desorption on wheat straw affecting sustainability in vineyards

Journal of Cleaner Production 139 (2016) 1496e1503

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Competitive and non-competitive cadmium, copper and lead sorption/desorption on wheat straw affecting sustainability in vineyards
voa-Mun ~ oz b, Gustavo F. Coelho a, Affonso C. GonÇalves Jr. a, Juan Carlos No b b
vez , María J. Ferna
ndez-Sanjurjo c,
ndez-Calvin ~ o , Manuel Arias-Este David Ferna c c , *
~ ez-Delgado , Avelino Nún Esperanza Alvarez-Rodríguez ^ndido Rondon, Parana
, Rua Pernambuco, 1777, CEP 85960-000, State University of West Parana
, Brazil Center for Agricultural Sciences, Marechal Ca Department of Plant Biology and Soil Science, Faculty of Sciences, Campus Ourense, Universidade de Vigo, 32004, Ourense, Spain c Department of Soil Science and Agricultural Chemistry, Engineering Polytechnic School, Universidade de Santiago de Compostela, Lugo, 27002, Spain a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2016 Received in revised form 3 September 2016 Accepted 3 September 2016 Available online 5 September 2016

The novelty of this work lies on the consideration of wheat straw to retain Cu and other heavy metals in vineyards, in addition to its known potential to decrease erosion, thus facilitating the growth of new vine plants and contributing to sustainability in vineyard production. In this study we used batch-type experiments to investigate Cd, Cu and Pb competitive and non-competitive sorption/desorption on wheat straw. In non-competitive experiments, sorption sequence was Pb > Cd > Cu when the lowest molar concentrations (0.5 mmol L
1) were added, and Pb > Cu > Cd when the highest molar concentrations (6.0 mmol L
1) were added. Sorption curves indicated clearly higher sorption for Pb, lower initial sorption in the case of Cu, and certain trend to saturation of sorption sites for Cd. Data showed good adjustment to the Langmuir model just for Cd, whereas the Freundlich equation fitted well for all three metals. Desorption rates were low, in the order Pb < Cd < Cu. In the competitive experiment, the sorption sequence was Pb > Cu > Cd. The results indicate that competition clearly affected to Cd sorption, especially when the highest concentrations (6 mmol L
1) of the three heavy metals were added. The highest percentage of desorption in the competitive system corresponded to Cd, whereas Pb and Cu experienced clearly lower release. Comparing competitive and non-competitive experiments, Pb sorption was equivalent in the non-competitive and competitive trials, Cu sorption was slightly higher in the non-competitive than in the competitive experiment, and Cd sorption was clearly higher in the noncompetitive trial. Percentage desorption decreased for Pb and for Cu in the competitive trial, whereas it was clearly higher for Cd in the competitive than in the non-competitive experiment. The overall results indicate that Pb, Cu and Cd can be retained by wheat straw (especially Cu and Pb), thus decreasing risks of pollution, which could be used to treat polluted waters, and could also give additional value to wheat straw mulching used to protect vineyards from erosion and Cu (and other heavy metals) pollution, thus contributing to sustainability in this productive sector. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Desorption Heavy metals Sorption Sustainability Vineyard Wheat straw

1. Introduction Heavy metals pollution is a global concern, and in the specific case of vineyards is mainly focused on Cu contamination due to the
rek traditional use of Cu-based fungicides in these areas (Koma

* Corresponding author. ~ ez-Delgado). E-mail address: [email protected] (A. Nún http://dx.doi.org/10.1016/j.jclepro.2016.09.021 0959-6526/© 2016 Elsevier Ltd. All rights reserved.

et al., 2010). High Cu concentration may even difficult the growth of new vine plants (Romeu-Moreno and Mas, 1999), thus compromising sustainability of this productive sector (sustainability understood as the quality of not being harmful to the environment or depleting natural resources, and thereby supporting long-term ecological balance). However, other heavy metals, such as Cd and Pb, have also been studied in these environments (Kom
arek et al.,
ndez et al., 2012; Duplay et al., 2014). In 2008; Herrero-Herna addition to heavy metals pollution, many vineyards are associated

