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Technosols as a novel valorization strategy for an ecological management of dredged marine sediments
Ecological Engineering 67 (2014) 182–189
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Technosols as a novel valorization strategy for an ecological management of dredged marine sediments P. Macía a , C. Fernández-Costas a , E. Rodríguez b , P. Sieiro c , M. Pazos a , M.A. Sanromán a,∗ a
Department of Chemical Engineering, University of Vigo, Campus As Lagoas-Marcosende s/n, E-36310 Vigo, Spain Department of Ecology and Animal Biology, University of Vigo, Campus As Lagoas-Marcosende s/n, E-36310 Vigo, Spain c Centro Tecnológico del Mar-Fundación CETMAR, Eduardo Cabello s/n, E-36208 Vigo, Spain b
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 9 January 2014 Accepted 24 March 2014 Available online 4 May 2014 Keywords: Composting Dredging Marine sediment Phytotoxicity Valorization Technosol
a b s t r a c t An alternative environmental friendly management of marine dredged material is presented via elaboration of a technosol. Technosols are a new group of soils that are strongly influenced by the technical human activity. Futhermore, they could be elaborated from wastes and employed in the subsequent regeneration of degraded or polluted soils. Thereby, these materials are no longer considered as waste and a value-added product is generated. In this work, preliminary assays have shown that the dredged sediment itself cannot be used as a soil due to its texture, which causes a bad aeration and poor drainage together with crusting phenomena. Consequently, amendments for the dredged sediment using different materials were tested in order to improve physicochemical properties of the manufactured soil. However, these enhancements barely overcome the growing difficulties of the dredged sediment and do not represent a feasible economical option. Thus, a composting process of the dredged material with a waste of high organic content was evaluated in order to obtain suitable properties and composition of technosol. Furthermore, phytotoxicity tests have revealed that the salts content in the studied dredged sediment inhibited the growth of the seeds. Therefore, desalination of the sediments appeared to be mandatory in the elaboration of the technosol. Finally, the technosol obtained after the composting of the desalinated sediment has led to germination indices higher than 320%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dredging operations are performed all around the world in order to keep waterways navigable and to sustain the intense current international trade in goods. Hundreds of millions of cubic meters of sediments are dredged annually, especially in coastal areas (Netzband and Adnitt, 2009). After that, these dredged materials become a waste and, accordingly an environmental concern, they must be disposed following the mandatory regulations. For technical and/or economical reasons, most of the non-contaminated or low-contaminated dredged sediments will be finally dumped into the sea. Nevertheless, ocean floor disposal of these materials have several environmental impacts in the marine ecosystem, namely perturbance of natural habitats, production of
∗ Corresponding author at: Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas-Marcosende, 36310 Vigo, Spain. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M.A. Sanromán). http://dx.doi.org/10.1016/j.ecoleng.2014.03.020 0925-8574/© 2014 Elsevier B.V. All rights reserved.
huge quantities of suspended solids, smothering of benthic organisms and physiological harm to fish (Abam, 2001; Orisakwe et al., 2001; Robinson et al., 2005). In particular cases, according to the mandatory regulations that may concern, these materials can also be disposed into landfills or similar sites. This management does not offer a long-term solution; their location needs a significant large surface of space and causes the destruction of the existing ecosystem and, in addition, represents a future vector of contamination (Regadío et al., 2013). In this framework, development of new alternatives for dredged material disposal is desirable. Guidelines for the management of dredged sediments of international conventions for the protection of the marine environment agree on the need of assessing potential productive uses for the utilization of the mentioned materials. However, there is not a specific regulation for such purposes. Valorization is also hampered by the organic and inorganic pollution that could be present in dredged materials due to industrial and anthropogenic origin. Spanish guidelines offer different alternatives for dredged sediments and they are mainly based on their use as filling material for the creation of beaches, reclaimed
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land, defense items, harbor works, etc. and as provision of aggregates for the construction industry (CEDEX, 1994). The scientific community is also trying to provide technological solutions to change this environmental concern into an economically and environmentally sustainable alternative (Pazos et al., 2013). In fact, there are an important number of attempts to obtain added-value products which could offer an economic profit. One of the most studied productive uses focuses on the preparation of materials for the construction. It has been reported their employment in the production of cementitious materials (Agostini et al., 2007), pavement base materials (Dubois et al., 2009) and bricks (Zoubeir et al., 2007). Recently, an interesting beneficial use of dredged materials as manufactured soils has been reported by Sheehan et al. (2010), who mixed these materials with an organic amelioration. Other researchers combine stabilization treatment and composting to amend heavy metals polluted sediments and to obtain fertilizers (Guangwei et al., 2009). In fact, it has been reported that the amendment of soil leads to an improvement in the physical properties of the soil as well as nutrient availability to the plants (Aggelides and Londra, 2000; Middleton and Jiang, 2013). Recently, technosols have emerged as a new reference soil group and it was included by the International Union of Soil Science (IUSS) in the World Reference Base for Soil Resources in the 2006 edition. This term makes reference to soils whose properties and pedogenesis are dominated by their technical origin. This fact is reflected by the presence of artifacts (materials created or substantially modified by humans, e.g. bricks, pottery, glass, industrial waste, garbage and mine spoil) or because they are sealed by technic hard rock (material created by humans, e.g. pavement). In summary, it is a soil dominated or strongly influenced by human-made materials and more frequently found in urban and industrial areas (IUSS, 2006). Despite its novelty as a reference soil group, technosols have already arisen interest in some research fields and the number of published articles has increased during the last five years (Baumgarten et al., 2013; Huot et al., 2013; Jangorzo et al., 2013; Krzaklewski et al., 2012; Novo et al., 2013; Wanat et al., 2013). Technosols can be elaborated intentionally from wastes, allowing in this context, an ecological management of the wastes. For that purpose, liming, adsorbent, reducing and fertilizer capabilities are desirable in the wastes employed. As well, it is recommended a composting process. Composting is an ex situ solid-phase bioremediation treatment which leads to a biological stabilization of the organic fraction and occurs in aerobic conditions, accomplished principally by microorganisms. The product obtained after this treatment is a stabilized and mature organic matter, rich in humic acids, which are the main and one of the most stable components of the organic matter and contribute to the essential functions of soils (Campitelli and Ceppi, 2008). Furthermore, composting enhances the function of soil as sink for organic carbon (Aira and Domínguez, 2008) and it is also a bioremediation strategy suitable for dredged sediments as several studies have shown (Beolchini et al., 2011; Colacicco et al., 2010). Technosols made from wastes need to reach some quality and safety criteria and, afterwards, they could be used to amend ˜ degraded or polluted areas (Camps Arbestain et al., 2008; Fandino et al., 2010; Punshon et al., 2002; Yao et al., 2009). Once applied, technosols should allow the rapid integration of the residual anthropogenic components into the biogeochemical cycles. For that purpose, a deep knowledge of the starting materials is required in order to obtain an environmentally sound mixture and to constitute a final product which fulfills the main soil functions. Thus, phytotoxicity (delay on seed germination, growth inhibition or other adverse effect on plants caused by phytotoxic substances or cultivation conditions) should be evaluated.
