This study offers the opportunity to utilize Undaria pinnatifida and Phragmites australis to remove lead from water in permeable reactive barrier (PRB) technology. Its efficacy was tested using batch experiments and PRB column systems. From the batch experiment results, a higher adsorption capacity was observed for Undaria pinnatifida. Nevertheless, Phragmites australis in the column system efficiently removed lead and the breakthrough occurred at the same time for both biomaterials. To dissipate this difference, a sequential extraction for metal speciation analysis was used for both columns. The results have shown that each biomaterial has a dominant mechanism. Phragmites australis removed lead by physical adsorption, whereas Undaria pinnatifida showed a higher tendency to bind lead due to organic matter, primary and secondary minerals.

INTRODUCTION

The release of large quantities of heavy metals has affected our environment in recent decades. Since heavy metals are not biodegradable, their tendency to accumulate in living organisms paves the way for health problems (Fu 2011). Lead (Pb2+), especially, among various toxic metals has been more discharged than any other heavy metals. According to the previous studies, it has been calculated that the total amount of Pb2+ anthropogenic discharge to the oceans of the world is four times greater than the natural flux (Cardwell et al. 2002; Duruibe et al. 2007). When it comes to its presence in drinking water, it causes serious health problems which end in extreme cases such as death (Volesky 1990). Thus, Pb2+ is considered to be one of the most toxic metal ions, which has the risk of damaging mostly the brain, kidneys, circulatory system and nervous system (Babel & Kurniawan 2004).

Owing to this spectrum of contaminants emanating from a variety of sources including agriculture, industries and especially mining, groundwater resources have been under serious threat. Conventional methods, such as pumping and treatment, are considered as expensive.

A novel passive treatment such as PRBs (permeable reactive barriers) is convenient and innovative for in situ remediation. This technology consists of permanent, semi-permanent or replaceable media placed across the flow of a plume of contaminated water.

As the contaminated plume passively migrates through the media under the influence of the natural hydraulic gradient, the contaminants react with the media leading to either their transformation to less harmful compounds or fixation to the reactive materials (Thiruvenkatachari et al. 2007; Obiri-Nyarko et al. 2014).

The reactive media may depend on the contaminant to be treated and how the reactive material interact with the contaminant, should be examined. Heavy metals removal fit into the classification of sorption, with the immobilization within the reaction zone by adsorption or complex formation (Roehl et al. 2005). The types of reactive materials for PRB are those changing pH or redox potential, those causing precipitation, materials with high sorption capacity, and those releasing nutrients/oxygen to enhance biological degradation (PEREBAR 2002). The reactive media as a solid phase (adsorbent) and a liquid phase consist of a separation method where the solvent which contains any contaminant could be sorbed (adsorbate, e.g. metal ions) by solid phase. The fluid molecules, atoms or ions are attracted to the surface of a solid through physical interactions (physical adsorption – physisorption), with electrostatic attractive force or weak forces like Van der Waals and dipole–dipole force, or chemical adsorption (chemisorption) interactions, due to chemical binding like covalent and hydrogen bonding. The most important characteristic in this process is the high affinity of the sorbent for the species in the sorbate, as the solutes are removed from the solution until the amount of remaining solute in solution is in equilibrium with that at the surface. This equilibrium is described by qe, by expressing the amount of solute absorbed per unit weight of adsorbent as a function of Ce, the residual, final or equilibrium concentration left in the solution (Walter & Weber 1974; Dabrowski 2001).

Although the most common media is the zero-valent iron, nowadays, there is a tendency to use hybrid technology for pollutants removal, and this study utilizes biosorbents as materials for PRB technology. Biosorbents have an established method which tests the biomass to determine its affinity of the contaminants and its high adsorption capacity, and previous studies have focused only on the determination of kinetics and equilibrium parameters in batch systems. However, its application as biomaterials in the PRBs remains little explored.

The main reason why biological materials have gained attention during recent years is because of its good performance and low cost as alternative and innovative treatment techniques (Scott 1992; Kurniawan et al. 2006). Materials locally available in large quantities, such as raw materials (marine algae) or agricultural wastes (reed), can be utilized as low-cost adsorbents for heavy metal removal; helping industries to reduce the cost of water remediation and providing a potential alternative to the activated carbon and zero-valent iron (Volesky 1994; Kurniawan et al. 2006; Wang & Chen 2009). In recent years, investigations have shown that brown algae is the most effective and promising substrate and although many biomaterials have been studied, its feasibility on PRB application remains unexploited (Davis et al. 2003; Romera et al. 2007). Undaria pinnatifida and Phragmites australis were selected to be tested as PRB materials for their availability of almost unlimited amounts, and also because of their regeneration without damaging their sorption capacity which was proven to be an important factor for their success in previous studies (Southichak et al. 2009). In this context, we study the identification of the main removal mechanism to elucidate the interaction between heavy metals and selected sorbents for a hybrid application of biosorbents under PRB technology conditions. Therefore, this study tested biosorbents not only using batch experiments but PRB columns and the sequential extraction procedure analysed the chemical form of the cations bound to the sorbent for determination of the media mechanism.