G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

to high risks of erosion (Kosmas et al., 1997), especially those located on steep slopes, taking into account the absence of vegetal cover on most soil surface for extended periods (Ramos and Martínez-Casasnovas, 2004). In this context, Cu may be transported from vineyards to water courses by means of runoff (Ribolzi et al., 2002). These facts suggest the convenience of implementing management practices aiding to retain Cu and other heavy metals, as well as to prevent soil erosion and drift of pollutants by runoff from vineyard areas. In fact, promoting sustainability in vineyards has been considered and environmentally and economically sound goal
zquez-Rowe et al., 2012; Villanueva-Rey et al., 2014). (Va Regarding control of erosion, barley straw has been recently studied as a means to reduce soil erosion in vineyards, with good results (Prosdocimi et al., 2016). Previous studies had reported the use of straw (mainly wheat straw) as an appropriate material for protecting soils from erosion after forest fires in sloped areas (Vega
ndez and Vega, 2016). et al., 2015; Ferna As regards heavy metal removal or immobilization, certain soils and bio-sorbents have shown variable retention potential (CutillasBarreiro et al., 2014), aiding to fight pollution. Taking into account sustainability, different industrial and agricultural waste and byproducts have been studied as potential bio-sorbents, promoting ~ ez-Delgado et al., 2015a; Quint
its recycling with this aim (Nún ansFondo et al., 2016). Examples of this are Jatropha curcas biomass (Nacke et al., 2013, 2016), cashew nut shell (Coelho et al., 2014), mussel industry waste (Seco-Reigosa et al., 2013, 2014; Otero et al., ~ ez-Delgado et al., 2015b). 2015) and wood industry waste (Nún Generally, these materials are locally and easily available in large quantities. Therefore, they have low economic cost, and even its recycling should be encouraged (Cutillas-Barreiro et al., 2014; Coelho et al., 2014). Straw (and specifically wheat straw) can be considered within these low cost sorbents, as other cellulosic materials (Malik et al., 2016). Furthermore, the recycling of this by-product should be promoted, as concluded in life-cycle studies that put in evidence even its implications on climate change (Song et al., 2016). As indicated in Tye et al. (2016), it has been estimated that annual total cereal straws production is about 1580 million tons, mainly from Europe (barley and oat), the United States (corn and sorghum) and China (rice and wheat). Searle and Malins (2016) indicate that total agricultural residue production (above ground biomass) is 315.9 million tons per year (dry basis) in the European Union, with 23.1 million tons per year produced in Spain, which is placed in fourth place within the EU countries (preceded by France, Germany and Poland). With regards to heavy metals retention potential, Dang et al. (2009) studied Cd and Cu adsorption on wheat straw, finding around 87% removal when 50 mg L
1 of the metals were added, whereas Pehlivan et al. (2009) found maximum retentions of 69 and 88% for Cu and Pb on barley straw. Mahmood-ul-Hassan et al. (2015) found that wheat straw could present interesting potential for Cd and Pb removal, with rapid metal sorption (>80% in 20 min). Regarding the simultaneous presence of different heavy metals, Wang et al. (2011) studied Cd, Cu and Pb competitive adsorption on black carbon from the burning residue of wheat straw, finding that the selectivity sequence was Pb > Cu > Cd. Park et al. (2016) studied competitive adsorption of heavy metals onto sesame straw biochar, finding that multi-metal adsorption behaviors differed from monometal adsorption due to competition. However, as long as we know, no previous studies have dealt with Cu, Cd and Pb retention on wheat straw in competitive and non-competitive trials, adding up to 6 mmol L
1 of each metal, all this focusing on the potential of straw mulching in vineyards, which could aid to simultaneously control heavy metals pollution

