adsorption by monocarboxylic acid-modified microalgae residuals in

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Enhanced Hg(II) adsorption by monocarboxylic acid-modified microalgae residuals in simulated and practical industrial wastewater. Yang Peng, Xiangjiang Liu, Xun Gong, Xiaomin Li, Yifu Liu, Erwei Leng, and Yang Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03094 • Publication Date (Web): 01 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Title：：Enhanced Hg(II) adsorption by monocarboxylic acids modified microalgae residuals in simulated and practical industrial wastewater.

Author names and affiliations Yang Penga, Xiangjiang Liub, Xun Gonga,*, Xiaomin Lia, Yifu Liua, Erwei Lenga, Yang Zhanga,* a

State Key Laboratory of Coal Combustion, School of Energy and Power

Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, PR China b

College of Fisheries, Huazhong Agricultural University, No.1 Shizi Shan Street,

Wuhan 430074, PR China

*Corresponding author Tel: +86-27-87542417-8301, Fax: +86-27-87545526 (X. Gong) E-mail address: [email protected] (X. Gong), [email protected] (Y. Zhang)

ACS Paragon Plus Environment

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Abstract In this study, three kind of monocarboxylic acids, formic acid, acetic acid and propionic acid, are firstly applied to modified microalgae residuals (M1-RD, M2-RD, M3-RD, respectively) after lipid extraction, aiming to enhance the adsorption capacity and selective binding ability for Hg(II) ions in simulated wastewater. Effect of pH, temperature, initial Hg(II) concentration was detailed investigated to identify the optimum adsorption conditions. Batch adsorption tests showed that maximum adsorption efficiency obtained was 96.7% of M1-RD, 91.1% of M2-RD and 84.4% of M3-RD at pH 4.05, comparing to 48.5%, 57.6% of raw and residual microalgae at pH 5.01, respectively. The adsorption capacity of raw, residual and modified microalgae increased with the increase of temperature. Langmuir and Freundlich isotherms model tests showed the maximum equilibrium adsorption capacity among three modified adsorbents reached to 63 ± 3 mg/g of M1-RD, in contrast to 17 ± 1 mg/g and 25 ± 2 mg/g of raw and residual microalgae, respectively. The characterization of modified sample by FTIR, BET, SEM showed that the fibrous structure of the algae was decomposed by carboxyl groups in organic acids involved in microalgae, resulting in larger surface area and more binding sites. In consideration of ions interference in actual process, a kind of actual desulfurization wastewater from a 1000 MW coal fired power plant (Guangdong, China) was introduced in simulated adsorption system. Verification tests in desulfurization wastewater showed up to 96 % of Hg(II) ions was removed, probably due to co-precipitation of mercury and other coexisting ions.


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1. Introduction the emissions of Hg and its compounds are serious environmental issues because of high toxicity, global migration and enrichment in food chain all over the world. Various of physiological toxicity caused by mercury, such as reproductive disturbance, mental retardation, antibiotic resistance and other diseases have been confirmed over several decades1. Several advanced technologies, including ion-exchange, flocculation, precipitation and electric chemistry and so on, have been developed to overcome this problem. However, high costs and potential secondary pollutions are still great challenges for widely commercial utilization in industrial process2. Microalgae are promising biomass resources with rapid growth rate and high lipid content3 which can be applied for biodiesel production, hydrogen production and other bio-processing. As the representative of the 3rd generation feedstock of biodiesel, microalgae show extraordinary advantages for energy demands. However, the costs of thermal pretreatments and chemical transfer process onto microalgae for energy utilization are still fairly high, comparing with traditional fossil fuels4,

5

.

Cost-effectiveness is one of the most severe challenges to overcome for biodiesel production by microalgae6. One of the most potential approach to increase the economic feasibility is making full use of residual biomass after lipid extraction as a low-cost biosorbent for adsorption of heavy metals7. With the study of bioremediation mechanism for heavy metals, an amount of research on biosorption of heavy metals by microalgae based adsorbents have been published, by forms of active cells,


