PHYTOEXTRACTION OF PB, CR AND CD BY HEMP DURING SUGAR INDUSTRY ANAEROBIC SEWAGE SLUDGE TREATMENT

Agnieszka Piotrowska-Cyplik¹, Zbigniew Czarnecki^2
1 Institute of Food Technology of Plant Origin, Faculty of Food Technology, Agricultural University of Poznań
² Department of Fermentation and Biosynthesis, Institute of Food Technology of Plant Origin, The August Cieszkowski Agricultural University of Poznan, Poland

ABSTRACT

The aim of the research was to estimate efficiency of monoaecian fibrous hemp (Cannabis sativa L., "Benico" variety) for phytoextracting heavy metals from soil-sludge substrate during anaerobic sewage sludge treatment at non-industrial sites. Hemp is known to be an efficient lead, chromium and cadmium phytoextractor from soils, though these heavy metals cannot be found among plants nutrients. Heavy metals analysis of the substrate has shown that an increase in hemp biomass led to reducing lead, chromium and cadmium concentrations by a factor of six, twelve and nearly three, respectively. The effect was observed in light soil fertilised with sewage sludge and compared against respective metals concentration at the beginning of the pot experiment.

Key words: dewatered anaerobic sewage sludge, hemp, chromium, lead, cadmium, biocumulation.

INTRODUCTION

Among many technologies for purifying substrates from heavy metals, including ferlisation with sewage sludge, phytoextraction occupies a special place [8]. Phytoextraction is an environmentally friendly biological method which reduces contaminants concentrations without causing any side-effects that usually accompany other chemical-technical technologies. The consequence of applying aggressive technologies is most often destruction of all forms of biological life in the purified soil [21]. Degree of soil degradation by heavy metals depends on soil properties to a considerable extent. Trace quantities of these elements occurring in cation forms (primarily heavy metals), as a rule, undergo strong sorption in the surface level of the soil profile [3, 19]. The following heavy metals are among the most noxious factors for the majority of plants, apart from their diversified effect and range of toxic concentrations: Ni, Hg, Cu, Pb, Co, Cd, Ag, Be and Sn [21]. The main factor influencing the mobility of heavy metals in the soil environment is its reaction, which affects the stability of humus complexes with metals. The stability of such complexes is illustrated by the series presented below:

In soils with pH considerably above 7 (alkaline and carbonate soils of high sorption capacity), the transfer of heavy metals into non-soluble forms takes place without any additional agrotechnical treatments. In case of acid and very acid soils, liming neutralises soil acidity, removes the action of physiologically acid mineral fertilisers and renders trace elements harmless. The process of rendering them harmless consists in limiting their mobility or their complete immobilisation following their transfer into non-soluble forms. A strong and always positive role is played by organic fertilisers causing, among others, a reduction in acidity and in the level of concentrations of toxic aluminium compounds [3, 22].

The efficiency of phytoextraction significantly depends on the choice of plants that are to be applied for the purpose of phyto-sorption of a given contamination from the soil into their overground parts [10, 14]. With the end of the vegetation period, plant residues constitute secondary sources of heavy metals uptaken earlier from the soil. They constitute one of the main elements of the metal bio-cycle in the ecosystem [5, 17]. However, the very same crop plants cultivated and later harvested at the appropriate stage of growth and development can remove contaminants from the environment [9, 8]. In case of hemp, when it is grown as a single purpose plant only for high-quality fibre, its harvesting is performed at the beginning of the flowering period. In addition, hemp is characterised by a considerable resistance to diseases, small susceptibility to weeding and a considerable capacity to utilise the natural soil fertility [16].

The cost of phytoextraction is relatively low and estimated at the range of 15-40$ to decontaminate 1 m³ of soil [16]. Introductory studies on the possibility of application of phytoextraction should include the assessment of the land contamination (type, level, properties) and the impact of the contamination on the physiological state of plants according to the principle that only healthy and high-yielding plants are capable of guaranteeing high effectiveness of the remediation treatment and it is widely known that the excessive concentrations of nutrients uptaken by plants leads to disturbed plant metabolism, checking of their growth and production of small quantities of the biomass [6, 20]. It is also commonly accepted that in Poland the process of phytoextraction of heavy metals could be applied on a microscale to remediate contaminated areas of exceptional value because of their location or those which pose high hazards [11, 13, 17].

Therefore, during research of the influence of anaerobic sewage sludge on fertilization of light soil at organic matter and by extension on the value of hemp yield, phytoextraction efficiency for three heavy metals, namely Cr, Pb, and Cd, by this plant have been described. The content of heavy metals in anaerobic sewage sludge and in soil was low and not overstepped. The aim of our research was to gauge the efficiency of hemp in removing these low values of heavy metals. In order to attain this organic carbon and nitrogen quantity and proportions between these two, macroelements at the beginning and at the end of the pot experiment were compared. The values of hemp yield and content of chromium, lead and cadmium in the substratum and in hemp (root, stem and leaf) have been designated.