1497

and erosion. All this could be of relevance and with clear scientific value. In view of that, the aim of this work was to study Cd, Cu and Pb retention/release on wheat (Triticum aestivum) straw, in individual and competitive systems, adding up to 6 mmol L
1 of each metal. Our research hypothesis is that the results of the study could demonstrate the potential of wheat straw to reduce risks of soil and water pollution, specially focusing on vineyards where straw could be used to simultaneously control erosion and heavy metal contamination, thus promoting sustainability in this productive sector. 2. Materials and methods 2.1. Characteristics of the wheat straw used as bio-sorbent The wheat (Triticum aestivum) straw was obtained from a local provider in Cospeito (Lugo, Spain). The material was transported to the laboratory, where it was dried, milled and sieved, retaining particles between 2.0 and 0.5 mm, which were used for chemical determinations and further retention/release experiments. The following parameters were determined in the 2e0.5 mm fraction: pH in distilled water (pHwater), and pH in 0.1 M KCl (pHKCl) (ratio solid:solution 1:5), using a pH-meter (model 2001 Crison, Spain). The point of zero charge (pHPZC) was determined as per Mimura et al. (2010). Total C and N contents were quantified by elemental analysis (CHNS Truspec, Leco, USA). Available P by means of the Olsen method (Olsen and Sommers, 1982), quantifying by UVevisible spectrophotometry (UV-1201, Shimadzu, Japan). Exchangeable cations were extracted using 1 M NH4Cl (Peech et al., 1947), quantifying by atomic absorption/emission spectrophotometry (Perkin Elmer AAnalyst 200, USA). The effective cation exchange capacity (eCEC) was calculated as the sum of exchangeable Ca, Mg, Na, K and Al. Total contents of different elements (Ca, Mg, Na, K, Al, Fe, Mn, Cu, Zn, Ni, Cd, Cr and Co) were determined after microwave digestion in a 65% nitric acid solution, quantifying
brega et al., 2012). Total nonby ICP Mass (Varian 820-NS, USA) (No crystalline Al and Fe (Alo, Feo) were quantified spectroscopically after extraction on 1-g samples using ammonium oxalate solutions
acidified to pH 3 with oxalic acid (Alvarez et al., 2012). All determinations were performed by triplicate. High C and low N contents (43.48% and 0.55% respectively) resulted in a C/N ratio of 79.05. pH in water and in KCl were near neutrality, both with very similar values (7.12 and 6.92, respectively), which indicates a low exchangeable acidity. Total concentrations were much higher for K (10.91 g kg
1) than for Ca (2.21 g kg
1), Mg (0.49 g kg
1), and Na (0.25 g kg
1). P levels were also high (314.09 mg kg
1). Most elements showed similar values to those obtained by Nunes et al. (2008) in other straw samples. Wheat straw showed a high effective cation exchange capacity (eCEC) (35.20 cmolc kg
1), where K is the dominant element (27.54 cmolc kg
1), followed by Ca (4.44 cmolc kg
1), Mg (2.22 cmolc kg
1) and Na (1.03 cmolc kg
1). Exchangeable Al was not detected, which would be related to the pH values and scarce exchangeable acidity previously commented. However, the concentration of noncrystalline Al (Alo) was relevant, constituting 40.6% of total Al in the straw. Non-crystalline Fe (Feo) constituted 57.3% of total Fe in the sorbent material. Both Alo and Feo can be of importance in the retention of heavy metals (Violante et al., 2003). In addition to the chemical characteristics commented above, the main functional groups present in wheat straw structure were determined by infrared spectroscopy on a FTIR-Bomen MB102 equipment (ABB, Switzerland). The spectra were obtained by transmittance using KBr pellets, performing determinations in the region between 400 and 4000 cm
1, with a resolution of 4 cm
1.

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G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

2.2. Non-competitive sorption/desorption experiments For sorption experiments, 3.0 g of wheat straw were weighed and added with 30 mL of a 1 M NaNO3 solution, with increasing concentrations (0, 0.5, 1.5, 3.0 and 6.0 mmol L
1) of Cd, Cu, or Pb, respectively, prepared from CdN2O6.4H2O (Sigma-Aldrich, USA), Cu(NO3)2.3H2O (Panreac, Spain), and Pb(NO3)2 (Scharlau, Germany). The resulting suspensions were stirred for 24 h, centrifuged at 4000 rpm (6167g) for 15 min, and filtered through acid washed paper (pore size 2.5 mm). Cd, Cu, Pb and pH values were quantified in the filtrated liquid using ICP Mass (Varian 820-NS, USA) and pHmeter (model 2001 Crison, Spain) equipment. To determine desorption, after ending the sorption experiments each sample was added with 30 mL of 0.01 M NaNO3, then the samples were stirred for 24 h, centrifuged at 4000 rpm (6167g) for 15 min, and filtered through acid washed paper (pore size 2.5 mm). Cd, Cu, Pb and pH were quantified in the filtrated liquid as indicated above. Percentage desorption was calculated after determining concentration released to the equilibrium solution, referring to that previously retained by sorption. All determinations were performed by triplicate. 2.3. Competitive sorption/desorption experiments For competitive experiments, 3.0 g of wheat straw were weighed and added with 30 mL of a 1 M NaNO3 solution containing equal concentrations of Pb, Cd and Cu in the same solution. Specifically, the following five solutions were used: 0-0-0 mmol L
1 (control), 0.5-0.5-0.5 mmol L
1, 1.5-1.5-1.5 mmol L
1, 3.0-3.03.3 mmol L
1 and 6.0-6.0-6.0 mmol L
1 of each of the three metals in each solution. Then the samples were stirred for 24 h, centrifuged at 4000 rpm (6167g) for 15 min, and filtered through acid washed paper (pore size 2.5 mm). Cd, Cu, Pb and pH were quantified in the filtrated liquid as indicated above. To determine desorption, after ending the sorption experiments each sample was added with 30 mL of 0.01 M NaNO3, then the samples were stirred for 24 h, centrifuged at 4000 rpm for 15 min (6167g), and filtered through acid washed paper (pore size 2.5 mm). Cd, Cu, Pb and pH were quantified in the filtrated liquid as indicated above. All determinations were performed by triplicate. 2.4. Data analyses The statistical package SPSS 21 (IBM, USA) was used to perform basic statistical treatment (descriptive statistics, correlation analysis) and fitting to adsorption models. Adsorbed quantities (Qe) were calculated for Cd, Cu and Pb by means of Equation (1):