MATERIALS AND METHODS

Materials

A waste product of brown seaweed biomaterial (Undaria pinnatifida) was obtained in March 2012 from a seafood industry in Miyagi Prefecture, Japan. According to the company the seaweed comes from Dalian, China. This biomaterial passed through many previous processes in the industry that includes water wash and removal of impurities. The seaweed is selected by separating the stem and the base, which is not included as part of the food product. The biomaterial is cut in certain particle sizes defined by the factory to prevail the characteristics of the product. The waste utilized in this study is a product of the first residue obtained during the selection process, therefore the quality of the material could be considered almost the same as the one obtained in the commercial product. Conversely, reed as biomaterial (Phragmites australis) was harvested near Hirose River in Sendai City, Japan in April 2012. During this period of the year, this large perennial rhizomatous grass was completely dry. In size the range was 1.5–3 m tall, its leaves are broad and sheath about 1–4 cm wide at their base. For this study, both biomaterials were freeze dried, reduced to a particle size of 1–2 mm, and stored in a desiccator (Figure 1).

Figure 1

Biomaterials powder.

Figure 1

Biomaterials powder.

Methods

For equilibrium studies the amount of biomaterial was 0.1 g mixed with 50 mL of heavy metal solution agitated for 24 hours. The Langmuir adsorption model was used to describe the equilibrium in the adsorption process. The Langmuir equation can be used for describing equilibrium conditions for sorption behaviour in different sorbate–sorbent systems, or for varied conditions within any given system. The hyperbolic form expressing the equilibrium metal uptake (qe mg/g) and the concentration of metal ions in solution at equilibrium (Ce mg/l) is as follows: 
formula
1
where Qmax (mg/g) is the maximum uptake by the biomass and b is the affinity constant to the isotherm.

Laboratory scale columns were set to estimate biomaterials performance on PRBs, the design of these were based on previous considerations made by other researchers (Gavaskar 1999). The water circulated from bottom to top in order to simulate the flow rate as it happens in the real field. The two PRB columns were carried out for reed (R) and brown seaweed (W) to state the removal performance of both biomaterials. Langmuir isotherm was used to model the performance of the columns. The column design was 20 cm in height with a diameter of 5 cm. The samples of the outlet were collected every day in order to state the removal performance of both biomasses; pH was also measured in this period of time.

A sequential extraction experiment was applied to the Pb(II) loaded biomaterials in the column. Samples were taken by dividing the reed column into three: bottom (B), medium (M) and top (T). Metal speciation content from the sequential extraction method was performed in order to determine four fractions:

  • Water/acid soluble and exchangeable (F1).

  • Bound to Fe and Mn oxides (F2).

  • Bound to sulphides and organic matter (F3).

  • Residual (F4).

F1 samples (0.5 g each) were shaken with 20 mL of 0.11 M CH3COOH for 16 hours at room temperature (Reciprocal Shaker NR–1, Taitec). For F2 samples, residues from previous step were shaken with 20 mL of 0.5M HONH2•HCl for 16 hours at room temperature (Reciprocal Shaker NR–1, Taitec). Between each successive extraction, separation was effected by centrifuging (Sorvall, Model RC2-B) at 3000 rpm for 20 min. The supernatant was removed with a pipette and analysed for trace metals, whereas the residue was washed with deionized water; after centrifugation for 30 min, this second supernatant was discarded. The volume of rinse water was kept to a minimum in order to avoid excessive solubilization of solid material, particularly organic matter. Deionized water used in preparing stock solutions and in each step of the leaching procedure was obtained from a Millipore Milli-Q3RO/Milli-Q2 system. In the case of F3, residues from previous step were shaken with 5 mL of 8.8 M H2O2 for 1 hour at room temperature (by Reciprocal Shaker NR-1, Taitec) and 1 hour at 85 °C (water bath — Personal H-10 Incubator, Taitec). After the second hour, 10 mL of 8.8 M H2O2 was added to the sample and continued heating at 85 °C for 1 more hour (Personal H-10 Incubator, Taitec). Once the samples were cooled down, they were shaken with 30 mL of 1 M CH3COONH4 for 16 hours at room temperature (by Reciprocal Shaker NR-1, Taitec), then samples were centrifuged at 3,000 rpm for 20 min, the supernatant was removed and analysed for trace metals. Finally, the sample residues were digested using 8 mL of HNO3 in a microwave digestion system (Start D, Milestone). The digest volume was allowed to cool and filtered through filter paper (ϕ = 110 mm, Advantec 5B), and afterwards samples were made up to 50 mL with deionized water to be analysed for trace metals. Digest blank was conducted for each acid batch in which the acid digest process is repeated with no biomass present.

