Introduction
A large volume of sediments are dredged from water bodies such as ports, waterways and wetlands and deposited on land. Sediments are typically polluted by contaminants such as heavy metals. Metals in sediments are in soluble, carbonate bound, Fe-Mn oxide, sulfide/organic matter bound and residual fractions. The metal solubility and mobility are mainly controlled by organic matter content, clay minerals, pH and redox potential. In exposure to air, metals release from loosely bound fractions and become available. One impact of dredging and on-land deposition of sediments is metal release enhancing their bioavailability and mobility. After sediments are deposited, plants start to grow; thus, metals can be up-taken by plants and become further available to the food chain. In case of human contact, there is a potential for direct accessibility of metals. Besides, long term leachability of metals can contaminate surface and ground water. According to former investigations, the mobility, availability and toxicity of metals cannot be assessed based on their total contents, and those are usually controlled by the chemical forms of metals. The metal speciation can be determined by sequential extraction procedures, and various single extraction methods can be used to evaluate their bioavailability and mobility. Other authors have studied metal leachability, bioavailability, bioaccessibility and speciation using various methods.
Study area
The Anzali International wetland was registered in the Ramsar Convention in 1975. It covers an area of 193 km², located at the southwest coast of the Caspian Sea, in Guilan, Iran. Due to the excessive discharge of different contaminants such as heavy metals, most of which are carried by rivers, into the wetland, bed sediments have become a sink for metals. The Pasikhan River is one of the most polluted rivers leading into the south east zone of Anzali wetland. A sediment trap in placed on the entry of the river, and sediments are being dredged and deposited in places adjacent to the trap; exposing to air, heavy metals might release from soluble and bioavailable phases.
Material and methods
Sampling was carried out after dredged sediments were deposited in the area for four months.
pH of sediments, the solid density, grain-size distributions, moisture content, Atterberg limits, mineralogical composition, major elements and total metal contents were determined. A sequential extraction procedure was applied for metal speciation. The detailed scheme for 1 g sample is as follows:
 
 




Table 1 Sequential extraction procedure




Step


Fraction


Reagent


Experimental conditions




F1


Exchangeable


10 ml of 1 molL^-1 MgCl₂


Room temp, 1h, shaking




F2


Carbonate


10 ml of 1 molL^-1 NaOAc


Room temp, 5h, shaking




F3


Fe-Mn oxide


20 ml of 0.04 molL^-1 NH₂OH·HCl in 25% (v/v) HOAc


96 °C, 6h agitation




F4


Organic matter


3 ml HNO₃ 0.02 molL^-1 + 5 ml 30% w/v, H₂O₂ 3 ml of 30% w/v, H₂O₂
5 ml  NH₄Ac  3.2 molL^-1 in 20% (v/v)  HOAc


85 °C, 2h agitation
85 °C, 3h agitation
Room temp, diluted, 30mins shaking




F5


Residual


Digestion


Method 3050B



 



The phytoavailability, bioaccessibility and mobility of metals were assessed using CaCl2 and EDTA, SBET and TCLP, respectively. Due to frequent precipitations in the area, SPLP test was conducted to simulate the amount of metals which can be washed and re-enter to the wetland.




Table 2 Leaching tests applied to assess dredged sediment samples




Test


Reagent


Contact time




CaCl₂


0.01 molL^-1 CaCl2, L/S= 10:1


3h, room temp




EDTA


0.05 molL^-1 EDTA, L/S= 10:1


1h, room temp




SBET


0.4 molL^-1 Glycine_, L/S= 100:1


1h, 37 °C




TCLP


AcOH, L/S= 20:1


18h




SPLP


H₂SO₄/HNO₃ (60:40), L/S= 20:1


18h




To determine release risk of metals, modified risk assessment code (mRAC) based on metal fractionations was applied. To evaluate the metal bioavailability and bioaccessibility, a new bioavailability/bioaccessibility index (BRAI) is used based on EDTA and SBET results.
Results and discussion
Sediments were classified as ML. Quartz was the dominant mineral observed in the XRD analysis which accorded with XRF. Concentrations of most metals exceeded those of earth’s crust, global average, Shijan zone of wetland and the Caspian Sea. Thus, these fine-grained sediments contained a high amount of metals. The sequential extraction showed that the highest percentages of metal associations with exchangeable, carbonate bound, Fe-Mn oxide, organic matter and residual fractions were related to Pb and Cd, Mn, Zn, Cu and Cr, respectively. Using the sum of metal extractions in exchangeable and carbonate fractions, the mRAC value was equal to 44.09 indicating high potential adverse impact.
The actual bioavailability of metals evaluated by CaCl₂ was low due to low concentrations of extracted metals (Fig. 1), and the concentrations of Pb and Cd, which were mainly associated with exchangeable fraction, were higher than those of other metals. EDTA extracts the potential bioavailable fraction of metals. Compared to other studied metals, high amounts of Cu, Mn, Pb and Cd were extracted by EDTA (Fig. 1); Cu and Mn were mainly associated with organic matter and carbonate bound fractions, respectively.  Based on results obtained from EDTA extraction, the calculated BRAI value of 2.4 showed medium risk of bioavailability. The concentrations of metals extracted by SBET method were high (Fig. 1). The highest concentrations were reported for Pb and Cd, almost all fractions of which were extracted. Based on SBET results, the calculated BRAI value was equal to 7.14 indicating very high risk of bioaccessibility. The release of Pb, Cd and Mn by TCLP method was higher than release of other metals (Fig. 1). Pb, Cd and Cr concentrations were below the USEPA regulatory limits indicating that sediments were not toxic and beneficial use of them is viable. The contents of Pb and Cd in the SPLP leachate were high compared to other metals with low concentrations (Fig. 1). Metal concentrations in SPLP leachate were commonly lower than drinking water standards.
 
Figure 1. Metal extraction by bioavailability/bioaccessibility and mobility tests.
In all extractions, the highest metal contents were reported for Pb and Cd and the lowest for Cr. The bioavailability of metals was in the decreasing order of Cd ~ Pb > Cu > Mn > Zn > Fe > Ni > Cr. Metal extractability of methods was in the order of SBET > TCLP > EDTA > SPLP > CaCl₂ for Pb, Ni, Cd and SBET > EDTA > TCLP > SPLP > CaCl₂ for the rest. The potential bioavailability of metals was higher than their actual bioavailability while the bioaccessibility of them was the highest. The concentrations of metals extracted by SBET were higher than those of TCLP, which was due to acidic pH and higher temperature in SBET. Although TCLP and SPLP methods are very similar, metal concentrations in TCLP were higher than SPLP. TCLP represents metal leaching under landfill conditions while SPLP simulates their release owing to precipitation which is an easier condition.
Conclusion
In this study, bioavailability, mobility and speciation of heavy metals in dredged sediments of Anzali wetland are assessed. The metal speciation and the mRAC index showed high potential adverse impacts. BRAI index using bioavailability and bioaccessibility test results represented medium and very high risks. Metal concentrations in TCLP test were lower than USEPA limits and in SPLP test were occasionally higher than standards. Results showed that metals in sediments of Anzali wetland can be up-taken by plants. Moreover, metals can leach to the underlying soil and contaminate ground water. They can also be washed due to the precipitation and re-enter to the wetland. On the other hand, sediments are not toxic and can be used for beneficial purposes. It can be concluded that unless properly managed, to deposit sediments can cause adverse effects on the environment and terrestrial organisms of Anzali wetland.