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In this work, technosols are presented as a valorization strategy for wastes such as dredged materials and sewage sludge from an alimentary industry. In this manner, technosols may be a novel and prospective option for the utilization of wastes and, at the same time, for the restoration of degraded areas avoiding the use of natural soils. First, the direct use as a soil of the dredged material, itself or amended, was evaluated. Secondly, a biotechnological approach was developed through the elaboration of a technosol after a composting process. Due to the absence of mandatory regulations for the elaboration and application of technosols, this work was evaluated according to the local recommendations given by the ITR/01/08 (Xunta de Galicia, 2008). 2. Materials and methods 2.1. Materials 2.1.1. Dredged sediment The marine sediment was collected from the maintenance dredging operations of a harbor located in the Northwest of Spain. It is an area of high maritime and industrial activity and, therefore, the presence of pollutants (as trace metals and organic compounds) should be a factor to evaluate. 2.1.2. Sewage sludge and wood chips A waste from poultry products manufacturing industry was chosen as the organic carbon supply to the composting mixture. In particular, it was sewage sludge from the water treatment plant of the industry itself. Table 1 shows its high organic carbon content (49%). Furthermore, this sludge has passed the acceptance criteria to elaborate technosols (Table 1). As bulking agent, wood chips were used. They came from pruning plant debris and forest cleaning and were kindly supplied by Compost Galicia S.A. A small particle size (0.5–1 cm) was chosen in order to obtain a suitable size particle and porosity in the composting mixture. The bulking agent is required to confer to the composting mixture optimal physical characteristics, so that the correct development of biological activity is allowed. 2.1.3. Amendments Different substrates were used to amend the dredged sediment: compost, peat moss, perlite and wood chips. Compost was supplied by Compost Galicia S.A. The employed peat moss was provided by Shangai S.A. Perlite was obtained from ADOA S.L. and wood chips as mentioned in Section 2.1.2. 2.2. Composting The composting trials were performed according to Pérez et al. (2008) in an aerated static bioreactors integrated system that operated with an air flow of 5 l/min. A maximum temperature of 60 ◦ C was reached. The composting mixture was made up of dredged sediment: sewage sludge in a ratio of 1.7:1 (in fresh weight). This ratio was chosen to provide twice the organic content of the dredged material to the final mixture. As well, the ratio of wood chips: composting mixture of 2.5:1 (volume:volume) was added. After 25 days of composting process, the final product was sieved through a 5mm sieve. In order to remove the dusty fraction, then, it was sieved again by 2 mm mesh. 2.3. Phytotoxicity assays Phytotoxicity tests were based on the standard method OECD (2000) and were developed cultivating directly over the samples.
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Table 1 Chemical characterization of the materials used in the composting process. Parameter
Dredged sediment
Sewage sludge
Wood chips
Acceptance criteriaa
pH Conductivity (dS/m) Elemental analysis Total nitrogen (%) Total carbon (%) Ratio C/N Organic matter (%) Nutrient analysis (mg/kg) NO3 − PO4 3− SO4 2− Cl− SO3 2− CN− Trace metals (mg/kg) As Cd Cr Cu Ni Pb Zn Hg Organic pollutants (mg/kg) Total hydrocarbons PCBsc
7.8 24.3
6.2 1.0
5.98 0.13
– –
0.5 7.1 14.2 8.7
5.1 55.4 10.9 84.5
0.5 46.5 87.7 51.7
– – – –
22.1 <0.5 6,813 43,854 8.9 n.d.b
n.d.b n.d.b 1,020 n.d.b 45.3 0.14
n.d.b n.d.b 527.8 n.d.b 14.9 <0.02
– – – – – –
35.8 <1.2 33.4 52.3 19.3 43.9 135.2 1.1
13.2 <1.2 7.3 33.4 12.3 <3.7 174.8 0.03
0.8 0.3 5.3 9.5 4.9 <3.7 29.6 <0.01
300 20 1,000 1,000 300 750 2,500 16
205.8 <0.02
2,400 <0.02
1,873.7 <0.02
a b c
– –
Values according to local regulations (Xunta de Galicia, 2008). No data. PCBs congeners number 28, 52, 101, 118, 138, 153 and 180.
There is a large number of species that could be applied in the assessment of phytotoxicity. In this work, two different species were tested in order to lead to the best results due to differences in sensitivity to the different tested materials and toxins. The dicot Lepidium sativum and the monocot Lolium perenne were chosen for the growth trials due to their fast growing, slight resistance to salinity and resistance to the fungal attack. Lepidium sativum (cress) seeds were provided by Vilmorin S.A. and Lolium perenne (ray grass) seeds were supplied by Fitó S.A. The peat moss described in Section 2.1.3 was employed as a control sample in the growth trials. As it is just a physical support of plant growth; therefore, it allows establishing a reference of the germination and growth. A minimum germination of 80% in control samples was required to consider the test valid. The seedbeds used in the phytotoxicity assays had no holes in their bottom to avoid leaching of the contaminants and to evaluate the real toxicity of the soil samples. Every seedbed was filled with 50 g of fresh material. The number of Lepidium sativum seeds per seedbed was 10 and the cultivation period was 7 days. For Lolium perenne, 0.3 g (around 180 seeds) were sown per seedbed and cultivated for 10 days, as they germinate later. All tests were performed in triplicate. Germination and growth were carried out in an environmental control chamber (SANYO Versatile Environmental Test Chamber) at 25 ± 2 ◦ C and at a relative humidity of a 60 ± 5%. They were kept in darkness for the first 48 h, then, a photoperiod of 16:8 light/dark (4000 lx of light) was established. Once a day, seedbeds were watered with distilled water, depending on irrigation requirements of the materials.
delays in germination over the control were quantified in days. Germination and growth of the test plants were studied through a methodology based on the germination index (GI) proposed by Zucconi et al. (1985) according to Eq. (1): GI (%) = [(SG · SL)/(CG · CL)] × 100
(1)
where SG and CG are sample and control germination percentage, respectively; and SL and CL are lengths of the roots (cm) of the sample and control, respectively. This modified methodology was chosen because it allows evaluating the presence of phytotoxic substances together with physical fertility of the technosol. Due to the presence of artifacts in technosols, the evaluation of a good drainage, humectation and texture of the material is crucial. Thus, the length of the seedlings gives information about these problems which, otherwise, would pass unperceived if only the number of germinated seeds is taken into account. 2.5. Analytical procedures Both initial wastes and composting final product were analyzed in order to fulfill the regulatory compliance and also to explain the changes undergone by the composting process. Except for the granulometric assays, which were made with the raw material, the analyses were made with the material dried at 40 ◦ C in an oven and milled with a mortar and pestle. All samples were stored in a cool dry place.
2.4. Phytotoxicity evaluation
2.5.1. Granulometric analysis Granulometric analysis of the dredged material was performed with a digital electromagnetic sieve shaker FTL-0150 from Filtra Vibracion S.L.
After the cultivation period, plants were removed and germination and growth parameters of the seedlings were examined (number of germinated seeds and length of the roots). Furthermore,
2.5.2. pH and conductivity pH was determined agitating 25 ml of ultrapure water with 5 ml of the sample for 5 min. After that, suspensions were left to decant
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for 2 h and pH was measured into the supernatant using a Jenway 3520 pH-meter (UNE 77305:1999). Conductivity determination was accomplished adding 100 ml of water to 20 g of the sample and shaking them for 30 min. After that, suspensions were filtered and conductivity was measured into the liquid fraction with a conductivity meter Crison Basic 30 (UNE 77308:2001). 2.5.3. Trace metals Trace metals were quantified with an Inductively Coupled Plasma-Optical Spectrometer Perkin Elmer Optima 4300 DV for As, Cd, Zn, Cr, Cu, Pb, and Ni and a Cold Vapor-Atomic Absorption Spectrometer Perkin Elmer FIMS 400 for Hg. Samples were prepared in triplicate, digesting 0.5 g of sample with 10 ml of HNO3 7 M and diluting to a final volume of 50 ml with ultrapure water. 2.5.4. Nutrient and elemental analysis Nutrient analysis was developed with an ionic chromatography equipment Metrohm (733 IC separation kit, 709 IC pump and 732 IC detector). Samples were prepared to shake 5 g of the test material with 25 ml of ultrapure water for 2 h. Finally, they were filtered and stored at 4 ◦ C. Elemental analysis was accomplished with an elemental analyzer LECO CN-2000 to quantify total carbon, organic matter and total nitrogen (Wright and Bailey, 2001). All combustions were made in triplicate. A traditional factor of 1.724 was used to convert organic carbon to organic matter that permits an approximation of the organic matter of this surface soil (Nelson and Sommers, 1996).
