Seasonal Bioavailability of Heavy Metal Contaminants From the University of Lagos Lagoon, Nigeria

Heavy metals have been implicated as Lagoon pollutants from sources such as agriculture, mining and manufacturing industries and waste water treatment works. A study was carried out in the University of Lagos lagoon to investigate the seasonal bioavailability of the heavy metal contaminants Cd, Cr, Cu, Pb and Zn. The physical parameters pH, redox potential, temperature, TDS and conductivity were measured on site. Dried sediment samples were extracted using the Community Bureau of Reference (BCR) sequential procedure and analysed by ICP-OES. A certified reference material (CRM), BCR 701 (lake sediment) was used for quality assurance with recoveries ranging between 80-120%. Statistical analysis (ANOVA) showed that there was a significant difference between metal distribution in the dry and wet seasons. Cu, Pb and Zn were in the available fractions (carbonate, Fe/Mn oxide and organic). Cu was highest in the Fe/Mn oxide and organic fractions. This indicated that an increase in organic matter and reducing agents will avail the Cu. Zn was distributed in all fractions while Pb was found in the Fe/Mn oxide fraction (3.93- 21.3 %). Results showed that the bioavailability of Cu, Pb and Zn was high. Metal bioavailability by BCR indicates a potential risk of pollution in lagoon sediments as the available metals exceeded the permissible Sediment Quality Assessment Guidelines (SQAG’s) from Environmental Protection Agency (EPA).


Introduction
Heavy metals (Cd, Cr, Cu, Pb, Hg) are toxic at elevated concentrations and may cause health problems to users of water from lagoon, reservoirs, dams and wells which has been exposed to metal input (Musyoka et al., 2013). Sources of metal pollutants include weathering of soil, waste water discharge, solid waste deposited into the lagoon by surface runoff transporting pollutants (Grabowski et al., 2001;Jain, 2004;Singh et al., 2005). Metals settle in sediments and bind through processes of adsorption, ion exchange and precipitation (Filgueiras et al., 2002). They are released depending on a number of factors, that include pH, conductivity, salinity, total dissolved solids, temperature and turbidity (Ramirez et al., 2005) (Calmano et al., 1993). For example a slightly acidic environment may increase the availability of metals, as most metals are soluble in acid more than alkaline, however Cr is reported to have high mobility in alkaline environments (Babale et al., 2011).
It is well established that total metal concentration is not an indicator of mobility or toxicity to living organisms. Bioavailability studies are used to measure metal mobility and bioavailability in the environment (Adriano, 2001). This involves fractionation of the total metal contaminant using chemical extraction procedures (Jain, 2004). To assess how metals are partitioned and their availability for uptake, single or selective extractants are used (Adriano, 2001). The extractants are chosen based on selectivity, precipitation and adsorption. Chemical sequential extractions are most commonly used to evaluate metal partitioning and mobility in sediment (Davidson et al., 1998;Marguı et al., 2004).
The most common and widely used sequential extraction was first developed by Tessier et al. (1979) (Baffi et al., 1998;Tessier et al., 1979). This method involves exposing the sample to the selected extractants in order to leach out the metals depending on their partitioning. The Tessier sequential procedure releases metals from five fractions. The fractions are metals bound to the exchangeable, carbonates, Fe/Mn oxides, organic matter and residual. A standardised sequential extraction procedure, the BCR sequential extraction is used for extraction of metals in soils and sediments (Ho et al., 1997). The BCR sequential extraction assists in identifying and quantifying different defined species, forms or phases in which an element occurs in the material (Van Herreweghe et al., 2003).
There may be the release of the metals due to ion exchange as these weakly adsorbed metals are held by weak electrostatic force. Low pH accelerates the mobility as it breaks the carbonate structure and increases the solubility of metals making them bioavailable to the living organisms (Atwell et al., 1999;Kazi et al., 2005). Metals bound to other fractions typically require a change in environmental conditions to become available. The metals can be mobilised by an influx of reducing conditions in the environment (Kazi et al., 2005). Metals bound to the organic phase are more difficult to mobilise as a result of metals forming strong complexes with organic compounds in an oxidising environment (Atwell et al., 1999). The residual phase is the unreactive or the non-available phase, which is mostly bound to the silicate and can be made available from weathering (Filgueiras et al., 2002) (Farkas et al., 2007Ianni et al., 2000;Ianni et al., 2001). Numerous studies have been carried out using the BCR sequential extraction procedure for measuring the mobility of Pb, Cd, Cr, Cu and Zn (Coetzee, 1993;Filgueiras et al., 2004;Singh et al., 2005). In Nigeria sediment quality studies have been carried out in some rivers, dams and estuaries. These were done by analysis of the physical parameters, particle size distribution and metal pollutants. The amount of metal contaminant that is bioavailable was measured by the amount accumulated in an organism (especially fish) and this was used as a measure of toxicity (Wepener et al., 2011). There is limited data on the bioavailability of metals in Nigeria lagoon sediments. Analysis must be undertaken to understand the chemical behaviour of metals and their impact on the environment. Therefore, it will require study into its pollution status and potential impact using bioavailable metal information (Howard et al., 1995). This study investigates the seasonal bioavailability of Cd, Cr, Cu, Pb and Zn. To the best of our knowledge, no data is available in literature on the bioavailability of these metals in the University Lagoon.

