.

Another, mild method of remediation applies to mobile, bioavailable heavy metal fractions in soil. It facilitates heavy metal migration to ground waters and, in that manner, limits their availability and toxicity to plants. It involves supplementing either natural or synthetic sorbent materials directly to the soil [8]. The soil cleaning by means of phytoremediation and immobilization is advantageous as it can be performed in situ without the necessary removing the topsoil and involvement of advanced technical devices.

This paper presents immobilization of selected heavy metals with derivatives of dialdehyde starch, which bind metal ions form sparingly water soluble complexes.

MATERIALS AND METHODS

Materials. Potato starch was manufactured in PPZ Trzemeszno Potato Enterprise, Poland. Semicarbazide hydrochloride of analytical grade was purchased from Fluka. Hydrazine sulfate, concentrated nitric and chlorate(VII) acids of reagent grade, and NaIO₃, Na₂CO₃, 3CdSO₄ · 8 H₂O, CuSO₄ · 5H₂O, NiSO₄ · 7H₂O, ZnSO₄ˇ 7H₂O, Pb(NO₃)₂, NH₄NO₃, KH₂PO₄, and KCl all analytical grade, were purchased from POCh, Gliwice, Poland.

Methods. Synthesis of dialdehyde starch derivatives. Dialdehyde starch (DAS) of 25% oxidation degree was prepared from potato starch oxidized with sodium iodate(VII) in the one-step process with simultaneous electrolytic regeneration of iodate(V) [9]. Water solubility of the product at 25°C (according to Richter method [10]) was 1.0%. Solutions (100cm³) containing either semicarbazide hydrochloride (20g) or hydrazine sulfate (24g) were added to aqueous suspension of dialdehyde starch (50 g DAS in 100cm³ H₂O) and left for 2 hours on intensive agitation and continuous control of acidity of the reaction mixture. The latter should be maintained on the level of pH 5 by admixing solid sodium carbonate. Subsequently, the suspension containing relevant nitrogen dialdehyde starch derivative was isolated by suction filtration. The crude products were washed with redistilled water (3 x 100 cm³) then dried at 100°C in 5 hours. Results of elemental and thermograwimetric analyses and IR spectra of derivatives fitted results formerly reported [9].

Soil preparation. A mineral soil of a texture of loamy sand was collected from the top layer of arable land. The soil reaction determined by potentiometric method in 1M KCl was 4.66. The soil contained 71% of sand, 16% of silt, 13% of floatable particles and 0.59 g · kg^-1d.m. nitrogen. Total heavy metal content, extracted with an a mixture of concentrated nitric and chlorate(VII) acids (3:2) were as follows: cadmium – 0.43 mg, lead – 25.25 mg, nickel – 11.43 mg, copper – 4.35 mg and zinc – 65.25 mg · kg^-1 d.m. soil. According to domestic regulations on soil quality [11] that soil could be classified as non-polluted with heavy metals. Thus, it was contaminated with solution containing the following salts: 3CdSO₄ · 8H₂O, CuSO₄ · 5H₂O, NiSO₄ · 7H₂O, ZnSO₄ · 7H₂O, Pb(NO₃)₂. These doses provided the following quantities of metal ions in 1 kg of dry soil: 2 mg Cd, 150 mg Pb, 50 mg Ni, 75 mg Cu and 250 mg Zn. Total amount of individual metals respond to II degree of soil pollution according to limit numbers issued by the Institute of Soil Science and Plant Cultivation in Puławy, Poland [12]. Uniform NPK (nitrogen/phosphorus/potassium) fertilization (in kg^-1 dry soil) was applied for all pots, i.e.: 0.3 mg nitrogen, 0.08 mg phosphorus, and 0.2 mg potassium by addition of solution of NH₄NO₃, KH₂PO₄, and KCl. The experimental design comprised of six treatments (each run in triplicate): I – control – soil free of heavy metals, II and III containing respectively 20 g · kg^-1 of DSC and DHZ, IV treatment with added heavy metal compounds, V and VI treatments with added heavy metal compounds and respectively 20 g · kg^-1 of DSC and DHZ.

Plant seeding, vegetation, and harvesting. Seeds of maize, KOKA cv. (bred at Plant Breeding Station at Mikulice) were sown into pots containing 1.5 kg prepared soil. During vegetation the plants were watered with redistilled water and moisture was maintained at 60% maximum water capacity. Vegetation period for maize was 50 days. After harvesting the plants were dried in a dryer with forced air flow at 75°C and, subsequently, the yield of maize aboveground parts and roots was determined.

Determination of metal concentrations. Samples of analysed plants were dry mineralised at 450°C in a muffle furnace [13]. After mineralization the analysed metal concentrations were essayed by the ICP (Inductively Coupled Plasma) method with ICP-AES, JY 238 ULTRACE JOBIN-YVON (Longjumeau, France) spectrometer.

Statistical computations were conducted using Microsoft Excel 7.0 calculation sheet. Significance of differences between compared means of maize yield were determined by Duncan method. Analysis of variance and Duncan test were conducted on the α = 0.01 significance level.