  Q e ¼ V C0
Cf =m

(1)

where Qe is the quantity of heavy metal adsorbed in the equilibrium (mg g
1), m is the mass of adsorbent used (g), C0 represents the initial concentration of each heavy metal in solution (mg L
1), Cf is the final concentration of each heavy metal in solution (mg L
1), and V is the volume of solution used (L). Data from sorption experiments were adjusted to the Langmuir and Freundlich models. In the Langmuir model, a maximum adsorption value (Qm) can be calculated from Equation (2):

Q eq ¼ Q m KL Ce =ð1 þ KL Ce Þ

(2)

where Qeq is the quantity of each heavy metal adsorbed (mmol kg
1), KL is the Langmuir constant related to the adsorption energy, Ce is the concentration of each heavy metal in the equilibrium solution (mmol L
1) and Qm is the maximum adsorption capacity (mmol kg
1). The Freundlich model can be expressed by means of Equation (3): 1=n

Q eq ¼ KF Ce

(3)

where Qeq is the quantity adsorbed of each heavy metal (mmol kg
1), KF is the Freundlich constant related to the energy of adsorption, Ce is the concentration of each metal in the equilibrium (mmol L
1), and n is a constant related to the adsorption intensity. 3. Results and discussion 3.1. Characteristics of the wheat straw used Table 1 shows chemical characteristics of the wheat straw used in this work. Regarding FTIR spectra for wheat straw, it demonstrated the presence of carboxylic, hydroxyl and amine functional groups on the surface of the adsorbent (Fig. S1, Supplementary Material), which can also be relevant in the retention of cationic Cd, Cu and Pb. 3.2. Non-competitive sorption as a function of Cd, Cu and Pb concentrations added Fig. 1 shows adsorbed quantities (Qe, in mmol kg
1) for Cd, Cu and Pb as a function of the molar concentrations added (mmol L
1)

Table 1 General characteristics of wheat straw (average values for 3 replicates, with coefficients of variation always <5%). Xe: exchangeable concentration of the element; XT: total concentration of the element; Alo and Feo: Al and Fe extracted with ammonium oxalate. C (%) N (%) pHwater pHKCl pHpz Cae (cmolc kg
1) Mge (cmolc kg
1) Nae (cmolc kg
1) Ke (cmolc kg
1) Ale (cmolc kg
1) eCEC (cmolc kg
1) POlsen (mg kg
1) PT (mg kg
1) CaT (mg kg
1) MgT (mg kg
1) NaT (mg kg
1) KT (mg kg
1) AsT (mg kg
1) CdT (mg kg
1) CoT (mg kg
1) CrT (mg kg
1) CuT (mg kg
1) NiT (mg kg
1) PbT (mg kg
1) ZnT (mg kg
1) MnT (mg kg
1) AlT (mg kg
1) FeT (mg kg
1) Alo (mg kg
1) Feo (mg kg
1)

43.48 0.55 7.12 6.92 6.68 4.44 2.22 1.03 27.51 0.00 35.20 314.1 979.8 2210.4 490.2 250.1 10907.0 0.05 0.08 0.25 3.47 2.93 2.36 0.25 17.57 304 147.7 76.79 60.03 44.01

G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

in non-competitive experiments. When the lowest molar concentrations (0.5 mmol L
1) were added, the sorption sequence was Pb > Cd > Cu, with values 4.53, 4.51 and 3.48 mmol kg
1, respectively (Fig. 1), corresponding to 92%, 90.6% and 69.9% percentage sorption (Table 2), calculated as the difference between initial concentration added (C0) and that remaining in the equilibrium solution (Cf), expressed as percentage of the initial concentration added: %Sorption ¼ 100 e [(Cf/C0)100]. When the highest molar concentrations (6.0 mmol L
1) were added, the sequence changed to Pb > Cu > Cd, with values of 57.25, 49.01 and 48.55 mmol kg
1, representing 96.1%, 81.8% and 81.5% of sorption expressed as percentage (Table 2). This last retention sequence coincides with that previously found by Cutillas-Barreiro et al. (2014) using pine bark as adsorbent for various heavy metals, as well as with that of electronegativity (Mcbride, 1989). The fact that Pb is the metal always showing the highest retention on wheat straw can be related to its strong affinity with carboxyl groups present in the adsorbent (Krishnami et al., 2008). Furthermore, Park et al. (2016) indicate that the adsorption preference for Pb over other heavy metals could be attributed to different factors, such as greater hydrolysis constant, higher atomic weight, smaller hydrated radius, and larger Misono softness value, providing Pb with improved efficacy for inner-sphere surface complexation or sorption reactions. Regarding the change in the sequence of sorption for the lowest and highest metal concentrations added, Park et al. (2016) found that maximum adsorption capacities were in the order of Pb [ Cd [ Cu (coincident with that corresponding to the lowest concentrations added in the present work) in a mono-metal noncompetitive experiment, whereas it changed to Pb [ Cu [ Cd in a multi-metal competitive trial. In the present non-competitive experiment, this last trend was also found when the highest metal concentrations were added. Taking into account that this sequence is coincident with the reverse order of the hydrated ionic radii of the three heavy metal species (Pb2þ, Cu2þ and Cd2þ), it could indicate that steric limitations to adsorption would be less important when the added metal concentrations are low, due to many adsorption sites been still available, whereas the opposite would occur when the added metal concentrations are high. In previous studies, Coelho et al. (2014) found up to 95% and 87% adsorption for Pb and Cd, respectively, when adding 120 mg L
1 of each metal to cashew nut shell used as bio-sorbent. Rubio et al. (2013) studied Crambe abyssinca H. as bio-sorbent for low concentrations of Pb and Cd (0.1 and 0.05 mg L
1), finding percentage adsorptions of 65% for Pb, and 71.2% for Cd, respectively, which can be considered poor results when compared to that of the wheat straw here studied, mostly taken into account that clearly higher heavy metal concentrations (up to 6 mmol L
1) were added in the present study. The high performance of wheat straw in Pb and Cu