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dredged material, therefore this sediment is classified as severely saline). Elemental and nutrient analyses show that this marine sediment has a content of organic matter of 8.7%, a value higher than the expected for marine sediments. In fact, it is considered as a pollutant load and has an anthropogenic origin. Moreover, remains of shells and marine organisms made of CaCO3 provide buffering capacity. Regarding other pollutants such as trace metals and organic compounds (Table 1), the dredged material has proved to fulfill the acceptance criteria to develop a technosol according to the ITR/01/08 (Xunta de Galicia, 2008). Throughout this work, different manufactured soils were evaluated until the attainment of a technosol suitable for the restoration of degraded areas. 3.1. Evaluation of the dredged sediment A first attempt of employing the dredged material as a soil was evaluated. Phytotoxicity tests revealed that none of the studied species was able to germinate. This result could be attributed to the high salinity of the dredged material (Table 1). It is well-known that the salinity is an increasingly important process of land degradation (Wichern et al., 2006) and causes adverse effects both preventing germination and reducing growth. Consequently, the starting point in the technosol elaboration was the desalination of the dredged sediment. 3.2. Desalination of the dredged material and amendments
2.5.5. Organic pollutants Total hydrocarbons concentration was determined with an oil analyzer Horiba OCMA 310. Polychlorinated biphenyls (PCBs) were determined by gas chromatography–mass spectrometry (Finnigan Trace GC Ultra). Samples were prepared by means of accelerated solvent extraction with a DIONEX ASE 200 employing an extraction solution of hexane:acetone (1:1). As well, samples were cleaned up with a sequential extraction of dimethyl sulfoxide and hexane. Finally, they were resuspended with hexane prior to analysis. 2.6. Leaching assays In triplicate, samples were suspended in ultrapure water in a liquid/solid ratio of 10 and shaken for 24 h. Finally, they were filtered and stored at 4 ◦ C until the pollutant content was evaluated following the method described before (Sections 2.5.3 and 2.5.4). 3. Results and discussion Characterization of the dredged sediment is crucial and will provide the clues to identify causes of phytotoxicity and viability of the material for its valorization. Granulometric analysis of the dredged sediment has shown the predominance of the clay and silt fraction (84.3% < 0.06 mm), whereas sands and gravels are in a lower proportion. Chemical characterization is reflected in Table 1. The sediment has a pH of 7.8 which corresponds to a slightly alkaline material and it is suitable for the soil. The salinity of the soil can be estimated roughly from an electrical conductivity (EC) measurement of the saturation extract. Values of EC less than 2 dS/m show negligible salinity effects; values between 2 and 4 dS/m imply that yields of very sensitive crops may be limited; values among 4 and 8 dS/m are related with yields of many crops restricted; values between 8 and 16 dS/m allow the growth of only tolerant crops; and finally, values over 16 dS/m permit only a few very tolerant crops to develop satisfactorily (USDA, 1954). A conductivity of 24.3 dS/m was found in the
Bedell et al. (2013) and Sheehan et al. (2010) showed a decrease of the toxicity of marine sediments after percolation with water. For this reason, desalination at laboratory scale was carried out with the dredged material in order to assess the impact of salinity from the dredged material. Dredged material was mixed with tap water in a 50 l container in a ratio solid:liquid of 1:10 (volume:volume). After 10 min of agitation, the suspension was left to decant for 24 h. Finally, the supernatant was removed, and the solid fraction was dried at room temperature in a fume hood until a moisture content of around 60% was obtained. Salinity reduction was assessed and a conductivity of 2.8 dS/m was reached after desalination, revealing the efficiency of the treatment. Desalinated dredged material was tested, in terms of phytotoxicity, itself and amended with different substrates (5% commercial compost, 5% peat moss, 5% perlite, 5% wood chips, and mixtures of 5% commercial compost + 5% peat moss, 5% peat moss + 5% perlite and 5% commercial compost + 5% perlite). These percentages are weight-based and grounded on the results obtained by Sheehan et al. (2010), who produced a manufactured topsoil with an organic amelioration around 5%. Furthermore, higher percentages would not have an economic viability. Commercial compost was used, mainly, for organic amelioration; while perlite, peat moss and wood chips were employed to evaluate a physical improvement of the properties of the dredged material such as aeration and drainage. Phytotoxicity tests revealed a slight improvement with respect to non-desalinated sediment. Nevertheless, in all cases, germination index did not exceed a 10% and germination suffered from a 2-day delay with respect to the control, showing a high phytotoxicity. Consequently, the amendments were not effective to improve the characteristics of the manufactured soil. Difficulties in plant growth can be easily explained in Fig. 1, as it is a perfect reflection of the problems found in the amended sediment. Predominance of the fine fraction of the dredged
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Fig. 3. Germination index of Lepidium sativum and Lolium perenne grown on the different evaluated materials.
through a composting was developed to obtain a stabilized product with enhanced properties.
3.3. Composting process
Fig. 1. Crusting phenomena and harmful rooting of Lepidium sativum grown on an amendment of the desalinated sediment with perlite.
sediment prevents from an optimal wetting and rooting, together with crusting phenomena. Even when perlite amended sediment was used due to sealing of the sediment, roots cannot penetrate into the deeper areas and they are forced to crawl through the surface. As a consequence, roots are bad fixed to the manufactured soil and the availability of nutrients and water is limited. On the other hand, fine particles of the dredged material get stacked into the leaves of the plants (crusting phenomena) and difficult their growing, together with a hampering in photosynthesis. As it has been previously reported, crusting conditions resulted in poor germination or emergence (Bresson and Boiffin, 1990). All these results had pointed to particle size distribution and soil compaction as the other major problems to face while elaborating the technosol. In this framework, a biological treatment
Dredged material and sewage sludge from an alimentary industry were composted as described in Section 2.2. After maturation, the texture of the final product is shown in Fig. 2b. Aggregates were established during composting and compactness of the dredged sediment was lost (Fig. 2a). This porous texture made during the composting process is expected to increase permeability of the soil and, also, to benefit its water retention capacity. Furthermore, the composting process did not change the pH of the new developed soil and salinity was reduced due to the dilution caused by the mixture with the sewage sludge. The composted sediment had a pH of 7.4 and a conductivity of 8.9 dS/m. As it can be classified as moderately saline, the composted sediment was subjected to phytotoxicity tests. The germination indexes obtained for Lepidium sativum and Lolium perenne were 11.0% and 16.4%, respectively. Furthermore, a 2-day delay in germination was observed with respect to the control. These two facts reveal that the high phytotoxicity still remains. Although the physical properties of the manufactured soil were improved, phytotoxicity could be attributed to the salinity content. In order to check this hypothesis, the composted sediment was desalinated.
Fig. 2. Physical appearance of (a) dredged sediment and (b) composted product.
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Table 2 Chemical characterization of the proposed technosol. Parameter pH Conductivity (dS/m) Nutrient analysis Total nitrogen (%) Total carbon (%) Ratio C/N Organic matter (%) NO3 − (mg/kg) PO4 3− (mg/kg) SO4 2− (mg/kg) Cl− (mg/kg) SO3 2− (mg/kg) Trace metals (mg/kg) As Cd Cr Cu Ni Pb Zn Hg a
Technosol 7.4 2.37 1.4 13.4 9.3 18.8 80.9 <0.5 5,629 1,893 15.4 10.4 <0.5 26.2 49.4 16.6 37.4 179.2 0.6
Acceptance criteriaa – – – – 12 4 – – – – – 100 0.5 150 200 140 100 250 1
Values according to local regulations (Xunta de Galicia, 2008).
Table 3 Analysis of the leachates from the technosol. Fig. 4. Differences in the growth on the technosol and on the control of (a) Lolium perenne and (b) Lepidium sativum.
3.4. Desalination of the composted sediment Composted sediment was desalinated as described previously in Section 3.2. Phytotoxicity tests developed with the desalinated composted sediment have achieved a 69% of germination index for both plant species (Fig. 3), so this material is considered moderately phytotoxic. This fact might be revealing that the salinity was responsible of the toxicity and also that the texture provided by the composting process was appropriate. Nevertheless, the desalination process of the composted material reduced the porous net, changing partially the texture of the material. Thus, composting should be undertaken with the desalinated sediment in order to increase the manufactured soil properties. 3.5. Composting of the desalinated sediment Desalination of the dredged sediment was carried out as described in the previous sections and after that the composting process was accomplished. Figs. 3 and 4 show the results of phytotoxicity texts of the obtained manufactured soil. As can be seen in these figures, favorable results were obtained for the technosol in the growth trials. Germination index (GI) for both Lepidium sativum and Lolium perenne exceeds the results obtained in the control, being 322% and 150%, respectively. This high GI can be explained taking into account that the manufactured technosol has more matured stable organic matter and nutrients than peat moss control sample. GI higher than 100% are related to phyto-stimulant and phytonutrient capability, and indicate the positive influence of some compounds present in the sample (Moldes et al., 2006). Furthermore, there were not delays in germination with respect to the control. Moreover, Lolium perenne was more sensitive than Lepidium sativum in all the assays (Fig. 3). This fact is in agreement with other researchers (Bedell et al., 2013) and has no relation with the cultivation substrate. A detailed characterization of the obtained manufacture soil was performed in order to evaluate if it overcomes the local
Parameter (mg/kg) As Cd Cr Cu Ni Pb Zn Hg Ba Mo Sb Se F− SO4 2− NO3 − a
Technosol 1.4 0.05 <0.1 1.1 0.4 0.3 1.1 0.01 0.14 1.6 <0.4 0.5 <1 5,690 3.2
Acceptance criteriaa 2 1 10 50 10 10 50 0.2 100 10 0.7 0.5 150 20,000 50
Values according to local regulations (Xunta de Galicia, 2008).