Study Area
The University of Lagos also called UNILAG, situated within Lagos Mainland LGA of Lagos state lies between latitude 03.2343°E -03.34554°E and longitude 06.2135°N -06.4323°N. It is bounded on the north by Bariga, at the south by Onike and Iwaya, the east by Lagos Lagoon and at the west by Yaba. The University of Lagos is an institution of higher learning founded in 1962. It presently has three campuses in Yaba and Surulere. The main campus which is of interest in this research is located at Akoka North eastern part of Yaba in Lagos Mainland LGA. It is largely surrounded by the scenic view of the Lagos Lagoon and with an area of 802 acres of land.

Experimental
1. Exchangeable 20mL of 0.11 molL-1 HOAc, was added to the samples and shaken overnight for 16h at 25°C. The samples were centrifuged at SPEED and supernatant was refrigerated for analysis.

Fe/Mn oxide
The residue from step 1 was treated with 20mL of 0.5 molL-1 NH2OH.HCL and shaken overnight for 16h at 25°C. Centrifuging was done as in step 1.
3. Organic 5mL of H2O2 at 85°C was added to the residue from step 2: then first hour manual shaking with uncovered vessel and last hour with a covered vessel, 25mL of 1.0 molL-1 NH4OAc was later added and the samples were shaken for 16h at 25°C. Centrifuging was done

Residual
Samples were digested with aqua regia 3:1 (HCl: HNO3) The extracts from each step were analysed using ICP-OES (Perkin Elmer 5300 DV). Analysis of variance (ANOVA) was done for seasonal variations, p < 0.05.

Results and discussion
Statistical analysis (one way-ANOVA) for the total metals showed that there was no significant difference between spring and summer and between winter and autumn. However, there is a significant difference between the dry season and wet season. Therefore we present data from 1 season from each grouping. The data presented shows wet and dry season extractions to represent the dry and wet season respectively.

Quality Assurance
The method was validated (Table 4) with the CRM. The percentage RSD was typically <3%.

Physico-chemical parameters
The results (Table 5) show the physico-chemical parameters. The pH ranges from 6 to 8 which is typical for river water (Babale et al., 2011).

Extractable metals
The BCR extraction showed Cd and Cr mostly in the residual fraction. The behaviour has been shown to be typical for these two metals (Babale et al., 2011;Kartal et al., 2006). Seasonally winter had the highest number of metals in the potentially available fractions and when exposed to changes in the environment these metals will become available. High temperatures in the wet season increase the rate of chemical weathering which in turn release metals and through runoff are deposited in solubilised form into the river (Dekov, Komy, Araújo, et al., 1997). Some data presented will be explained using the physical parameters as studies have shown that they influence the bioavailability of metals (Sauve et al., 2000).

Cadmium
The distribution of Cd was in the following order; residual> organic> exchangeable~ Fe/Mn oxide. This was observed in the wet season where Cd was in high concentration whereas in the dry season most of the measurements were below the detection limit. Though the Cd concentration was high in the wet season, it was mostly in the residual fraction (not available). The Cd concentration that was in the available fractions (carbonate, Fe/Mn oxide and organic) were consistent throughout ranging from 2.0 to 3.0 mg kg-1. All sites showed Cd input from residential to industrial areas as Cd ions immediately discharged from point sources are absorbed into the sediment as they are not mobile over long distances. When the ions are detected in high concentrations in the carbonate phase, it generally implies that a nearby source had some influence (Tsai et al., 2007). Cd in the exchangeable fraction meant that it was held by weak electrostatic forces and can be removed by ion exchange (Babale et al., 2011). The EPA Sediment Quality Assessment Guideline (SQAG) value for Cd in river sediment is 0.676 mg kg-1 (FDEP, 1994). This indicates that Cd in the exchangeable fraction has a possibility of negatively impacting the biological processes of the ecosystem as it is exceeding the limit value (Klavinš et al., 2000). It has also been shown in other bioavailability studies that even though Cd exists in low concentrations, it is found mostly in the exchangeable phase and is bound to carbonates (Gunther et al., 2005;Tsai et al., 2002). Metals bound to carbonates (acid soluble fraction) are susceptible to pH changes but in this case Cd bioavailability will not be highly influenced by pH because it is in the optimum level (6-8).