70

Cd

Sorption (mmol kg-1)

60

1499

Table 2 Percentage of Cd, Cu and Pb sorption on wheat straw for different initial concentrations added (C0) (0, 0.5, 1.5, 3.0 and 6.0 mmol L
1) in the non-competitive experiments. Average values for 3 replicates, with coefficients of variation always <5%. C0

0.5

1.5

3.0

6.0

% Sorption Cd Cu Pb

90.6 69.9 92.0

90.4 76.6 91.6

89.3 74.3 93.5

81.5 81.8 96.1

sorption (and, to a lower extent, in Cd sorption) can be related to the presence of organic groups with high affinity for cationic heavy metals (Uchimiya et al., 2011; Wang et al., 2011; Jiang et al., 2012). As shown in Table 2, the highest value for Cu sorption on wheat straw was 81%, and occurred when the highest molar concentration (6.0 mmol L
1) was added. This result is close to those found by Cutillas-Barreiro et al. (2014) using pine bark (84%), and by
rez et al. (2013) using mussel shell (86%). Ramírez-Pe Regarding pH, Table 3 shows that it decreased in the equilibrium solution after the sorption of Cd, Cu and Pb, suggesting the release of Hþ and/or of substances generating OHþ 3 in solution when sorption takes place. In this way, the surface of wheat straw has functional groups (such as hydroxyl or carboxylic) which are able to participate in ion exchange (Mohan and Pittman, 2007). Cu was associated to the highest acidification in the equilibrium solution after the sorption process, reaching a pH value of 4.47 when the highest concentration (6.0 mmol L
1) was added, followed by Pb and Cd. Regarding the effect of changing the pH value of the equilibrium solution, Dang et al. (2009) found that sorption capacity of wheat straw increased about 130% and 60% for Cd and Cu when pH increased from 4.0 to 7.0. Pehlivan et al. (2009) also found that Cu and Pb adsorption on barley straw was affected by pH: Cu removal increased from 60% to 69% when pH rose from 3.0 to 6.4, then decreasing to 50% with an increase of pH to 9.0, while Pb removal increased from 70% to 88% with an increase of pH from 2.5 to 6.0, then decreasing to 25% when pH rose to 9.0. Negative correlations between Qe (adsorbed quantities of each heavy metal) and pH in the equilibrium solution were found for all three heavy metals, being significant (p < 0.05) for Cu. Coefficients of correlation were
0.930 for Cd, -0.892 for Cu, and
0.949 for Pb. This means that higher sorption was associated to higher release of Hþ, thus decreasing pH. 3.3. Cd, Cu and Pb sorption curves in non-competitive experiments Fig. 2 shows sorption curves for Cd, Cu, and Pb, indicating clearly higher sorption for Pb (without evidence of saturation of adsorption sites, in accordance with the Freundlich model), as well as a certain trend to saturation of adsorption sites in the case of Cd (in accordance with the monolayer adsorption process described by

Cu

50 Pb

40

30 20 10

0 0

1

2 3 4 5 6 Concentration added (mmol L-1)

Fig. 1. Cd, Cu and Pb adsorbed (mmol kg
1) on wheat straw for different concentrations added (0, 0.5, 1.5, 3.0 and 6.0 mmol L
1) in the non-competitive experiments. Average values for 3 replicates, with coefficients of variation always <5%.