recommendations (Table 2). As can be seen, there were not significant differences in pH (7.4) and conductivity (2.37 dS/m), comparing to the manufactured soil obtained in Section 3.5. With respect to the elemental analysis (Table 2), the technosol has a proper organic matter content, 18.85%, and has complied with the rest of the local regulations. However, the C/N ratio found, 9.3, is slightly lower than the recommended value, 12. This can be attributed to the high protein content of the sewage sludge because of its poultry industry origin. Nevertheless, during the process of composting, the C/N ratio decreases until values close to 10 (Haug, 1993), so, this ratio can be considered suitable. Furthermore, the reduction obtained of total hydrocarbons content (around 40%) has proved that the composting process has also acted as a biological treatment. In addition, leaching assays of trace metals and nutrients were assessed, and they also satisfied the local recommended values as it is showed in Table 3. Leaching assays were made to the feasible technosol in order to assess if it could represent an environmental risk once applied in a real place. In summary, this technosol not only overcomes the local recommendations (Xunta de Galicia, 2008) but even might play the natural soil role. So, the product obtained after both processes,
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Fig. 5. SWOT analysis of the elaboration of technosols from dredged materials.
desalination and composting, represents the technosol proposed in this work. 3.6. SWOT analysis of the technosol obtained An SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis is a methodology employed to assess the internal and external factors that are favorable and unfavorable in a project or a business venture. To criticize our proposal and evaluate the real feasibility of the proposed management system, an SWOT analysis of the technosol elaborated from desalinated dredged materials was performed and summarized in Fig. 5. One of the strengths found in the internal analysis of the technosol is that allows, simultaneously, the valorization of dredged materials and other wastes such as sewage sludge together with the elaboration of a manufactured soil that replaces the use of natural fertile soils in restoration ecology. Recently, the European Union is promoting the development of new waste recovery proposals and, particularly, when the final objective is the treatment of soils. Other strengths are the low cost of the technosol proposed and minimum energy and technological requirements. With respect to the weaknesses found, the main problem lies in the dredging process as it is irregular and, depending on its location, will provide materials of different geochemical nature. Consequently, dredged sediments would have to be characterized, and the formulation of the technosol could be subjected to changes. External analysis has shown other opportunities, specifically the proposal of an alternative to the traditional waste management and the job promoting. The threats exhibit that the main disadvantage of the elaboration of this kind of technosols lies in the social vision of the processing plants. Social rejection
of the construction of a composting plant should occur due to the fact that these plants are associated with bad odors. Nevertheless, odors can be avoided with the proper design of the plant and the implementation of appropriate technology. This fact can be transferred to the population in order to not generate a social alarm. The SWOT analysis reflects that management of dredged sediments through the elaboration of technosols is a viable strategy and is also supported by governments of the area of study. The main drawback is the irregular delivery of dredged materials, though this could be solved designing a composting plant which elaborates both technosols from dredged marine sediments and other products. 4. Conclusions In this work, different strategies to manufacture a technosol from dredged material were evaluated. In this valorization process, the main problems were generated due to the high salinity and predominance of the fine particle size of the dredged sediments. The first problem has been solved with a desalination process with water washing. For the second, it has been assessed a composting processed in order to improve the texture by means of the formation of a porous matrix. The final product is in compliance with the local regulations besides being a stabilized organic material. In this manner, recycling of dredged materials together with other wastes was achieved and the produced technosol could be used in restoration ecology. In summary, in this study the viability of the ecological management of marine dredged sediments through the elaboration of technosols has been proved.
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Acknowledgements This research was funded by Spanish Ministry of Economy and Competitiveness and ERDF Funds (Project IPT-310000-2010-17) and for financial support of Marta Pazos under the Ramón y Cajal Program (RYC-2010-05756). References Abam, T.K.S., 2001. Regional hydrological research perspectives in the Niger Delta. Hydrol. Sci. J. 46, 13–25. Agostini, F., Skoczylas, F., Lafhaj, Z., 2007. About a possible valorisation in cementitious materials of polluted sediments after treatment. Cement Concr. Comp. 29, 270–278. Aggelides, S.M., Londra, P.A., 2000. Effects of compost produced from town wastes and sewage sludge on the physical properties of a loamy and a clay soil. Bioresour. Technol. 71 (3), 253–259. Aira, M., Domínguez, J., 2008. Optimizing vermicomposting of animal wastes: effects of rate of manure application on carbon loss and microbial stabilization. J. Environ. Manage. 88, 1525–1529. Baumgarten, W., Fleige, H., Peth, S., Horn, R., 2013. (Bio-)remediation of VCHC contaminants in a technosol under unsaturated conditions. Biodegradation 24, 455–469. Bedell, J.P., Bazin, C., Sarrazin, B., Perrodin, Y., 2013. Assessment of the phytotoxicity of seaport sediments in the framework of a quarry-deposit scenario: germination tests of sediments aged artificially by column leaching. Arch. Environ. Contam. Toxicol. 65, 1–13. Beolchini, F., Fonti, V., Dell’Anno, A., Vegliòa, F., 2011. Bio-mobilization of heavy metals from contaminated sediments by acidophilic microbial consortia. Chem. Eng. Trans. 24, 883–888. Bresson, L.M., Boiffin, J., 1990. Morphological characterization of soil crust development stages on an experimental field. Geoderma 47, 301–325. Campitelli, P., Ceppi, S., 2008. Effects of composting technologies on the chemical and physicochemical properties of humic acids. Geoderma 144, 325–333. Camps Arbestain, M., Madinabeitia, Z., Anza Hortalà, M., Macías-García, F., Virgel, S., Macías, F., 2008. Extractability and leachability of heavy metals in technosols prepared from mixtures of unconsolidated wastes. Waste Manage. 28, 2653–2666. CEDEX, Centro de Estudios y Experimentación de Obras Públicas, 1994. Recomenda˜ Puertos ciones para la gestión del material de dragado en los puertos Espanoles. del Estado, Madrid, Spain. Colacicco, A., De Gioannis, G., Muntoni, A., Pettinao, E., Polettini, A., Pomi, R., 2010. Enhanced electrokinetic treatment of marine sediments contaminated by heavy metals and PAHs. Chemosphere 81, 46–56. Dubois, V., Abriak, N.E., Zentar, R., Ballivy, G., 2009. The use of marine sediments as a pavement base material. Waste Manage. 29, 774–782. ˜ V.A., Andrade Couce, M.L., Vega, F.A., Covelo, E.F., 2010. Characterization of Fandino, different age technosols developed on a copper mine tailing. Fresenius Environ. Bull. 19, 1687–1693. Guangwei, Y., Hengyi, L., Tao, B., Zhong, L., Qiang, Y., Xianqiang, S., 2009. In-situ stabilization followed by ex-situ composting for treatment and disposal of heavy metals polluted sediments. J. Environ. Sci. 21, 877–883. Haug, R.T., 1993. The Practical Handbook of Compost Engineering. Lewis Publishers, Boca Raton, FL. Huot, H., Simonnot, M., Marion, P., Yvon, J., De Donato, P., Morel, J., 2013. Characteristics and potential pedogenetic processes of a technosol developing on iron industry deposits. J. Soil Sediment. 13, 555–568. IUSS, 2006. Working Group WRB. World Reference Base for Soil Resources 2006. World Soil Resources Reports 103, FAO, Rome, Italy. Jangorzo, N.S., Watteau, F., Schwartz, C., 2013. Evolution of the pore structure of constructed technosols during early pedogenesis quantified by image analysis. Geoderma 207–208, 180–192.