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inactive microalgae powders and residual biomass8, 9, 10. These microorganisms are reported to be efficient for binding varies of metal ions11, including Cd2+, Cu2+, Cr3+, Cr6+, Cr2O32-, Fe3+, Hg2+, Ni2+, Pb2+, Zn2+, on account of large amounts of specific functional groups out of the microalgae cells. It is feasible to overcome the heavy metals discharge, especially mercury pollution in industrial wastewater of coal combustion12, 13, 14, 15by microalgae residuals. However, low adsorption capacity and poor ability of interference rejection make it difficult to become an ideal absorbent material, comparing with commercial activated carbon and ion exchange resin. Further modifications of microalgae residual to enhance its adsorption capacity and adsorption stability in both simulated and industrial wastewater system are urgently required. Large amounts of literatures studies reported different chemical reagents were used in adsorbents preparation in last few decades16-19. Among them, H2SO420, HNO321, KMnO422, 23, MnOx24, KOH25, 26, NaOH27 and some metal salts28, 29 are the most widely used substances. However, there are still some shortages in chemical treatment, including the potential secondary pollution risk for corrosiveness and poisonousness properties come from the chemical reagents. A kind of environmentally friendly reagents are needed for biosorbent modification. Low molecular weight organic acids30 (LMWOAs), are a series of organic acids mainly derived from biomass and microorganisms’ decomposition, which was shown to be low toxic for surrounding. At the same time, the corrosiveness of LMWOAs are much lower than that of the inorganic acids. In this paper we propose the use of LMWOAs for


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modification of adsorbents. Few studies have demonstrated that certain organic acids are effective to improve the performance of adsorbents. Citric, tartaric, and acetic acids are proven to have significant impacts on methylene blue adsorption by biochar derived from eucalyptus saw dust31. It reported that citric acid modified corncob revealed higher capacity in Cd(II) adsorption32 than that modified with nitric acid, on account of carboxyl groups introduction33. Also, tartaric acid modified swede rape straw34, bagasse35 and rice husk36 were detailedly investigated in former references. These studies came to a similar conclusion that organic acids modified the raw feedstocks by means of carboxyl groups introduction onto biochar surface, which might be one significant factor for adsorption improvement. Meanwhile, no research on organic acid modification of microalgae residual were reported so far. Simultaneously, almost no accurate experimental data on the practical treatment effects of Hg(II) in industrial wastewater by algae based bio-sorbents were reported, which are need to be assessed furtherly. As a species of chlorophyte, Chlorella vulgaris (C. vulgaris) is a kind of single-cell autotrophic microscopic organisms with 2~10 µm diameter which reproduces asexually and rapidly. It is also considered as one of the most ideal microalgae for bioremediation of wastewater with a remarkable potential to entirely remove hazardous compounds in the aqueous system. In this work, three monocarboxylic acids, which are three of the most basic organic acids, formic acid, acetic acid and propionic acid were firstly applied in residual modification of C. vulgaris to remove Hg(II) in simulated wastewater. Moreover, a kind of actual wet


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flue gas desulfurization (WFGD) wastewater from a 1000 MW coal fired power plant was introduced to evaluate the adsorption performance and anti-interference ability of modified adsorbents in practical utilization. 2. Experimental section 2.1 Materials and adsorbents preparation A fresh kind of chlorophyte, Chlorella vulgaris, was obtained from College of Fisheries, Huazhong Agricultural University. The microalgae individuals were cultivated in BG11 medium at pH 7, 2000 lux light intensity, 25 ˚C for 20 days. After that, the cells were harvested by centrifugation and freeze drying. The dried feedstocks were soaked in by n-hexane (Aladdin Ltd., AR, China) at 85 ˚C for 48 h to completely extract the crude fat, using the Bligh-Dyer method37. Then, the extracted residuals were washed with deionized water until no extracting solvent remains, followed by freeze drying for 24 h. Thereafter, each 20 g dried algae residuals were impregnated in 1 mol/L prepared formic (M1-RD), acetic (M2-RD) and propionic (M3-RD) acid solution (Aladdin Ltd., AR, China), for 12 h at room temperature. After acid dipping, the residuals were separated and deionized water-washed up to pH of 6.5. Then, these three kind of acid modified residuals were freeze dried for testing. As comparison, dried microalgae feedstock (RAW) and unmodified residual (RD) were adopted in this study as well. The microtopography of prepared adsorbents were observed by a Scanning Electron Microscope (SEM) system, JSM-7100F (JOLE Co. Ltd, Japan). A VERTEX 70 (Bruker, German) Fourier Transform Infrared Spectroscopy (FTIR) system was employed to investigate the changes of functional