Chromium
Cr was found mostly in the residual fraction meaning that it is retained in the crystalline structure and not bioavailable (Kazi et al., 2005). It has relatively low amounts in the Fe/Mn oxide fraction, followed by organic and finally exchangeable fractions for all seasons (Figure 2). The available fractions do not exceed the EPA SQAG permissible value of 52.3 mg kg-1 therefore there is low toxicity. As shown in Figure 2, Cr in winter from the industrial zone was predominantly in the organic fraction whereas in summer it was in the Fe/Mn oxide fraction.
Cr is used in industries and if deposited as hazardous waste into the lagoon, the metal oxides that is CrO2 will flow with the water and some particulates settle in the sediments (Metze et al., 2005). There is a possibility of Cr input entering from the waste sludge which flushes down the catchment until it reaches the ocean.
Cr exists as Cr (III) and Cr (VI) states and its bioavailability is dependent on the chemical state. The highly soluble, more toxic and bioavailable form of Cr is Cr (VI) (Dirilgen et al., 2002). Both of them are predominant and highly available in humic environments, but Cr (VI) in acidic solution has a high redox potential which shows that it is strongly oxidising and unstable in the presence of electron donors for example organic matter (Metze et al., 2005). The pH in dry season is approximately 6.0 which is weakly acidic and high redox potential up to 130 mV, therefore these can avail Cr (VI) that is bound to the Fe/Mn oxide phase.

Copper
Sediments typically have high concentrations of Cu in the wet season (Dekov, Komy, Araújo, et al., 1997). Cu was predominantly in the organic and residual phase in the dry season ( Figure 3) making its potential mobility to be limited. In addition these partitioned stable complexes are highly mobile and toxic (Artiola, 2005). Cu bound to the organic fraction in the dry season has the highest percentage (32% of the total concentration). The stable Cu organic complex can be described by anion exchange from binding with hydroxyl and carbonyl functional groups (Iwegbue, 2011) (Coetzee, 1993). Cu in the dry season has a possibility of being bioavailable under oxidising conditions and can be toxic to the ecosystem. The metals bound to organic matter will be released into the environment by oxidising agents (decomposition processes) (Artiola, 2005). Cu2+ is usually adsorbed onto clays especially Fe/Mn oxide while the stable Cu is mostly in organic soils. In the wet season Cu was consistently associated with the Fe/Mn oxide fraction. The EPA SQAG permissible limit for Cu in river sediments is 18.7 mg kg-1 (FDEP, 1994). For highly mobile soluble Cu the levels are below the permissible EPA SQAG limit which indicates less toxicity.

Lead
Pb is predominantly in the Fe/Mn oxide phase (Figure 4) in the wet season and mostly unavailable in dry season. This shows that in winter Pb2+ is able to replace Fe2+ in the Fe/Mn oxide because of the adsorbing capacity and surface area (Iwegbue, 2011). High concentrations of Pb will be bioavailable depending on the reducing conditions and changes in redox conditions. Our data follows the reported trend that Fe/Mn oxide fraction has a high affinity of Pb in the dry season (Fonseca et al., 2013). The available Pb partitioned to carbonates in the dry season is below the EPA SQAG permissible limit (30.2 mg kg-1) hence there is low toxicity threat. In the wet season Pb was in the residual fraction indicating that it is not bioavailable. However if the Pb-Fe-Mn-oxide phase become available; the concentration of Pb would be above the permissible levels and would pose a threat to the environment. These results are in agreement with another study on seasonal bioavailability of Pb in sediments which showed that Pb was mostly in the Fe/Mn oxide phase in the dry season as compared to the wet season (Iwegbue, 2011).

Zinc
Zn is distributed in all the fractions (Figure 4). Soluble Zn attaches to Fe/Mn oxides or organic matter and the mobility and availability is dependable on pH (Balintova et al., 2012). When the pH is basic the metal is likely to be mobile and bioavailable.  FDEP, 1994) which shows that the Zn will be toxic to the ecosystem. Zn will be highly bioavailable as the pH and redox in the sediment changes (Iwegbue, 2011).

Conclusion
Due to the nature of the areas along the lagoon, it is difficult to point out the reasons for the variations. Many activities may be affecting the natural balance of the lagoon and these include informal settlements, industries, illegal dumpsites and commercial zones. Heavy metals Cd, Cr, Cu, Pb and Zn were extracted and bioavailable in different seasons and sampling sites. Cd and Cr were mostly in the residual fraction; this meant that the metals were not available. Low traces of Cr were found in the Fe/Mn oxide phase in summer while Cd was in the exchangeable phase only in winter. Generally seasonal variability of these metals showed winter having more metals that is Cd, Cu, Pb and Zn in the available fractions. Metals in the exchangeable fraction like Cd and Zn which is predominant in most sampling sites are toxic to organisms. This is so as they can easily be moved, therefore, bioavailable to organisms in the ecosystem. The organic phase is also toxic as the metals are easily solubilised when the environmental conditions change. Change in the environment pH, redox potential, conductivity and anion concentration will solubilise the metals making them bioavailable thus subjecting University Lagoon to be a danger zone to living organisms.

Acknowledgement
This research was made possible by the University of Lagos, Faculty of Science Akoka, Lagos.