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Krzaklewski, W., Pietrzykowski, M., Wo´s, B., 2012. Survival and growth of alders (Alnus glutinosa (L.) Gaertn. and Alnus incana (L.) Moench) on fly ash technosols at different substrate improvement. Ecol. Eng. 49, 35–40. Middleton, B.A., Jiang, M., 2013. Use of sediment amendments to rehabilitate sinking coastal swamp forests in Louisiana. Ecol. Eng. 54, 183–191. Moldes, A., Vázquez, M., Domínguez, J., Díaz-Fierros, F., Barral, T., 2006. Evaluation of mesophilic biodegraded grape marc as soil fertilizer. Appl. Biochem. Biotechnol. 142, 27–36. Nelson, D.W., Sommers, L.E., 1996. Total carbon, organic carbon, and organic matter. In: Page, A.L., et al. (Eds.), Methods of Soil Analysis, Part 2. Agronomy, vol. 9, 2nd ed. Am. Soc. Agron., Inc., Madison, WI, pp. 961–1010. Netzband, A., Adnitt, C., 2009. Dredging management practices for the environment: a structured selection approach. Terra Aqua J. 114, 3–7. Novo, L.A.B., Covelo, E.F., González, L., 2013. The use of waste-derived amendments to promote the growth of Indian mustard in copper mine tailings. Miner. Eng. 53, 24–30. OECD (Organization for Economic Co-operation and Development), 2000. Guideline for the Testing of Chemicals: Proposal for Updating Guideline 208. Orisakwe, O.E., Asomugha, R., Obi, E., Afonne, O.J., Dioka, C.E., Akumka, D., Ilondu, N.A., 2001. Ecotoxicological study of the Niger-Delta area of the River Niger. Bull. Environ. Contam. Toxicol. 66, 548–552. Pazos, M., Iglesias, O., Gómez, J., Rosales, E., Sanromán, M.A., 2013. Remediation of contaminated marine sediment using electrokinetic-Fenton technology. J. Ind. Eng. Chem. 19, 932–937. Pérez, D., Villar, I., Mato, S., Armesto, I., García, J.C., 2008. Design and construction of a replicated composting system at laboratory scale: technical characteristics and first homogeneity test. In: From ORBIT 2008 (Proceeding). Copyright: European Compost Network ECN e.V. Punshon, T., Adriano, D.C., Weber, J.T., 2002. Restoration of drastically eroded land using coal fly ash and poultry biosolid. Sci. Total Environ. 296, 209–225. Regadío, M., de Soto, I.S., Rodríguez-Rastrero, M., Ruiz, A.I., Gismera, M.J., Cuevas, J., 2013. Processes and impacts of acid discharges on a natural substratum under a landfill. Sci. Total Environ. 463–464, 1049–1059. Robinson, J.E., Newell, R.C., Seiderer, L.J., Simpson, N.M., 2005. Impacts of aggregate dredging on sediment composition and associated benthic fauna at an offshore dredge site in the southern North Sea. Mar. Environ. Res. 60, 51–68. Sheehan, C., Harrington, J., Murphy, J.D., 2010. A technical assessment of topsoil production from dredged material. Resour. Conserv. Recycl. 54, 1377–1385. UNE 77305:1999. Soil quality. Determination of pH. UNE 77308:2001. Soil quality. Determination of the specific electrical conductivity. USDA (United States Department of Agriculture), 1954. Diagnosis and improvement of saline and alkali soils. In: Richards, L.A. (Ed.), Washington, DC. Wanat, N., Austruy, A., Joussein, E., Soubrand, M., Hitmi, A., Gauthier-Moussard, C., Lenain, J., Vernay, P., Munch, J.C., Pichon, M., 2013. Potentials of Miscanthus giganteus grown on highly contaminated technosols. J. Geochem. Explor. 126–127, 78–84. Wright, A.F., Bailey, J.S., 2001. Organic carbon, total carbon, and total nitrogen determinations in soils of variable calcium carbonate contents using a LECO CN-2000 dry combustion analyzer. Commun. Soil Sci. Plan. 32, 3243–3258. Wichern, J., Wichern, F., Joergensen, R.G., 2006. Impact of salinity on soil microbial communities and the decomposition of maize in acidic soils. Geoderma 137, 100–108. Xunta de Galicia, 2008. Instrucción Técnica de Residuos ITR/01/08. Diario Oficial de Galicia. Yao, F.X., Macías, F., Santesteban, A., Virgel, S., Blanco, F., Jiang, X., Camps Arbestain, M., 2009. Influence of the acid buffering capacity of different types of technosols on the chemistry of their leachates. Chemosphere 74, 250–258. Zoubeir, L., Adeline, S., Laurent, C.S., Yoann, C., Truc, H.T., Benoît, L.G., Federico, A., 2007. The use of the Novosol process for the treatment of polluted marine sediment. J. Hazard. Mater. 148, 606–612. Zucconi, F., Monaco, A., Forte, M., De Bertoli, M., 1985. Phytotoxins during the stabilization of organic matter. In: Gasser, J.K.R (Ed.), Composting of Agricultural and Other Wastes. Elsevier, London, UK, pp. 73–80.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Technosols as a novel valorization strategy for an ecological management of dredged marine sediments P. Macía a , C. Fernández-Costas a , E. Rodríguez b , P. Sieiro c , M. Pazos a , M.A. Sanromán a,∗ a
Department of Chemical Engineering, University of Vigo, Campus As Lagoas-Marcosende s/n, E-36310 Vigo, Spain Department of Ecology and Animal Biology, University of Vigo, Campus As Lagoas-Marcosende s/n, E-36310 Vigo, Spain c Centro Tecnológico del Mar-Fundación CETMAR, Eduardo Cabello s/n, E-36208 Vigo, Spain b
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 9 January 2014 Accepted 24 March 2014 Available online 4 May 2014 Keywords: Composting Dredging Marine sediment Phytotoxicity Valorization Technosol
a b s t r a c t An alternative environmental friendly management of marine dredged material is presented via elaboration of a technosol. Technosols are a new group of soils that are strongly influenced by the technical human activity. Futhermore, they could be elaborated from wastes and employed in the subsequent regeneration of degraded or polluted soils. Thereby, these materials are no longer considered as waste and a value-added product is generated. In this work, preliminary assays have shown that the dredged sediment itself cannot be used as a soil due to its texture, which causes a bad aeration and poor drainage together with crusting phenomena. Consequently, amendments for the dredged sediment using different materials were tested in order to improve physicochemical properties of the manufactured soil. However, these enhancements barely overcome the growing difficulties of the dredged sediment and do not represent a feasible economical option. Thus, a composting process of the dredged material with a waste of high organic content was evaluated in order to obtain suitable properties and composition of technosol. Furthermore, phytotoxicity tests have revealed that the salts content in the studied dredged sediment inhibited the growth of the seeds. Therefore, desalination of the sediments appeared to be mandatory in the elaboration of the technosol. Finally, the technosol obtained after the composting of the desalinated sediment has led to germination indices higher than 320%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dredging operations are performed all around the world in order to keep waterways navigable and to sustain the intense current international trade in goods. Hundreds of millions of cubic meters of sediments are dredged annually, especially in coastal areas (Netzband and Adnitt, 2009). After that, these dredged materials become a waste and, accordingly an environmental concern, they must be disposed following the mandatory regulations. For technical and/or economical reasons, most of the non-contaminated or low-contaminated dredged sediments will be finally dumped into the sea. Nevertheless, ocean floor disposal of these materials have several environmental impacts in the marine ecosystem, namely perturbance of natural habitats, production of
∗ Corresponding author at: Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas-Marcosende, 36310 Vigo, Spain. Tel.: +34 986 812383; fax: +34 986 812380. E-mail address: [email protected] (M.A. Sanromán). http://dx.doi.org/10.1016/j.ecoleng.2014.03.020 0925-8574/© 2014 Elsevier B.V. All rights reserved.