Table 3 pH values in equilibrium solutions after Cd, Cu and Pb sorption on wheat straw, for different initial concentrations (C0) in the non-competitive experiments. Average values for 3 replicates, with coefficients of variation always <5%. C0 mmol L
1

Cd

0 0.5 1.5 3.0 6.0

6.99 6.66 5.83 5.48 5.05

Cu

Pb

pH in the equilibrium solution 6.33 6.07 5.43 4.6 4.47

6.34 6.24 5.93 5.32 5.02

1500

G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

the Langmuir model), and lower initial sorption in the case of Cu, but with a trend to increase (thus, following Freundlich rather than the Langmuir model). Table 4 shows the fitting to the Langmuir and Freundlich models, calculated using the experimental sorption data. Table 4 shows good adjustment to the Langmuir model just for Cd, whereas the Freundlich model fitted well for all three metals. Wang et al. (2011) also found good fitting for the Freundlich model studying Cd, Cu and Pb adsorption on wheat-residue derived black carbon, whereas Dang et al. (2009) found that Cd and Cu adsorption on wheat straw adjusted better to the Langmuir model. These findings suggest that wheat straw has different types of adsorption sites (especially for Cd), resulting in the occurrence of adsorption in mono- and multi-layer (Foo and Hameed, 2010), whereas adsorption would be mainly of multilayer type in the case of Cu and Pb. The Freundlich n parameter indicates the reactivity of active sites in the adsorbent. In the present study, values of the n parameter were higher than 1 for Pb and Cu, with Pb > Cu > Cd. According to Khezami and Capart (2005) and to Foo and Hameed (2010), n > 1 indicates the presence of high energy sites. These values can also suggest the occurrence of cooperative sorption, involving strong interactions between the molecules of the adsorbate itself. In the present study, Freundlich KF values were clearly higher for Pb than for Cd and Cu, with a good fitting that supports multilayer adsorption on wheat straw.

3.4. Cd, Cu and Pb desorption from wheat straw in non-competitive experiments Table 5 shows low desorption rates, especially for Pb and Cd. In fact, Pb had the lowest percentage desorption, followed by Cd and Cu. For Cu (and for Pb in most cases), desorption rate decreased as the initial concentration added was increased, whereas the opposite occurred to Cd. Low desorption from wheat straw can be a consequence of the strong affinity that these metals have with organic functional groups present in the adsorbent (Krishnami et al., 2008). Dupont et al. (2003) reported high affinity for bindings of heavy metals in lignocellulosic substrates extracted from wheat bran, specifically following the sequence Cu z Pb > Cd. Our data on sorption and desorption in non-competitive experiments indicate that the studied heavy metals (Cd, Cu and Pb) can be partially retained by wheat straw, thus decreasing risks of pollution. These results could be taken into account when designing filtration systems to treat polluted waters, and could also give additional value to certain mulching practices, as the use of wheat straw to protect vineyards from erosion and Cu (and other

70

Cd

Sorption (mmol kg-1)

60

Cu

50 Pb

40

30 20 10

0 0

0.25 0.5 0.75 1 1.25 Equilibrium concentration (mmol L-1)

Fig. 2. Sorption curves for Cd, Cu and Pb, corresponding to the non-competitive experiments using wheat straw. Average values for 3 replicates, with coefficients of variation always <5%.