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groups during modification. In addition, surface characterization and microporous structure were tested by Brunauer, Emmett, Teller's (BET) analysis by ASAP 2460 (Micromeritics, USA). 2.2 Adsorption testing Batch adsorption testing was conducted in conical flasks which containing 10 mg each kind of prepared adsorbent and 30 mL Hg(II) ion solution of certain concentration (1, 2, 5, 10, 15, 20 mg/L) at adjusted pH value (2.10, 3.07, 4.07, 5.01, 6.06, 7.05), stirring at 150 rpm for 3 hours. Three adsorption temperature, 30, 40 and 50 ˚C was conducted for Adsorption experiments. Afterwards, the suspensions were subsequently separated at 1800 g for 15 min, and then the supernatant was filtered by a 0.45 microporous membrane filter for testing. With accurate dilution, the remaining Hg(II) ions in supernatant was determined by an Atomic Fluorescence Spectrophotometer (AFS) system (RGF6300, BOHUI Co. Ltd, China). Each case was tested for 3 times to ensure the accuracy. Mass balance of Hg in batch system was guaranteed to be more than 90%. The equilibrium adsorption capacity of adsorbents was calculated by the following formula: qe =

(C0 − Ce ) ⋅ V m

(1)

where qe is the equilibrium adsorption capacity in mg/g, Co and Ct is the initial and the equilibrium Hg(II) solution concentration in mg/L, respectively. V is the volume of Hg(II) solution in L and m is the mass weight of adsorbent in g. The adsorption efficiency was calculated by:


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η=

Co − Ce ×100% Co

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(2)

where η is the adsorption efficiency in %. 2.3 Adsorption test in WFGD wastewater A WFGD wastewater sample obtained from a typical 1000 MW coal-fired power plant in south China, was employed to assess the practical adsorption performance of organic acid modified algae residuals. Batch tests with similar conditions as above mentioned were conducted in this work. Full elements sweep by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (ELAN DRC-e, PerkinElmer Co. Ltd, USA) was applied to measure the concentration of metal species in wastewater. 3. Results and discussion 3.1 Characterization of modified adsorbents. To observe the surface morphology changes by organic acid at the 5 µm magnification scale, SEM images of microalgae feedstock (RAW), unmodified residual (RD) and three monocarboxylic acids modified residuals were obtained in this study, as shown in Fig. 1. From these images it can be clearly recognized that the organic acids obviously changed the microstructure of algae residuals, while no significant changes were observed after lipid extraction by n-hexane on the surface. This is because the extract can enter cellular tissue without damaging the cell wall38. Before acid modification, the unmodified residual, RD, shown in Fig. 1(b), was collapsed by the extraction of n-hexane on the surface. With the impregnation of formic, acetic and propionic acid, the extracellular fibrous structure and the


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polysaccharides was destructed gradually, these drastic changes formed abundant porous structures between intercellular spaces39. Meanwhile, the extent of modification varies from acid species. M1-RD, the sample modified by formic acid, exhibited more irregular pores and space than those of M2-RD as well as M3-RD. This might be caused by that smaller molecular mass and shorter molecular chain length in formic acid, which could provide more protonation, comparing with other two acids, acetic and propionic acid. The FTIR spectra are shown in Fig. 2, aiming to investigate the changes of functional groups by monocarboxylic acids modification. Besides, the major peaks associated with modification are signed in Table 1. For five tested adsorbents, RAW, RD, M1-RD, M2-RD and M3-RD, a broad peak at 3276 cm-1 for -NH stretching vibration of amide was clearly observed. Sharper peaks at 1625 cm-1 for C=C stretching vibration of amide II in M1-RD, M2-RD and M3-RD were obtained in spectrum, comparing with that in RAW and RD. This transformation might be caused by the conversion of mainly polysaccharides and proteins to carbon based biochar in the presence of organic acids40. Another new peak was obtained at 1520 cm-1 for C=O stretching vibration, which might be caused by the interaction (including combination or precipitation) of organic acids with remaining some basic groups, i.e. proteins and micro-molecule polypeptides after lipid extraction from algae biomass, which was confirmed in previous literatures41-42.. Similar changes can be observed at 1452 cm-1 and 1224 cm-1, which are associated with -OH bending in -COOH symmetrical stretching vibration and -COOH bending vibration, respectively.