huge quantities of suspended solids, smothering of benthic organisms and physiological harm to fish (Abam, 2001; Orisakwe et al., 2001; Robinson et al., 2005). In particular cases, according to the mandatory regulations that may concern, these materials can also be disposed into landfills or similar sites. This management does not offer a long-term solution; their location needs a significant large surface of space and causes the destruction of the existing ecosystem and, in addition, represents a future vector of contamination (Regadío et al., 2013). In this framework, development of new alternatives for dredged material disposal is desirable. Guidelines for the management of dredged sediments of international conventions for the protection of the marine environment agree on the need of assessing potential productive uses for the utilization of the mentioned materials. However, there is not a specific regulation for such purposes. Valorization is also hampered by the organic and inorganic pollution that could be present in dredged materials due to industrial and anthropogenic origin. Spanish guidelines offer different alternatives for dredged sediments and they are mainly based on their use as filling material for the creation of beaches, reclaimed
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land, defense items, harbor works, etc. and as provision of aggregates for the construction industry (CEDEX, 1994). The scientific community is also trying to provide technological solutions to change this environmental concern into an economically and environmentally sustainable alternative (Pazos et al., 2013). In fact, there are an important number of attempts to obtain added-value products which could offer an economic profit. One of the most studied productive uses focuses on the preparation of materials for the construction. It has been reported their employment in the production of cementitious materials (Agostini et al., 2007), pavement base materials (Dubois et al., 2009) and bricks (Zoubeir et al., 2007). Recently, an interesting beneficial use of dredged materials as manufactured soils has been reported by Sheehan et al. (2010), who mixed these materials with an organic amelioration. Other researchers combine stabilization treatment and composting to amend heavy metals polluted sediments and to obtain fertilizers (Guangwei et al., 2009). In fact, it has been reported that the amendment of soil leads to an improvement in the physical properties of the soil as well as nutrient availability to the plants (Aggelides and Londra, 2000; Middleton and Jiang, 2013). Recently, technosols have emerged as a new reference soil group and it was included by the International Union of Soil Science (IUSS) in the World Reference Base for Soil Resources in the 2006 edition. This term makes reference to soils whose properties and pedogenesis are dominated by their technical origin. This fact is reflected by the presence of artifacts (materials created or substantially modified by humans, e.g. bricks, pottery, glass, industrial waste, garbage and mine spoil) or because they are sealed by technic hard rock (material created by humans, e.g. pavement). In summary, it is a soil dominated or strongly influenced by human-made materials and more frequently found in urban and industrial areas (IUSS, 2006). Despite its novelty as a reference soil group, technosols have already arisen interest in some research fields and the number of published articles has increased during the last five years (Baumgarten et al., 2013; Huot et al., 2013; Jangorzo et al., 2013; Krzaklewski et al., 2012; Novo et al., 2013; Wanat et al., 2013). Technosols can be elaborated intentionally from wastes, allowing in this context, an ecological management of the wastes. For that purpose, liming, adsorbent, reducing and fertilizer capabilities are desirable in the wastes employed. As well, it is recommended a composting process. Composting is an ex situ solid-phase bioremediation treatment which leads to a biological stabilization of the organic fraction and occurs in aerobic conditions, accomplished principally by microorganisms. The product obtained after this treatment is a stabilized and mature organic matter, rich in humic acids, which are the main and one of the most stable components of the organic matter and contribute to the essential functions of soils (Campitelli and Ceppi, 2008). Furthermore, composting enhances the function of soil as sink for organic carbon (Aira and Domínguez, 2008) and it is also a bioremediation strategy suitable for dredged sediments as several studies have shown (Beolchini et al., 2011; Colacicco et al., 2010). Technosols made from wastes need to reach some quality and safety criteria and, afterwards, they could be used to amend ˜ degraded or polluted areas (Camps Arbestain et al., 2008; Fandino et al., 2010; Punshon et al., 2002; Yao et al., 2009). Once applied, technosols should allow the rapid integration of the residual anthropogenic components into the biogeochemical cycles. For that purpose, a deep knowledge of the starting materials is required in order to obtain an environmentally sound mixture and to constitute a final product which fulfills the main soil functions. Thus, phytotoxicity (delay on seed germination, growth inhibition or other adverse effect on plants caused by phytotoxic substances or cultivation conditions) should be evaluated.
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In this work, technosols are presented as a valorization strategy for wastes such as dredged materials and sewage sludge from an alimentary industry. In this manner, technosols may be a novel and prospective option for the utilization of wastes and, at the same time, for the restoration of degraded areas avoiding the use of natural soils. First, the direct use as a soil of the dredged material, itself or amended, was evaluated. Secondly, a biotechnological approach was developed through the elaboration of a technosol after a composting process. Due to the absence of mandatory regulations for the elaboration and application of technosols, this work was evaluated according to the local recommendations given by the ITR/01/08 (Xunta de Galicia, 2008). 2. Materials and methods 2.1. Materials 2.1.1. Dredged sediment The marine sediment was collected from the maintenance dredging operations of a harbor located in the Northwest of Spain. It is an area of high maritime and industrial activity and, therefore, the presence of pollutants (as trace metals and organic compounds) should be a factor to evaluate. 2.1.2. Sewage sludge and wood chips A waste from poultry products manufacturing industry was chosen as the organic carbon supply to the composting mixture. In particular, it was sewage sludge from the water treatment plant of the industry itself. Table 1 shows its high organic carbon content (49%). Furthermore, this sludge has passed the acceptance criteria to elaborate technosols (Table 1). As bulking agent, wood chips were used. They came from pruning plant debris and forest cleaning and were kindly supplied by Compost Galicia S.A. A small particle size (0.5–1 cm) was chosen in order to obtain a suitable size particle and porosity in the composting mixture. The bulking agent is required to confer to the composting mixture optimal physical characteristics, so that the correct development of biological activity is allowed. 2.1.3. Amendments Different substrates were used to amend the dredged sediment: compost, peat moss, perlite and wood chips. Compost was supplied by Compost Galicia S.A. The employed peat moss was provided by Shangai S.A. Perlite was obtained from ADOA S.L. and wood chips as mentioned in Section 2.1.2. 2.2. Composting The composting trials were performed according to Pérez et al. (2008) in an aerated static bioreactors integrated system that operated with an air flow of 5 l/min. A maximum temperature of 60 ◦ C was reached. The composting mixture was made up of dredged sediment: sewage sludge in a ratio of 1.7:1 (in fresh weight). This ratio was chosen to provide twice the organic content of the dredged material to the final mixture. As well, the ratio of wood chips: composting mixture of 2.5:1 (volume:volume) was added. After 25 days of composting process, the final product was sieved through a 5mm sieve. In order to remove the dusty fraction, then, it was sieved again by 2 mm mesh. 2.3. Phytotoxicity assays Phytotoxicity tests were based on the standard method OECD (2000) and were developed cultivating directly over the samples.
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Table 1 Chemical characterization of the materials used in the composting process. Parameter
Dredged sediment
Sewage sludge
Wood chips
Acceptance criteriaa
pH Conductivity (dS/m) Elemental analysis Total nitrogen (%) Total carbon (%) Ratio C/N Organic matter (%) Nutrient analysis (mg/kg) NO3 − PO4 3− SO4 2− Cl− SO3 2− CN− Trace metals (mg/kg) As Cd Cr Cu Ni Pb Zn Hg Organic pollutants (mg/kg) Total hydrocarbons PCBsc
7.8 24.3
6.2 1.0
5.98 0.13
– –
0.5 7.1 14.2 8.7
5.1 55.4 10.9 84.5
0.5 46.5 87.7 51.7
– – – –
22.1 <0.5 6,813 43,854 8.9 n.d.b
n.d.b n.d.b 1,020 n.d.b 45.3 0.14
n.d.b n.d.b 527.8 n.d.b 14.9 <0.02
– – – – – –
35.8 <1.2 33.4 52.3 19.3 43.9 135.2 1.1
13.2 <1.2 7.3 33.4 12.3 <3.7 174.8 0.03
0.8 0.3 5.3 9.5 4.9 <3.7 29.6 <0.01
300 20 1,000 1,000 300 750 2,500 16
205.8 <0.02
2,400 <0.02
1,873.7 <0.02
a b c
– –
Values according to local regulations (Xunta de Galicia, 2008). No data. PCBs congeners number 28, 52, 101, 118, 138, 153 and 180.