RESULTS AND DISCUSSION

The Langmuir model was used because of its simplicity when considering the major binding sites on the biomass are homogeneous without interaction between sorbed molecules. The batch experiment fitted the Langmuir model with high correlation coefficients. Two equilibrium isotherms were plotted for each biomass at different temperatures (T = 4 and 20 °C) as Figure 2 shows.

Figure 2

Langmuir isotherm for (a) reed Phragmites australis (R) and (b) commercial seaweed Undaria pinnatifida (W) (initial concentration 10 mg/L, pH 5.5, volume of Pb2+ 50 mL, 0.1 g of biomass at 4 and 20 °C).

Figure 2

Langmuir isotherm for (a) reed Phragmites australis (R) and (b) commercial seaweed Undaria pinnatifida (W) (initial concentration 10 mg/L, pH 5.5, volume of Pb2+ 50 mL, 0.1 g of biomass at 4 and 20 °C).

For reed the maximum uptake at 20 °C is 19.5 mg/g with an affinity of 0.1 L/mg. When it is compared with 4 °C, the affinity becomes 0.106 L/mg and the maximum uptake is 17.9 mg/g. The amount of adsorbed lead slightly increases with variation of temperature from 4 to 20 °C. The constant b presents smaller values at both temperatures in comparison with other studies (Southichak 2002; Southichak et al. 2009); this could be attributed to its dependence on the number of active sites present and how easily they can be accessed (Hashim & Chu 2004). Therefore, the difference in affinity is accredited to the experiment conditions; common reed was only washed with water as an alternative material for PRBs. In this sense reed biomass was not treated because the elevated cost of chemical treatment would lower the advantages of PRB. The materials will become costly and hence unsuitable for this technology (Rocca et al. 2007; Park 2010). The Langmuir adsorption constants were evaluated with high correlation coefficients (R2).

As for commercial seaweed, Figure 2 shows that the maximum uptake could be found for a temperature 20 °C, in contrast with 4 °C conditions. Therefore, optimum temperature for biosorption is 20 °C, implying an endothermic process. As for b, higher affinity of lead to commercial brown seaweed can also be observed at 20 °C. As other studies confirmed, temperature is a factor which could lead to the variation of the biomass performance. In this study commercial seaweed has lower affinity at low temperatures. It was observed for both biomaterials that temperature of 20 °C had better results on removing lead compared with a temperature of 4 °C (Soto Rios et al. 2014). Therefore, all parameters were studied at this temperature.

Two constants were obtained from the Langmuir model, Qmax as the maximum amount of metal ions adsorbed per unit weight of adsorbent and b as the coefficient of affinity between the metal ion and the adsorbent. For R, Qmax is 19.5 mg/g and b equals to 0.1 L/mg, in the case of W 232 mg/g and 0.219 L/mg, respectively (Table 1). From the constants we can state that W has higher adsorption capacity and higher affinity with the contaminant. These results help us to find the theoretical value when column systems are tested. However, results from the column showed that the removal efficiency observed on water analysis was 99.7% for both biomaterials during the first days of removal until breakthrough. The final concentrations found in the outlet for reed column demonstrate that this biomass can efficiently achieve the standard limits. As for the case of seaweed column although its surface binding area remains to adsorb lead contaminant, it reduces to 66.2% (Figure 3).

Table 1

Langmuir constants

BiomassT (°C)Qmax (mg/g)b (l/mg)R2References
Reed (1–2 mm) 20 19.46 0.100 0.99 This study 
Treated reed (0.9 mm) room 17.18 1.789 0.99 Southichak (2002)  
Calcified reed (0.5–1 mm) room 10.35 2.275 0.99 Southichak et al. (2009)  
Commercial brown seaweed (1–2 mm) 20 227.3 0.22 0.96 This study 
BiomassT (°C)Qmax (mg/g)b (l/mg)R2References
Reed (1–2 mm) 20 19.46 0.100 0.99 This study 
Treated reed (0.9 mm) room 17.18 1.789 0.99 Southichak (2002)  
Calcified reed (0.5–1 mm) room 10.35 2.275 0.99 Southichak et al. (2009)  
Commercial brown seaweed (1–2 mm) 20 227.3 0.22 0.96 This study 
Figure 3

Removal efficiency for the reed column (a) and seaweed column (b).

Figure 3

Removal efficiency for the reed column (a) and seaweed column (b).