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3.2 Effect of pH on raw and modified adsorbents for Hg(II) adsorption. It is widely reported that pH value has a significant impact on the metal adsorption. As is shown in Fig. 3, the adsorption efficiency of Hg(II) by various feedstock increased with an increasing pH value from 2 to 4. While from 5 to 7, optimal efficiency values of five adsorbents were achieved followed by a slightly decrease, it is probably due to the occurrence of Hg(OH)2 precipitation, which normally disrupts the interaction of ionic Hg with algae-based materials24. Both cell surface metal binding sites and metal chemistry are influenced by the pH in solution. At low pH, cell wall ligands are tightly associated with the ions H3O+ and restricted the approach of metal ions by means of the repulsive force. As the pH increased, more ligands such as carboxyl and amino functional groups would be exposed, carrying negative charges with attraction of metallic ions with positive charge on the residual surface to the benefit of adsorption11, 43. As a comparison, the monocarboxylic acid modified residuals showed higher adsorption efficiency than that of raw algae powders and crude residuals in all ranges of tested pH values. The peak values of Hg(II) adsorption efficiency by M1-RD, M2-RD and M3-RD are 96.7%, 91.1% and 84.4% at pH 4.05, comparing to 48.5%, 57.6% of raw and residual microalgae at pH 5.01, respectively. With larger surface area and pores structures, the modified residuals provide more sites for -COOH introduction. These occupation of negative charge groups greatly enhanced the negative potential on surface of residual particles, which would exhibit stronger affinity for cations44.


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3.3 Improvement of Hg(II) adsorption. 3.3.1. Adsorption kinetics Fig. 4 illustrates the adsorption capacity of Hg (II) adsorbed by these five prepared adsorbents, with the passage of the contact time (0-120 min). The amounts of adsorbed Hg(II) rapidly increased with time and then plateaued to equilibrium for those five adsorbents. Two classic kinetic models, pseudo-first-order (PFO) and pseudo-second-order (PSO)45 were applied in this study. The kinetics expressions are: dqt = k1 (qe − qt ) dt dqt = k2 ( qe − qt ) 2 dt

(3) (4)

Where qt (mgg-1) is the adsorption capacity at time t, qe (mgg-1) is the adsorption capacity at equilibrium, k1 (min-1), k2 (gmg-1 min-1) are the kinetic coefficients of PFO and PSO, respectively. From the simulated kinetic parameters obtained in Table 2, the Hg(II) adsorption kinetic processes of RAW, RD, M1-RD, M2-RD and M3-RD are better fitted by Pseudo-second-order, in which R2 are 0.984, 0.995, 0.998, 0.989 and 0.999, respectively. This model assumes that two reactions took place, a rapid first one and a slower second one46, and the domination of adsorption rate was more likely controlled by the chemical sorption47. By contrast, the adsorption rate of Hg(II) by RD, M1-RD, M2-RD and M3-RD, was faster in the initial stages than that of RAW. These results indicate that the high adsorption rate is kept by these modified residuals compared to the raw biomass, which is more extensively applicable to the high concentration of heavy metal ions contaminated wastewater.


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3.3.2 Adsorption isotherms To optimize the design of biosorption process, obtaining appropriate correlation of adsorption isothermal curve is necessary. Two isothermal models, Langmuir and Freundlich were applied in this work, aiming to evaluate the experimental equilibrium data. The equation of Langmuir and Freundlich are:

Qm K LCe 1 + K LCe

Langmuir:

qe =

Freundlich:

qe = K FCe1/ n

(5) (6)

Where Ce (mgL-1) is the equilibrium concentration of metal ions in adsorption system, Qm (mg/g) is the theoretical maximum adsorption capacity of adsorbent, KL (Lmg-1) and KF (Lng-1mg1-n) are Langmuir and Freundlich constants, respectively. The fitting curves of isothermal models are presented in Fig. 5, associated data are shown in Table 3. For all these five adsorbents, both Langmuir and Freundlich models fitted well for the experimental data, which R2 > 0.99. To evaluate the maximum adsorption capacity, Qm is an essential parameter. From the fitting data of Langmuir model, the Qm of M1-RD, M2-RD, M3-RD are 63 ± 3 mg/g, 42 ± 2 mg/g and 31 ± 4 mg/g, respectively, much higher than that of RAW and RD, 25 ± 2 mg/g and 17 ± 1 mg/g, separately. Moreover, The Qm of M1-RD exhibits higher than that of some reported modified bio-wastes and adsorbents, as shown in Table 4. This enhancement implies that the modification significantly improve the adsorption capacity, and formic acid is verified to be the best modifying agent in this work. 3.3.3 Adsorption thermodynamics Thermodynamic behavior of Hg(II) adsorption by algal biomass was studied by