RESULTS AND DISCUSSION

Obtained derivatives of dialdehyde starch (Fig.) were sparingly water-soluble (DSC – 1.9%, DHZ – 0.9%).

DSC and DHZ supplied to the soil formed sparingly water-soluble complexes with metal ions. Quantities of metal atoms coordinating the ligands depended on the metal ion [14, 15].

Effect of the dialdehyde starch upon immobilization of heavy metal ions in soil was assessed from the yield of maize crops and heavy metal accumulation in aerial parts and roots of that plant. The yield of aboveground parts significantly depended on the level of soil pollution with heavy metals. It ranged between 5.1 and 29.2 g per pot (Table). Yield of the maize roots was much lower and was between 1.3 and 6.5 g per pot. Dialdehyde starch derivatives used for the experiment declined yield of maize aboveground parts and roots. Decrease in the aboveground part yield on treatment II (soil + disemicarbazone (DSC)) was 76% and on treatment III (soil + dihydrazone (DHZ)) it was slightly over 30% in respect to control. Toxic effect of dialdehyde starch derivatives might result from complexing activities between polysaccharide and protein fragments of cell membranes and polysaccharide fragment of dialdehyde derivatives. Such polysaccharide-polysaccharide and polysaccharide-protein interactions are well known [16]. They may change properties and functions of outer cell wall layers making possible simultaneous interaction between semicarbazone and hydrazone residues with the cell interior. As proven in separate experiments neither hydrazine nor semicarbazide were liberated from those derivatives by hydrolysis.

The most pronounced, 80% decline in maize yield was observed in treatment IV (soil polluted with heavy metals without DAS derivative share).

DSC added to contaminated soil (treatment V) and DHZ supplied to soil of treatment VI inhibited the yield decline in comparison to treatment IV, which was particularly remarkable for DHZ. Such treatment resulted in yield decreased by 59%, whereas without DHZ the decline reached 80%.

Bioaccumulation of metals in maize was highly diversified, ranging between 76.6 and 3300 mg Zn; 1.04-360 mg Pb; 1.40-370 mg Cu; 0.50-21.5 mg Cd; 0.19-635 mg Ni · kg^-1 dry mass (Tab.). Significantly the highest content of heavy metals was noted in maize in treatment IV (soil polluted with heavy metals without DAS derivatives). It explained the highest reduction of maize biomass yield. In aboveground parts, DAS derivatives supplied to the soil (DSC and DHZ in treatments V and VI, respectively) significantly reduced the heavy metal, particularly, cadmium and nickel accumulation. DSC the most efficiently reduced the level of zinc and cadmium, while DHZ was most efficient in that respect for nickel. Zinc, cadmium and nickel concentrations in maize aerial parts depended on DAS derivative, whereas concentrations of lead and copper were equally reduced by both derivatives.

Except zinc, DAS derivatives also decreased heavy metal uptake in roots. For lead, copper, and nickel DHZ appeared more efficient than DSC but both these derivatives exhibited comparable effect in respect to cadmium. An assessment of trace element concentration levels based on criteria for usable value of crops [11] showed that obtained biomass of the tested plant might be used for compost production.

Inspection of Table revealed that content of heavy metals in roots exceeded that in the aerial parts. Thus, in maize a barrier on the way of metal transport from roots to the aboveground parts could be assumed. The literature [1, 17, 18], pointed to lead and copper as the most readily uptaken and results presented in this report corroborate with that point (26-fold difference for lead and 28 – fold for copper in root and aboveground part content).

Inhibitory effect of heavy metals on the growth and development of the aerial parts and roots of plants is very important in reclamation processes in chemically degraded areas [17]. Presented results are compatible with formerly published data [2, 18], which pointed to maize as that distinguished from several other plant species in a relatively high heavy metal accumulating ability. For that sake maize could be used in soil phytoremediation. However, its relatively strong sensitivity to heavy metals is a drawback leading to considerable decrease in its yield. A partial solution to the problem, particularly at soil cleaning from zinc, lead, and copper, might come from application of DHZ on seeding maize. It causes some decrease in the metal concentrations in plant, but at the same time, a 100% growth of biomass is observed (treatments IV and VI) and, in consequence, better efficiency of phytoextraction process. For zinc, at low metal content, an additional effect of increased metal uptake by maize is observed after DHZ addition.

CONCLUSION

1. Administration of starch dialdehyde derivatives to soils of natural heavy metal content resulted in decrease in the maize crops. Particularly, it was observed in case of disemicarbazone. Simultaneously, these derivatives had no effect upon heavy metal accumulation in all parts of maize plants.

2. In heavy metal contaminated soils, starch dialdehyde derivatives, particularly dihydrazone, immobilized the heavy metals. It increased the maize crops which, at the same time, had lower level of bioaccumulated heavy metals.

3. In heavy metal contaminated soils dialdehyde starch dihydrazone can support phytoremediation, particularly, in case of zinc. In that case biomass increases and the content of zinc therein is only insignificantly reduced.

ACKNOWLEDGMENT

This work was supported by a grant from Polish State Committee for Scientific Research, Grant No. PBZ-KBN-070/T09/2001/19.

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