There is a large number of species that could be applied in the assessment of phytotoxicity. In this work, two different species were tested in order to lead to the best results due to differences in sensitivity to the different tested materials and toxins. The dicot Lepidium sativum and the monocot Lolium perenne were chosen for the growth trials due to their fast growing, slight resistance to salinity and resistance to the fungal attack. Lepidium sativum (cress) seeds were provided by Vilmorin S.A. and Lolium perenne (ray grass) seeds were supplied by Fitó S.A. The peat moss described in Section 2.1.3 was employed as a control sample in the growth trials. As it is just a physical support of plant growth; therefore, it allows establishing a reference of the germination and growth. A minimum germination of 80% in control samples was required to consider the test valid. The seedbeds used in the phytotoxicity assays had no holes in their bottom to avoid leaching of the contaminants and to evaluate the real toxicity of the soil samples. Every seedbed was filled with 50 g of fresh material. The number of Lepidium sativum seeds per seedbed was 10 and the cultivation period was 7 days. For Lolium perenne, 0.3 g (around 180 seeds) were sown per seedbed and cultivated for 10 days, as they germinate later. All tests were performed in triplicate. Germination and growth were carried out in an environmental control chamber (SANYO Versatile Environmental Test Chamber) at 25 ± 2 ◦ C and at a relative humidity of a 60 ± 5%. They were kept in darkness for the first 48 h, then, a photoperiod of 16:8 light/dark (4000 lx of light) was established. Once a day, seedbeds were watered with distilled water, depending on irrigation requirements of the materials.
delays in germination over the control were quantified in days. Germination and growth of the test plants were studied through a methodology based on the germination index (GI) proposed by Zucconi et al. (1985) according to Eq. (1): GI (%) = [(SG · SL)/(CG · CL)] × 100
(1)
where SG and CG are sample and control germination percentage, respectively; and SL and CL are lengths of the roots (cm) of the sample and control, respectively. This modified methodology was chosen because it allows evaluating the presence of phytotoxic substances together with physical fertility of the technosol. Due to the presence of artifacts in technosols, the evaluation of a good drainage, humectation and texture of the material is crucial. Thus, the length of the seedlings gives information about these problems which, otherwise, would pass unperceived if only the number of germinated seeds is taken into account. 2.5. Analytical procedures Both initial wastes and composting final product were analyzed in order to fulfill the regulatory compliance and also to explain the changes undergone by the composting process. Except for the granulometric assays, which were made with the raw material, the analyses were made with the material dried at 40 ◦ C in an oven and milled with a mortar and pestle. All samples were stored in a cool dry place.
2.4. Phytotoxicity evaluation
2.5.1. Granulometric analysis Granulometric analysis of the dredged material was performed with a digital electromagnetic sieve shaker FTL-0150 from Filtra Vibracion S.L.
After the cultivation period, plants were removed and germination and growth parameters of the seedlings were examined (number of germinated seeds and length of the roots). Furthermore,
2.5.2. pH and conductivity pH was determined agitating 25 ml of ultrapure water with 5 ml of the sample for 5 min. After that, suspensions were left to decant
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for 2 h and pH was measured into the supernatant using a Jenway 3520 pH-meter (UNE 77305:1999). Conductivity determination was accomplished adding 100 ml of water to 20 g of the sample and shaking them for 30 min. After that, suspensions were filtered and conductivity was measured into the liquid fraction with a conductivity meter Crison Basic 30 (UNE 77308:2001). 2.5.3. Trace metals Trace metals were quantified with an Inductively Coupled Plasma-Optical Spectrometer Perkin Elmer Optima 4300 DV for As, Cd, Zn, Cr, Cu, Pb, and Ni and a Cold Vapor-Atomic Absorption Spectrometer Perkin Elmer FIMS 400 for Hg. Samples were prepared in triplicate, digesting 0.5 g of sample with 10 ml of HNO3 7 M and diluting to a final volume of 50 ml with ultrapure water. 2.5.4. Nutrient and elemental analysis Nutrient analysis was developed with an ionic chromatography equipment Metrohm (733 IC separation kit, 709 IC pump and 732 IC detector). Samples were prepared to shake 5 g of the test material with 25 ml of ultrapure water for 2 h. Finally, they were filtered and stored at 4 ◦ C. Elemental analysis was accomplished with an elemental analyzer LECO CN-2000 to quantify total carbon, organic matter and total nitrogen (Wright and Bailey, 2001). All combustions were made in triplicate. A traditional factor of 1.724 was used to convert organic carbon to organic matter that permits an approximation of the organic matter of this surface soil (Nelson and Sommers, 1996).
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dredged material, therefore this sediment is classified as severely saline). Elemental and nutrient analyses show that this marine sediment has a content of organic matter of 8.7%, a value higher than the expected for marine sediments. In fact, it is considered as a pollutant load and has an anthropogenic origin. Moreover, remains of shells and marine organisms made of CaCO3 provide buffering capacity. Regarding other pollutants such as trace metals and organic compounds (Table 1), the dredged material has proved to fulfill the acceptance criteria to develop a technosol according to the ITR/01/08 (Xunta de Galicia, 2008). Throughout this work, different manufactured soils were evaluated until the attainment of a technosol suitable for the restoration of degraded areas. 3.1. Evaluation of the dredged sediment A first attempt of employing the dredged material as a soil was evaluated. Phytotoxicity tests revealed that none of the studied species was able to germinate. This result could be attributed to the high salinity of the dredged material (Table 1). It is well-known that the salinity is an increasingly important process of land degradation (Wichern et al., 2006) and causes adverse effects both preventing germination and reducing growth. Consequently, the starting point in the technosol elaboration was the desalination of the dredged sediment. 3.2. Desalination of the dredged material and amendments
2.5.5. Organic pollutants Total hydrocarbons concentration was determined with an oil analyzer Horiba OCMA 310. Polychlorinated biphenyls (PCBs) were determined by gas chromatography–mass spectrometry (Finnigan Trace GC Ultra). Samples were prepared by means of accelerated solvent extraction with a DIONEX ASE 200 employing an extraction solution of hexane:acetone (1:1). As well, samples were cleaned up with a sequential extraction of dimethyl sulfoxide and hexane. Finally, they were resuspended with hexane prior to analysis. 2.6. Leaching assays In triplicate, samples were suspended in ultrapure water in a liquid/solid ratio of 10 and shaken for 24 h. Finally, they were filtered and stored at 4 ◦ C until the pollutant content was evaluated following the method described before (Sections 2.5.3 and 2.5.4). 3. Results and discussion Characterization of the dredged sediment is crucial and will provide the clues to identify causes of phytotoxicity and viability of the material for its valorization. Granulometric analysis of the dredged sediment has shown the predominance of the clay and silt fraction (84.3% < 0.06 mm), whereas sands and gravels are in a lower proportion. Chemical characterization is reflected in Table 1. The sediment has a pH of 7.8 which corresponds to a slightly alkaline material and it is suitable for the soil. The salinity of the soil can be estimated roughly from an electrical conductivity (EC) measurement of the saturation extract. Values of EC less than 2 dS/m show negligible salinity effects; values between 2 and 4 dS/m imply that yields of very sensitive crops may be limited; values among 4 and 8 dS/m are related with yields of many crops restricted; values between 8 and 16 dS/m allow the growth of only tolerant crops; and finally, values over 16 dS/m permit only a few very tolerant crops to develop satisfactorily (USDA, 1954). A conductivity of 24.3 dS/m was found in the
Bedell et al. (2013) and Sheehan et al. (2010) showed a decrease of the toxicity of marine sediments after percolation with water. For this reason, desalination at laboratory scale was carried out with the dredged material in order to assess the impact of salinity from the dredged material. Dredged material was mixed with tap water in a 50 l container in a ratio solid:liquid of 1:10 (volume:volume). After 10 min of agitation, the suspension was left to decant for 24 h. Finally, the supernatant was removed, and the solid fraction was dried at room temperature in a fume hood until a moisture content of around 60% was obtained. Salinity reduction was assessed and a conductivity of 2.8 dS/m was reached after desalination, revealing the efficiency of the treatment. Desalinated dredged material was tested, in terms of phytotoxicity, itself and amended with different substrates (5% commercial compost, 5% peat moss, 5% perlite, 5% wood chips, and mixtures of 5% commercial compost + 5% peat moss, 5% peat moss + 5% perlite and 5% commercial compost + 5% perlite). These percentages are weight-based and grounded on the results obtained by Sheehan et al. (2010), who produced a manufactured topsoil with an organic amelioration around 5%. Furthermore, higher percentages would not have an economic viability. Commercial compost was used, mainly, for organic amelioration; while perlite, peat moss and wood chips were employed to evaluate a physical improvement of the properties of the dredged material such as aeration and drainage. Phytotoxicity tests revealed a slight improvement with respect to non-desalinated sediment. Nevertheless, in all cases, germination index did not exceed a 10% and germination suffered from a 2-day delay with respect to the control, showing a high phytotoxicity. Consequently, the amendments were not effective to improve the characteristics of the manufactured soil. Difficulties in plant growth can be easily explained in Fig. 1, as it is a perfect reflection of the problems found in the amended sediment. Predominance of the fine fraction of the dredged
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Fig. 3. Germination index of Lepidium sativum and Lolium perenne grown on the different evaluated materials.
through a composting was developed to obtain a stabilized product with enhanced properties.