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evaluation of thermodynamic parameters, free energy change (∆G˚, KJmol-1), enthalpy (∆H˚, KJmol-1) and entropy (∆S˚, Jmol-1K-1)48, the following equations are:

∆G o = − RT ln Ke, (Ke = qe / Ce ) ln Ke =

∆S o ∆H o − R RT

(7) (8)

Where R (8.314 J/molK) is the universal gas constant, T (K) is the adsorption temperature, Ke is the distribution coefficient. ∆G˚ can be calculated directly by equation (7), ∆H˚ and ∆S˚ can be obtained from the slopes and intercepts in the estimation curves in Fig. 6. The values of these parameters are presented in Table 4, ∆G˚ of five algal biomass are all found to be negative, which reveals that spontaneous sorption processes were conducted with increasing metal sorption at higher temperatures. While positive ∆H˚ values (between 10 and 30 KJmol-1 ) suggest that the sorption reactions are endothermic, supported by an increase in qe (mg/g) with increasing temperature39. It is observed that the modification enriched the affinity between Hg(II) ions and adsorbents by introduction of functional groups, due to the increasing of ∆H˚ and ∆S˚ values. Meanwhile, no significant changes on the adsorption type were observed after modification, which possibly indicated that the acid functional groups on the algae cells are the dominated binding sites for Hg(II) adsorption. 3.4 Modification improvement for porosity and surface characteristics The pore size distribution and surface characteristic are provided in Fig. 7 and Table 5, respectively. It can be observed that M1-RD, M2-RD, M3-RD adsorbent exhibits sharp pore volume peaks centered at around 3.14 nm, 4.74 nm and 6.68 nm,


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respectively, followed by uniformly decreasing pore volume distribution within the whole tested range of pore width, while no significant pore structures were measured in RD adsorbent. It indicates that the introduction of monocarboxylic acids largely completely changes the appearance and enriches the porosity of algae residuals. In addition, the modified adsorbent M1-RD possesses the highest specific surface area and pore volume with the value of 58.2 m2/g and 4.43E-2 cm3/g, in comparison to 3.0 m2/g and 1.70E-3 cm3/g in RD, 2.6 m2/g and 4.71E-3 cm3/g in RAW, respectively, which is consistent with the SEM observation. Meanwhile, the average particle size of M1-RD, M2-RD and M3-RD reveals finer size with the value of 0.10 µm, 0.09 µm and 0.11 µm, compared with the unmodified feedstocks, 2.17 µm and 2.03 µm of RD and RAW, respectively, which implies that more binding sites on the surface of M1-RD can be occupied during adsorption. 3.5 Practical performance of Hg(II) adsorption by raw and modified adsorbents in WFGD wastewater. In previous works31,

33, 34

, the adsorption of adsorbents in single-ions or

simulated multi-ions systems have been certified repeatedly, while few practical results have been reported about treatment capacity of adsorbents in actual industrial wastewater, especially in coal fired power plants. In this work, a WFGD wastewater sample was employed for the adsorption stability and persistence ability of modified algae biomass in Hg(II) removal. The element contents of the digested sample are shown in Table 6. From the multi-adsorption capacity showed in Fig. 8, the target ions, Hg(II) was removed thoroughly, approximate to 96% by M1-RD, 92% by M2-RD and


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90% by M3-RD comparing to that of RD, 58.8%. Moreover, other metal ions, including Ni, Zn, Cd, Al, Ti are significantly adsorbed by M1-RD, M2-RD and M3-RD whose capacity are still much higher than that of RD. On the other hand, some alkali and alkaline metals like K, Rb and Sr are significantly released during adsorption, which may be caused by the ion-exchanges49. The loss of P was observed in the adsorption process of these five biomass, which may due to the degradation of long-chain proteins and polysaccharides40.