MATERIALS AND METHODS

Dewatered anaerobic sewage sludge, of moisture 62%, after mezophilic fermentation from sugar industry wastewater treatment biological plant in Kościan was chosen as research material. The sewage sludge was dewatered by means of polyelectrolyte F-410 using Draimad-teknobag dewatered system type 06BCAVPK in the sewage sludge dewatering station near Leszno. Hemp was cselected because of its efficiency in removing organic and inorganic pollutants from the substratum [12]. Research with the application of hemp was conducted as a pot experiment in a greenhouse in 2000 and 2001 in the experimental station of the Institute of Natural Fibres in Pętkowo near Środa Wielkopolska. The hemp was seeded each year, when average ambient temperature ranged between 8-10°C. The harvest took place each year about two weeks after blooming, which gives strong, high quality fibres. The pot experiment was conducted in twenty-four plastic pots designed especially for hemp growing, with U-shape and a hole aeration system, of 17 dm³ capacity. The pots were filled a week before sowing with 10 kg of air-dried soil composed of poor clayey soil (further referred to as light soil) thoroughly mixed with organic additions such as organic manure and wheat straw at appropriate mass proportions. There were two doses of sewage sludge appropriately to nitrogen content in a pot and to hemp manurial needs of nitrogen:

a) a low dose - 5% d.m. sewage sludge /10 kg d.m. soil,
b) a high dose -10% d.m. sewage sludge /10 kg d.m. soil.

Both low and high dose were encompassed from three combinations of the substratum, namely: the first - raw anaerobic sewage sludge,
the second - anaerobic sewage sludge with wheat straw, and
the third - anaerobic sewage sludge with organic manure.

Each combination was repeated three times.

There were also soil-control pots (repeated three times) for each part of the experiment. i.e. low and high doses. The soil-control pots were supplemented fertilizers taking into account annual doses of fertilizers in ploughland and hemp manorial needs of nitrogen, phosphorus and potassium.

a) N in ammonium nitrate (NH₄NO₃) - 0.3 g/kg soil,
b) P in sodium dihydrogen phosphate (Na H₂PO_4·2H₂O) - 0.1 g/kg soil,
c) K in potassium sulphate (K₂SO₄) - 0.3 g/kg soil.

During vegetation the hemp was watered to 55-60% of water capacity. After harvesting the biomass (weight and height) of hemps was indicated.

The heavy metals were determined for the low dose of pot experiment. The content of mean lead, cadmium and chromium in raw materials used in the experiment, in pot samples at the beginning and in the end of the experiment, as well as in hemp (roots, leaves and stems) were determined by atomic absorption spectrophotometry (ASA). The samples were mineralised and burned using a mixture of spectra pure concentrated nitric acid and perchloric acid (1:1 v/v, Merck). The contents of Pb, Cd and Cr in the mineralized samples were determined by means of flame atomic absorption spectrometry (SpectrAA 250 Plus, Varian).

Chemical analyses of pot contents

The analyses were made at the beginning and in the end of the pot experiment. The analyses included:

• total nitrogen by Kjeldahl method,

• total carbon by Tiurin method,

• assimilable phosphorus and potassium by Egner-Riehm method,

• calcium by flame spectrometry,

• class and category of soil by Bouyoucosa in modification of Cassagrande and Prószynski method.

Accumulation and translocation indicators (%) of Pb, Cd and Cr in hemp

*     indicator of accumulation calculated as a ratio of average content of metal in plant from all combinations to its concentration in the control plant,

**   indicator of translocation in plant calculated with metal concentration in root set to 100%, and its concentration in other organs as % of this value,

*** indicator of translocation soil-plant calculated with the metal concentration in soil set as 100%, and its concentration in plant organs as a % of this value.

Statistical Methods

Statistical evaluation of the data has been performed with an analysis of variance, Levene´s test, Kruskall-Wallis test, LSD test. Calculations were made with program Statistica 5.0.

RESULTS AND DISCUSSION

In the anaerobic sewage sludge, i.e. the research material, the content of organic carbon was 9.8%. The total contents of macronutrients in this material were as follows: N - 2.24%, P - 0.5%, K- 0.5%, and Ca - 20.8%. The organic carbon to total nitrogen ratio was 11:1. Whereas contents of assimilable macronutrients in sewage sludge were : P - 0.097% (10% HCl), K - 0.06% (10% HCl), Ca - 6.5% (10% HCl). The pH was closed to neutral reaction at 6.5.

The soil examined in the pot experiment was slightly acid poor clayey soil, with pH of 5.6 in 1 M KCl and consisted of 66% sand, 21% dust and 13% floatable parts; the latter has 3% of colloidal clay.

The content of organic carbon in soil used in experiment was 0.9% and total nitrogen 0.11%. Concentration of phosphorus and potassium were very high: P₂O₅ - 25.6 mg/100 g a.d.m. and K₂O - 20.7 mg/100 g a.d.m. In the analysed soil organic carbon to total nitrogen proportion was 8:1. To preserve the optimal conditions for hemp to grow, the materials composing the substratum in pots in each combination were taken based on the required organic carbon to total nitrogen proportion.