3.3. Composting process
Fig. 1. Crusting phenomena and harmful rooting of Lepidium sativum grown on an amendment of the desalinated sediment with perlite.
sediment prevents from an optimal wetting and rooting, together with crusting phenomena. Even when perlite amended sediment was used due to sealing of the sediment, roots cannot penetrate into the deeper areas and they are forced to crawl through the surface. As a consequence, roots are bad fixed to the manufactured soil and the availability of nutrients and water is limited. On the other hand, fine particles of the dredged material get stacked into the leaves of the plants (crusting phenomena) and difficult their growing, together with a hampering in photosynthesis. As it has been previously reported, crusting conditions resulted in poor germination or emergence (Bresson and Boiffin, 1990). All these results had pointed to particle size distribution and soil compaction as the other major problems to face while elaborating the technosol. In this framework, a biological treatment
Dredged material and sewage sludge from an alimentary industry were composted as described in Section 2.2. After maturation, the texture of the final product is shown in Fig. 2b. Aggregates were established during composting and compactness of the dredged sediment was lost (Fig. 2a). This porous texture made during the composting process is expected to increase permeability of the soil and, also, to benefit its water retention capacity. Furthermore, the composting process did not change the pH of the new developed soil and salinity was reduced due to the dilution caused by the mixture with the sewage sludge. The composted sediment had a pH of 7.4 and a conductivity of 8.9 dS/m. As it can be classified as moderately saline, the composted sediment was subjected to phytotoxicity tests. The germination indexes obtained for Lepidium sativum and Lolium perenne were 11.0% and 16.4%, respectively. Furthermore, a 2-day delay in germination was observed with respect to the control. These two facts reveal that the high phytotoxicity still remains. Although the physical properties of the manufactured soil were improved, phytotoxicity could be attributed to the salinity content. In order to check this hypothesis, the composted sediment was desalinated.
Fig. 2. Physical appearance of (a) dredged sediment and (b) composted product.
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Table 2 Chemical characterization of the proposed technosol. Parameter pH Conductivity (dS/m) Nutrient analysis Total nitrogen (%) Total carbon (%) Ratio C/N Organic matter (%) NO3 − (mg/kg) PO4 3− (mg/kg) SO4 2− (mg/kg) Cl− (mg/kg) SO3 2− (mg/kg) Trace metals (mg/kg) As Cd Cr Cu Ni Pb Zn Hg a
Technosol 7.4 2.37 1.4 13.4 9.3 18.8 80.9 <0.5 5,629 1,893 15.4 10.4 <0.5 26.2 49.4 16.6 37.4 179.2 0.6
Acceptance criteriaa – – – – 12 4 – – – – – 100 0.5 150 200 140 100 250 1
Values according to local regulations (Xunta de Galicia, 2008).
Table 3 Analysis of the leachates from the technosol. Fig. 4. Differences in the growth on the technosol and on the control of (a) Lolium perenne and (b) Lepidium sativum.
3.4. Desalination of the composted sediment Composted sediment was desalinated as described previously in Section 3.2. Phytotoxicity tests developed with the desalinated composted sediment have achieved a 69% of germination index for both plant species (Fig. 3), so this material is considered moderately phytotoxic. This fact might be revealing that the salinity was responsible of the toxicity and also that the texture provided by the composting process was appropriate. Nevertheless, the desalination process of the composted material reduced the porous net, changing partially the texture of the material. Thus, composting should be undertaken with the desalinated sediment in order to increase the manufactured soil properties. 3.5. Composting of the desalinated sediment Desalination of the dredged sediment was carried out as described in the previous sections and after that the composting process was accomplished. Figs. 3 and 4 show the results of phytotoxicity texts of the obtained manufactured soil. As can be seen in these figures, favorable results were obtained for the technosol in the growth trials. Germination index (GI) for both Lepidium sativum and Lolium perenne exceeds the results obtained in the control, being 322% and 150%, respectively. This high GI can be explained taking into account that the manufactured technosol has more matured stable organic matter and nutrients than peat moss control sample. GI higher than 100% are related to phyto-stimulant and phytonutrient capability, and indicate the positive influence of some compounds present in the sample (Moldes et al., 2006). Furthermore, there were not delays in germination with respect to the control. Moreover, Lolium perenne was more sensitive than Lepidium sativum in all the assays (Fig. 3). This fact is in agreement with other researchers (Bedell et al., 2013) and has no relation with the cultivation substrate. A detailed characterization of the obtained manufacture soil was performed in order to evaluate if it overcomes the local
Parameter (mg/kg) As Cd Cr Cu Ni Pb Zn Hg Ba Mo Sb Se F− SO4 2− NO3 − a
Technosol 1.4 0.05 <0.1 1.1 0.4 0.3 1.1 0.01 0.14 1.6 <0.4 0.5 <1 5,690 3.2
Acceptance criteriaa 2 1 10 50 10 10 50 0.2 100 10 0.7 0.5 150 20,000 50
Values according to local regulations (Xunta de Galicia, 2008).
recommendations (Table 2). As can be seen, there were not significant differences in pH (7.4) and conductivity (2.37 dS/m), comparing to the manufactured soil obtained in Section 3.5. With respect to the elemental analysis (Table 2), the technosol has a proper organic matter content, 18.85%, and has complied with the rest of the local regulations. However, the C/N ratio found, 9.3, is slightly lower than the recommended value, 12. This can be attributed to the high protein content of the sewage sludge because of its poultry industry origin. Nevertheless, during the process of composting, the C/N ratio decreases until values close to 10 (Haug, 1993), so, this ratio can be considered suitable. Furthermore, the reduction obtained of total hydrocarbons content (around 40%) has proved that the composting process has also acted as a biological treatment. In addition, leaching assays of trace metals and nutrients were assessed, and they also satisfied the local recommended values as it is showed in Table 3. Leaching assays were made to the feasible technosol in order to assess if it could represent an environmental risk once applied in a real place. In summary, this technosol not only overcomes the local recommendations (Xunta de Galicia, 2008) but even might play the natural soil role. So, the product obtained after both processes,
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Fig. 5. SWOT analysis of the elaboration of technosols from dredged materials.
desalination and composting, represents the technosol proposed in this work. 3.6. SWOT analysis of the technosol obtained An SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis is a methodology employed to assess the internal and external factors that are favorable and unfavorable in a project or a business venture. To criticize our proposal and evaluate the real feasibility of the proposed management system, an SWOT analysis of the technosol elaborated from desalinated dredged materials was performed and summarized in Fig. 5. One of the strengths found in the internal analysis of the technosol is that allows, simultaneously, the valorization of dredged materials and other wastes such as sewage sludge together with the elaboration of a manufactured soil that replaces the use of natural fertile soils in restoration ecology. Recently, the European Union is promoting the development of new waste recovery proposals and, particularly, when the final objective is the treatment of soils. Other strengths are the low cost of the technosol proposed and minimum energy and technological requirements. With respect to the weaknesses found, the main problem lies in the dredging process as it is irregular and, depending on its location, will provide materials of different geochemical nature. Consequently, dredged sediments would have to be characterized, and the formulation of the technosol could be subjected to changes. External analysis has shown other opportunities, specifically the proposal of an alternative to the traditional waste management and the job promoting. The threats exhibit that the main disadvantage of the elaboration of this kind of technosols lies in the social vision of the processing plants. Social rejection
of the construction of a composting plant should occur due to the fact that these plants are associated with bad odors. Nevertheless, odors can be avoided with the proper design of the plant and the implementation of appropriate technology. This fact can be transferred to the population in order to not generate a social alarm. The SWOT analysis reflects that management of dredged sediments through the elaboration of technosols is a viable strategy and is also supported by governments of the area of study. The main drawback is the irregular delivery of dredged materials, though this could be solved designing a composting plant which elaborates both technosols from dredged marine sediments and other products. 4. Conclusions In this work, different strategies to manufacture a technosol from dredged material were evaluated. In this valorization process, the main problems were generated due to the high salinity and predominance of the fine particle size of the dredged sediments. The first problem has been solved with a desalination process with water washing. For the second, it has been assessed a composting processed in order to improve the texture by means of the formation of a porous matrix. The final product is in compliance with the local regulations besides being a stabilized organic material. In this manner, recycling of dredged materials together with other wastes was achieved and the produced technosol could be used in restoration ecology. In summary, in this study the viability of the ecological management of marine dredged sediments through the elaboration of technosols has been proved.
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