Conclusion Series of tests demonstrates that modification of algae residual by three monocarboxylic acids, formic acid, acetic acid and propionic acid is an efficient method for the preparation of algae-based adsorbent on Hg(II) removal. Moreover, formic acid is certified to be the best agent in these three acids. The capacity of modified biomass is proven to be significantly improved mainly due to the micro-porosity improvement by organic acids modification. Moreover, interaction of carboxylic groups with proteins and polypeptides in residuals might increase the content of oxygen-containing functional groups, which probably attribute to the enhancement of adsorption. And the adsorption investigation in WFGD wastewater reveals that the modified algae residual is still high efficient in practical industrial wastewater for Hg(II) removal.

Acknowledge This research was supported by the National Natural Science Foundation of China (51661125011, 51776085) and the Foundation of State Key Laboratory of Coal


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Combustion (FSKLCCB1406). The authors are grateful to the Graduates' Innovation Fund and the Analytical and Testing Center at Huazhong University of Science and Technology.

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15. Zhang, B.; Xu, P.; Qiu, Y.; Yu, Q.; Ma, J.; Wu, H.; Luo, G.; Xu, M.; Yao, H., Increasing oxygen functional groups of activated carbon with non-thermal plasma to enhance mercury removal efficiency for flue gases. Chemical Engineering Journal

2015, 263, 1-8. 16. Zhou, Y.; Wang, X.; Zhang, M.; Jin, Q.; Gao, B.; Ma, T., Removal of Pb (II) and malachite green from aqueous solution by modified cellulose. Cellulose 2014, 21 (4), 2797-2809. 17. Zhou, Y.; Gao, B.; Zimmerman, A. R.; Cao, X., Biochar-supported zerovalent iron reclaims silver from aqueous solution to form antimicrobial nanocomposite. Chemosphere 2014, 117, 801-805. 18. Zhou, Y.; Jin, Q.; Hu, X.; Zhang, Q.; Ma, T., Heavy metal ions and organic dyes removal from water by cellulose modified with maleic anhydride. Journal of Materials Science 2012, 47 (12), 5019-5029. 19. Zhou, Y.; Jin, Q.; Zhu, T.; Akama, Y., Adsorption of chromium (VI) from aqueous solutions by cellulose modified with β-CD and quaternary ammonium groups. Journal of hazardous materials 2011, 187 (1), 303-310. 20. El-Sikaily, A.; El Nemr, A.; Khaled, A.; Abdelwehab, O., Removal of toxic chromium from wastewater using green alga Ulva lactuca and its activated carbon. Journal of hazardous materials 2007, 148 (1-2), 216-28. 21. Xu, H.; Shen, B.; Yuan, P.; Lu, F.; Tian, L.; Zhang, X., The adsorption mechanism of elemental mercury by HNO 3 -modified bamboo char. Fuel Processing Technology

2016, 154, 139-146.


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22. Tavakoli Foroushani, F.; Tavanai, H.; Hosseini, F. A., An investigation on the effect of KMnO4 on the pore characteristics of pistachio nut shell based activated carbon. Microporous and Mesoporous Materials 2016, 230, 39-48. 23. Wang, H.; Gao, B.; Wang, S.; Fang, J.; Xue, Y.; Yang, K., Removal of Pb(II), Cu(II), and Cd(II) from aqueous solutions by biochar derived from KMnO4 treated hickory wood. Bioresource technology 2015, 197, 356-62. 24. Li, B.; Yang, L.; Wang, C. Q.; Zhang, Q. P.; Liu, Q. C.; Li, Y. D.; Xiao, R., Adsorption of Cd(II) from aqueous solutions by rape straw biochar derived from different modification processes. Chemosphere 2017, 175, 332-340. 25. Dehkhoda, A. M.; Gyenge, E.; Ellis, N., A novel method to tailor the porous structure of KOH-activated biochar and its application in capacitive deionization and energy storage. Biomass and Bioenergy 2016, 87, 107-121. 26. Regmi, P.; Garcia Moscoso, J. L.; Kumar, S.; Cao, X.; Mao, J.; Schafran, G., Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process. Journal of environmental management 2012, 109, 61-9. 27. Preparation of activated carbons from Spanish anthracite II Activation by NaOH. Carbon 2001, 39, 751-759. 28. Dehkhoda, A. M.; Ellis, N.; Gyenge, E., Effect of activated biochar porous structure on the capacitive deionization of NaCl and ZnCl2 solutions. Microporous and Mesoporous Materials 2016, 224, 217-228. 29. Li, G.; Shen, B.; Li, Y.; Zhao, B.; Wang, F.; He, C.; Wang, Y.; Zhang, M.,