A better analysis was conducted to explain these differences for both biomaterials. For this purpose Langmuir constants were used to calculate the adsorption capacity for reed column with a theoretical value of 97.5 mg-Pb. Conversely, experimental results showed its maximum adsorption before the breakthrough at 98.5 mg-Pb on Day 20. Thus, the reed column result is found to be similar to the one expected by theoretical value. As for seaweed column, under the same conditions, maximum theoretical value was calculated as 781.4 mg-Pb and experimental results showed a reduction on the removal performance on the day 23 with a maximum adsorption of 104.42 mg-Pb, within a concentration in the outlet of 0.6 mg/L. Although the seaweed column system continued removing the contaminant, its performance decreased gradually to 3.14 mg/L until day 47, which is not considered highly efficient considering actual application. Theoretical values helped to state when the breakthrough would happen. However, the dominant mechanism is the one which states the performance of the biomaterials.

The two leading mechanisms assumed for this study were physical adsorption and ion exchange (Figure 4) because of previous studies (Kim et al. 1995; Choong et al. 2002; Southichak et al. 2009), however from the results it can be inferred that the functional groups carry negative charge to the cell surface (Waals’ forces) at the beginning of the adsorption for both biomaterials. Nevertheless, the replacement of counter ions with the bivalent metal ions is also observed for seaweed along with physical adsorption. To state the quantity of Pb forms on both surfaces, sequence extraction was also carried out.

Figure 4

Differences in the removal mechanisms for commercial seaweed and reed. Cell wall structure after Schiewer & Volesky (2000).

Figure 4

Differences in the removal mechanisms for commercial seaweed and reed. Cell wall structure after Schiewer & Volesky (2000).

The metal speciation results from the sequential extraction method, showed that the dominant mechanism in the reed biomass column was F1 as for exchangeable lead forms. It can be observed in Figure 5(a) that the bottom (RB), the medium (RM) and the top (RT) of the column followed a homogeneous trend for the heavy metal removal. The high F1 of reed indicated that the biomaterial surface is high exchangeable to bind lead as was shown in previous studies (Southichak et al. 2009; Soto Rios et al. 2014). Thus, it can be stated that the dominant mechanism for the removal of lead using this biomaterial is physical adsorption. However, seaweed (Figure 5(b)) shows that there are two dominant mechanisms F3 and F4, which means that lead may be bound to various forms of organic matter and primary and secondary minerals (Tessier et al. 1979), which may hold it within their crystal structure. From the figure it can also be observed that the top of the seaweed column (WT) contained more quantity of lead forms than the bottom (WB) and medium (WM). This result showed that for Undaria pinnatifida, ion exchange and physical adsorption are the main mechanisms at the beginning of the treatment, however after breakthrough still can remove heavy metals bounded to their organic matter.

Figure 5

Lead concentration extracted from (a) reed column and (b) seaweed column.

Figure 5

Lead concentration extracted from (a) reed column and (b) seaweed column.

It was found by Yong et al. (1995) that the higher performance of seaweed can be attributed to the major cell wall component of brown algae known as alginic acid, which is responsible for the binding of lead ions in this species. From the results, this mechanism was low in comparison to the F3 and F4 binding. The alginic acid quantity could have been removed from its surface in time thus decreasing its efficiency, and the dominant mechanism found in the metal speciation results explained how this biomaterial continued to remove lead.

The results of the sequential extraction in both columns showed an approximation of lead forms adsorbed whereas reed contained 10,545 mg/kg and seaweed 19,934 mg/kg. Seaweed removed double the quantity of lead than that expected by the Langmuir constants, however it took a longer time and decreased its adsorption in the column as it is considered more difficult than the attachment to organic manner and minerals. Thus, these biomaterials offer the opportunity to remove heavy metals mainly depending on their dominant mechanism.

Therefore, the variation of results in the column system with respect to the batch experiments had been explained by this result. Furthermore, considering its removal mechanism, these biomaterials can be used efficiently as sequenced barriers for PRB by taking into consideration the reuse of one biobarrier by isolation for desorption while using the other sequenced biobarrier to continue the removal process.

CONCLUSIONS

This study highlighted the competently lead (Pb2+) removal using Undaria pinnatifida and Phragmites australis as alternative barriers for PRB. However, it is important to take into consideration the leading mechanisms when the breakthrough of these biomaterials occurs to be used efficiently. Regardless Undaria pinnatifida and Phragmites australis have been studied previously in batch experiments under different conditions, their suitability for PRB depends largely on their own properties and hence its different removal mechanism after a period of time. Langmuir constants gave an accurate approximation for the reed column system. The seaweed column removed double the quantity of lead than that expected by the Langmuir constants but its efficiency reduced in time due to its adsorption mechanism. It would be interesting to use both materials in a PRB sequenced column system because of its different mechanisms to remove lead in different stages.

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