heavy metals) pollution, thus contributing to sustainability. 3.5. Cd, Cu and Pb competitive sorption/desorption on wheat straw Fig. 3 shows Cd, Cu and Pb adsorbed (mmol kg
1) on wheat straw in the competitive experiment, as a function of added concentrations. The layouts and overall results are similar to those in the non-competitive trial (Fig. 1), unless in the case of Cd when the highest concentration (6 mmo L
1) was added, showing clearly lower Cd sorption in the competitive experiment. Similarly, Park et al. (2016) found that Cd was easily exchanged and substituted by other metals during multi-metal adsorption, when studying competitive adsorption of heavy metals onto sesame straw biochar. These authors also found that maximum adsorption capacities were in the order of Pb [ Cd [ Cu in their mono-metal noncompetitive experiment, whereas it changed to Pb [ Cu [ Cd in their multi-metal competitive trial. The latter sequence was also found by Wang et al. (2011) studying competitive adsorption on wheat-residue derived black carbon, indicating that this trend is coincident with the reverse order of the hydrated ionic radii of the three heavy metal species, and that the reason of that sequence could be the selectivity of ion exchange in multicomponent systems. As overall results, in our competitive experiment Pb presented the highest value for Qe (adsorbed quantity), followed by Cu and Cd. Qe values ranged from 4.74 to 57.58 mmol kg
1 for Pb, increasing as a function of the initial Pb concentration added in the competitive solution, although the highest Pb sorption percentage (99.3%) corresponded to 3.0 mmol Cu L
1 as initial concentration added (Table 6). For Cu, Qe values ranged from 3.37 to 45.88 mmol kg
1, also increasing as a function of the initial concentration of this metal added in the competitive solution, but obtaining the highest percentage sorption (81.9%) with an initial concentration of 3.0 mmol Cu L
1. For Cd, sorption increased for initial concentrations from 0.5 to 3.0 mmol L
1, with Qe values ranging from 4.57 to 20.51 mmol kg
1, decreasing to 14.04 mmol kg
1 when the highest Cd concentration was added. These results indicate that competition clearly affected to Cd sorption on wheat straw, especially when the highest concentrations (6 mmol L
1) of the three heavy metals were added simultaneously. Thus, comparing competitive and non-competitive experiments, Cd sorption was clearly higher in the non-competitive trial (from 4.51 to 48.55 mmol kg
1, for 0.5e6.0 mmol L
1 added) than in the competitive experiment (from 4.57 to 14.04 mmol kg
1, for 0.5e6.0 mmol L
1 added), whereas Cu sorption was slightly higher in the non-competitive (from 3.48 to 49.01 mmol kg
1) than in the competitive trial (from 3.37 to 45.88 mmol kg
1), and Pb sorption was equivalent in the non-competitive (from 4.53 to 57.25 mmol kg
1) and competitive experiments (from 4.74 to 57.58 mmol kg
1). Fig. 4 shows sorption curves for Cd, Cu and Pb in the competitive experiment. Comparing with the non-competitive experiment (Fig. 2), more pronounced differences between the layout and behavior of Cd and each of the other two metals were evidenced. As in the non-competitive experiment, in this competitive trial Cd showed saturation of adsorption sites, with a behavior in accordance with the Langmuir model, whereas sorption did not show a maximum in the case of Cu and Pb, in accordance with the Freundlich model. In a previous work, Wang et al. (2011) found a good fitting to the Freundlich model studying competitive Cd, Cu and Pb adsorption on wheat-residue derived black carbon. Table 7 shows that the highest percentage of desorption in the competitive system corresponded to Cd, which can be easily released to the solution (especially at high concentrations), whereas Pb and Cu had clearly lower desorption percentages,

G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

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Table 4 Parameters of the Langmuir and Freundlich models, corresponding to Cd, Cu and Pb non-competitive sorption on wheat straw. Qm: maximum adsorption capacity; KL: parameter related to the strength of interaction adsorbent/adsorbate; R2: coefficient of determination; KF: parameter related to the adsorption capacity; n: parameter related to the heterogeneity of the sorbent; -: error values too high for fitting. Langmuir Qm (mmol kg

Freundlich
1


1

)

KL (L mmol

76.3 ± 3.5 e e

Cd Cu Pb

R

KF (mmol kg
1)

n

R2

0.999 e e

46.4 ± 2.3 41.0 ± 2.8 1931.0 ± 630.0

0.60 ± 0.03 1.65 ± 0.33 2.39 ± 0.21

0.986 0.972 0.993

2

)

1.6 ± 0.1 e e

Table 5 Cd, Cu and Pb desorption in mmol kg
1 (and percentage between brackets) from wheat straw for different initial concentrations (C0) added in the non-competitive experiment. Average values for 3 replicates, with coefficients of variation always <5%. Cd

0.5 1.5 3.0 6.0

0.25 1.29 1.12 4.24

Cu

Pb

Desorption (5.5) (6.0) (5.9) (8.7)

0.62 1.66 3.13 6.62

(17.9) (14.5) (14.1) (13.5)

70

(4.0) (5.0) (3.2) (2.4)

Cu

50 Pb

40 30

20 10 0 0

1 2 3 4 5 Equilibrium concentration (mmol L-1)

Fig. 4. Sorption curves for Cd, Cu and Pb corresponding to the competitive experiment using wheat straw. Average values for 3 replicates, with coefficients of variation always <5%.

Cd

60 Sorption (mmol kg-1)

0.21 0.67 0.91 1.40

Cd

60

Sorption (mmol kg-1)

C0 mmol L
1

70

Cu

50 Pb

40 30

20 10 0 0

1

2 3 4 5 6 Concentration added (mmol L-1)

Fig. 3. Cd, Cu and Pb adsorbed (mmol kg
1) on wheat straw, in the competitive system, for different initial concentrations added (0, 0.5, 1.5, 3.0 and 6.0 mmol L
1). Average values for 3 replicates, with coefficients of variation always <5%.

Table 6 Cd, Cu and Pb adsorbed (%) on wheat straw in the competitive experiment for different initial concentrations added (C0) (0, 0.5, 1.5, 3.0 and 6.0 mmol L
1). Average values for 3 replicates, with coefficients of variation always <5%. C0

0.5

1.5

3.0

6.0

Sorption (%) Cd Cu Pb

91.5 67.4 94.8

85.8 80.8 98.2

68.4 81.9 99.3

23.4 76.5 96.0

indicating that they were more strongly retained. Comparing with the non-competitive experiment, percentage desorption decreased for Pb and for Cu in the competitive trial, whereas it was clearly higher for Cd in this competitive trial (especially when metal concentrations of 3 and 6 mmol L
1 were added), evidencing a pronounced effect of competition just in the case of Cd.