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Removal of element mercury by medicine residue derived biochars in presence of various gas compositions. Journal of hazardous materials 2015, 298, 162-9. 30. Sun, B.; Lian, F.; Bao, Q.; Liu, Z.; Song, Z.; Zhu, L., Impact of low molecular weight organic acids (LMWOAs) on biochar micropores and sorption properties for sulfamethoxazole. Environmental Pollution 2016, 214, 142-148. 31. Sun, L.; Chen, D.; Wan, S.; Yu, Z., Performance, kinetics, and equilibrium of methylene blue adsorption on biochar derived from eucalyptus saw dust modified with citric, tartaric, and acetic acids. Bioresource technology 2015, 198, 300-8. 32. Zhou, Y.; Hu, X.; Zhang, M.; Zhuo, X.; Niu, J., Preparation and characterization of modified cellulose for adsorption of Cd (II), Hg (II), and acid fuchsin from aqueous solutions. Industrial & Engineering Chemistry Research 2013, 52 (2), 876-884. 33. Leyva-Ramos, R.; Bernal-Jacome, L. A.; Acosta-Rodriguez, I., Adsorption of cadmium(II) from aqueous solution on natural and oxidized corncob. Separation & Purification Technology 2005, 45 (1), 41-49. 34. Feng, Y.; Zhou, H.; Liu, G.; Qiao, J.; Wang, J.; Lu, H.; Yang, L.; Wu, Y., Methylene blue adsorption onto swede rape straw ( Brassica napus L.) modified by tartaric acid: Equilibrium, kinetic and adsorption mechanisms. Bioresource technology 2012, 125 (12), 138-144. 35. Low, L. W.; Teng, T. T.; Rafatullah, M.; Morad, N.; Azahari, B., Adsorption Studies of Methylene Blue and Malachite Green From Aqueous Solutions by Pretreated Lignocellulosic Materials. Separation Science & Technology 2013, 48 (11), 1688-1698.


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36. Wong, K. K.; Lee, C. K.; Low, K. S.; Haron, M. J., Removal of Cu and Pb from electroplating wastewater using tartaric acid modified rice husk. Process Biochemistry

2003, 39 (4), 437-445. 37. Peng, Y.; Deng, A.; Gong, X.; Li, X.; Zhang, Y., Coupling process study of lipid production and mercury bioremediation by biomimetic mineralized microalgae. Bioresource technology 2017, 243, 628-633. 38. Safi, C.; Zebib, B.; Merah, O.; Pontalier, P.-Y.; Vaca-Garcia, C., Morphology, composition, production, processing and applications of Chlorella vulgaris: A review. Renewable and Sustainable Energy Reviews 2014, 35, 265-278. 39. Inyang, M. I.; Gao, B.; Yao, Y.; Xue, Y.; Zimmerman, A.; Mosa, A.; Pullammanappallil, P.; Ok, Y. S.; Cao, X., A review of biochar as a low-cost adsorbent for aqueous heavy metal removal. Critical Reviews in Environmental Science and Technology 2015, 46 (4), 406-433. 40. Nautiyal, P.; Subramanian, K. A.; Dastidar, M. G., Adsorptive removal of dye using biochar derived from residual algae after in-situ transesterification: Alternate use of waste of biodiesel industry. Journal of environmental management 2016, 182, 187-97. 41. Piyushi Nautiyal, K. A. S., M.G. Dastidar*, Adsorptive removal of dye using biochar derived from residual algae after in-situ transesterification: Alternate use of waste of biodiesel industry. Journal of environmental management 2016, 182, 187-197. 42. LIU Yuan, X. M.-x., KANG Juan, JIANG Min, Spectroscopic studies on the