The carbon to nitrogen ratios in particular combinations were changed from 8.8-10.4 to 10.2-11.8 at the end of the pot experiment, which was caused by taking more nitrogen towards carbon (p<0.005) (Tab. 1). In relatively short vegetative experiments only small parts of organic carbon are used by plants. The greatest amount of organic carbon is released and taken by plants in the last part of the vegetation season, or in case of many growing seasons - in the second or third vegetation season. The content of total nitrogen assimilated by plants already in the firsts months of vegetation is significantly reduced [1, 7].

After harvesting hemp height and weight for particular combinations were determined (Fig. 1). For a low dose the addition of organic matter radically affected the hemp yield (p<0.001). For high dose of sludge radical differences resulting from the increased organic matter content in the substrate (p> 0.58) were found neither in hemp weight nor in its height for particular combinations of pots. It can be concluded from the results, however, that in comparison to the yield of hemps from the minerally fertilized soil (NPK) (p<0.01), which was a control substrate, where height and weight amounted to 2.14 m and 117 g, respectively, the addition of anaerobic sewage sludge in low dose drastically affected hemp height and weight since the obtained height was equal to 2.61 m, while weight to 177 g. The greatest weight and height of hemp was found for substrates whose soil was enriched by adding sewage sludge and manure at low dose. The addition of straw also increased weight and height of plants in comparison with the addition of sewage sludge at low dose, but not so radically as the addition of manure (p<0.01).

For the high dose of sewage sludge no radical differences in hemp yield were observed; average weight and height were equal to 319.7 g, and 3.16 m, respectively. The addition of manure and straw at this dose did influence neither weight nor height of hemp (p<0.23). These values were comparable with weight and height of plants fertilized with low dose of sewage sludge and manure (p<0.10). It can be explained by sewage sludge added excessively in relation to manurial needs of hemp [12].

The positive reactions to sewage sludge introduction to soil, resulting in the increase of field plants weight and height have been presented in many papers [15, 18]. It can be concluded from the results that adding sewage sludge has become very efficient in the utilization aspect. Data on positive impact of sewage from agricultural-food industry was confirmed by the research of industrial plant-hemp.

The total concentration of heavy metals in anaerobic sewage sludge was estimated as 66.43 mg, 1.4 mg and 6.31 mg per kg d.m. sewage sludge for Pb, Cd, and Cr, respectively. The respective concentration of three heavy metals in soil were 0.51 mg, 0.023 mg and 0.17 mg/kg d.m. soil. The content of the concerned metals in soil was low and corresponded to soils of rustic lands [4]. The total contents of heavy metals in organic manure used in the experiment were: Pb - 1.03 mg, Cd - 0.46 mg and Cr - 1.17 mg/kg d.m. manure. In the wheat straw concentration of analyzed heavy metals was: Pb - 0.02 mg, Cd - 0.08 mg and Cr - 0.019 mg/kg d.m. straw.

At the beginning and in the end of the pot experiment the heavy metals in soil-sludge substratum in combination with low sewage sludge dose were determined in order to estimate hemp phytosorption efficiency (Fig. 2). Statistical analysis (a=0.05) revealed dramatic changes for all three analysed heavy metals in substratum, which were: Pb (p<0.001), Cr (p<0.001), Cd (p<0.005). The concentrations of Pb -3.7 mg, Cr -0.52 mg and Cd -0.08 mg/dm³ can be removed from soil-sludge substratum by growing fibrous hemp without reducing its height and weight.

Lead in hemp roots was accumulated at the greatest degree in all combinations. The organic matter addition, particularly sewage sludge, increased lead accumulation in hemp stems. No detectable content of lead was observed in leaves (Fig. 3).

The greatest sorption in case of cadmium and chromium took place by roots and then by leaves of hemps (Figs. 4 and 5).

Neither manure nor straw addition had radical influence on heavy metals accumulation by hemp (p<0.15) (Figs. 3-5).

The greatest concentration of the analysed heavy metals was found in roots (Tab. 2). When particularly high concentrations in soil are observed, heavy metals tend to be cumulated in peripheral parts of plant. With very low heavy metal concentrations in substrates, as it was in the presented experiment, the majority of heavy metals is cumulated in roots.

With greater concentrations of heavy metals in the substratum, they tend to cumulate also in hemp stem, as reported by Bragato et al. [2]. Thus, after several hemp growing seasons, concentration of heavy metals in substratum were reduced to satisfactory levels [4].

CONCLUSIONS

1. The addition of anaerobic sewage sludge in high dose to a pot experiment increased both height and weight of hemp by a factor of one and a half and two to seven, respectively.

2. With very low heavy metals concentrations in the substratum, as in the presented experiment, most of heavy metals are cumulated in roots.

3. The obtained results showed that it is advisable to clean of mesophilic anaerobic sewage sludge of heavy metals by fibrous hemp growing. The concentration of 3.7 mg, 0.08 mg and 0.52 mg/dm³ for Pb, Cd, and Cr, respectively, were removed from soil-sludge substratum by growing fibrous hemp and it did not cause any reduction of hemp height and weight.

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