3.6. Implications and future research As regards future research, determining the effect of pore sizes, surface characteristics, surface area and other physical

characteristics of wheat straw on both adsorption and desorption can be very interesting. In addition, it should be taken into account that Mahmood-ul-Hassan et al. (2015) carried out kinetic studies showing rapid initial adsorption for Cd and Pb onto wheat straw, thus justifying further research using the stirred flow chamber technique in order to elucidate rapid kinetics in a more detailed
ndez-Calvin ~ o et al., 2016). In addition, future research way (Ferna focusing on the fractionation of the heavy metals retained would give details about the expected stability of the bindings responsible of retention processes. Fractionation of the retained metals would be also interesting in order to investigate effects of prolonged incubation times (day, weeks, and even one month or more) for wheat straw and for combinations of wheat straw and soils in contact with heavy metals, which could give information on what can be expected regarding metal retention/release in certain field situations, for example in vineyards where straw mulching is applied. Moreover, taking into account that some aspects (such as the effect of the pH value of the equilibrium solution that affect heavy metals retention/release on wheat straw) have been previously studied (Dang et al., 2009), it could be interesting to perform further research on the competitive effects of additional cations (and even certain anions), at equal and at different metal concentrations for bi-metallic and multi-metallic combinations. Finally, studying the retention/removal potential of combinations

Table 7 Cd, Cu and Pb desorption in mmol kg
1 (and percentage between brackets) from wheat straw for different initial concentrations (C0) added in the competitive experiment. Average values for 3 replicates ± standard deviation, with coefficients of variation always <5%. C0 mmol L
1

Cd

0.5 1.5 3.0 6.0

0.16 ± 0.007 (3.4) 0.97 ± 0.03 (7.5) 4.31 ± 0.21 (21.0) 11.95 ± 0.63 (86.1)

Cu

Pb

Desorption 0.60 1.00 2.23 4.04

± ± ± ±

0.02 0.04 0.16 0.29

(17.9) (8.3) (9.1) (8.8)

0.11 0.15 0.13 0.64

± ± ± ±

0.003 (2.3) 0.004 (1.0) 0.003 (0.5) 0.01 (1.1)

1502

G.F. Coelho et al. / Journal of Cleaner Production 139 (2016) 1496e1503

including wheat straw and other bio-sorbents (such as pine bark, mussel shell or wood ash) would be also of interests as regards added and/or complementary effects on the sorption/desorption of multi-element solutions. 4. Conclusions We investigated Cd, Cu and Pb competitive and non-competitive retention/release on wheat straw by means of batch-type experiments. In the non-competitive experiments, sorption was in the order Pb > Cd > Cu (92%, 90.6% and 69.9%) when the lowest molar concentrations (0.5 mmol L
1) were added, and Pb > Cu > Cd (96.1%, 81.8% and 81.5%) when the highest concentrations (6.0 mmol L
1) were added. Sorption data fitted well to the Langmuir model just for Cd, whereas all three metals showed good adjustment to the Freundlich equation. Desorption rates were low, in the sequence Pb < Cd < Cu. In the competitive experiment, sorption was in the order Pb > Cu > Cd, with the highest Pb and Cu sorption percentages (99.3% and 81.9%) corresponding to 3.0 mmol L
1 of each metal as initial concentration added. Competition clearly decreased Cd sorption, especially when the highest concentrations (6 mmol L
1) of the three heavy metals were added. Cd also showed the highest desorption percentage. Comparing competitive and non-competitive trials, Pb sorption was equivalent in both kind of experiments, Cu sorption was slightly higher in the non-competitive trial, and Cd sorption was clearly higher in the non-competitive experiment. In the competitive trial desorption was clearly higher for Cd, but decreased for Pb and for Cu. In view of that all, wheat straw could be used to remove Cd, Cu and Pb from polluted waters, as well as to retain these heavy metals (specially Cu and Pb) in vineyards, thus giving additional value to wheat straw mulching used to protect vineyards from erosion, which could contribute to sustainability in this productive sector. In fact, the results of this study could be of aid in the management of vineyards, taking into account that the retention of Cu and other heavy metals on wheat straw can facilitate the growth and development of new vine plants, as well as of other complementary vegetal covers (for example ryegrass) that may be also used to control erosion. Acknowledgements Funding: This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) [grant numbers CGL2012-36805-C02-01 and CGL2012-36805-C02-02]; and it was also partially financed by the European Regional Development Fund (ERDF) (FEDER in Spain). The research of Ms. Gustavo F. Coelho, and Dr. Affonso C. GonÇalves Jr. was also supported by the Improving Coordination of Senior Staff (CAPES) of the Brazilian Government. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2016.09.021. References

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