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interaction of Human Serum Albumin with Low Molecular Organic Acids. Spectroscopy and Spectral Analysis 2004, 24 (11), 115-116. 43. Gupta, V. K.; Rastogi, A., Equilibrium and kinetic modelling of cadmium(II) biosorption by nonliving algal biomass Oedogonium sp. from aqueous phase. Journal of hazardous materials 2008, 153 (1-2), 759-66. 44. Houben, D.; Evrard, L.; Sonnet, P., Mobility, bioavailability and pH-dependent leaching of cadmium, zinc and lead in a contaminated soil amended with biochar. Chemosphere 2013, 92 (11), 1450-1457. 45. Lagergren, S., Zur theorie der sogenannten adsorption geloster stoffe. Kungliga Svenska Vetenskapsakademiens. Handlingar 1898, 24 (pp), 1-39. 46. Markou, G.; Mitrogiannis, D.; Çelekli, A.; Bozkurt, H.; Georgakakis, D.; Chrysikopoulos, C. V., Biosorption of Cu2+ and Ni2+ by Arthrospira platensis with different biochemical compositions. Chemical Engineering Journal 2015, 259, 806-813. 47. Sarı, A.; Uluozlü, Ö. D.; Tüzen, M., Equilibrium, thermodynamic and kinetic investigations on biosorption of arsenic from aqueous solution by algae (Maugeotia genuflexa) biomass. Chemical Engineering Journal 2011, 167 (1), 155-161. 48. Sari, A.; Tuzen, M., Equilibrium, thermodynamic and kinetic studies on aluminum biosorption from aqueous solution by brown algae (Padina pavonica) biomass. Journal of hazardous materials 2009, 171 (1-3), 973-9. 49. Davis, T. A.; Volesky, B.; Mucci, A., A review of the biochemistry of heavy metal biosorption by brown algae. Water Research 2003, 37 (18), 4311-4330.


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50. Kong, H.; He, J.; Gao, Y.; Wu, H.; Zhu, X., Cosorption of phenanthrene and mercury(II) from aqueous solution by soybean stalk-based biochar. J Agric Food Chem 2011, 59 (22), 12116-23. 51. Lohani, M. B.; Singh, A.; Rupainwar, D. C.; Dhar, D. N., Studies on efficiency of guava (Psidium guajava) bark as bioadsorbent for removal of Hg (II) from aqueous solutions. Journal of hazardous materials 2008, 159 (2-3), 626-9. 52. Tuzun, I.; Bayramoglu, G.; Yalcin, E.; Basaran, G.; Celik, G.; Arica, M. Y., Equilibrium and kinetic studies on biosorption of Hg(II), Cd(II) and Pb(II) ions onto microalgae Chlamydomonas reinhardtii. Journal of environmental management 2005, 77 (2), 85-92. 53. Sinha, A.; Pant, K. K.; Khare, S. K., Studies on mercury bioremediation by alginate immobilized mercury tolerant Bacillus cereus cells. International Biodeterioration & Biodegradation 2012, 71, 1-8. 54. Cataldo, S.; Gianguzza, A.; Pettignano, A.; Villaescusa, I., Mercury(II) removal from aqueous solution by sorption onto alginate, pectate and polygalacturonate calcium gel beads. A kinetic and speciation based equilibrium study. Reactive and Functional Polymers 2013, 73 (1), 207-217. 55. Zhang, Y.; Kogelnig, D.; Morgenbesser, C.; Stojanovic, A.; Jirsa, F.; Lichtscheidl-Schultz, I.; Krachler, R.; Li, Y.; Keppler, B. K., Preparation and characterization of immobilized [A336][MTBA] in PVA-alginate gel beads as novel solid-phase extractants for an efficient recovery of Hg (II) from aqueous solutions. Journal of hazardous materials 2011, 196, 201-9.


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56. Jia*, J. H. C. a. C. Q., Mercury Removal from Aqueous Solution Using Coke-Derived Sulfur-Impregnated Activated Carbons. Ind.eng.chem.res 2010, 49, 2716-2721. 57. Hadavifar, M.; Bahramifar, N.; Younesi, H.; Li, Q., Adsorption of mercury ions from synthetic and real wastewater aqueous solution by functionalized multi-walled carbon nanotube with both amino and thiolated groups. Chemical Engineering Journal 2014, 237, 217-228.


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Figure captions Fig. 1. SEM images of RD, M1-RD, M2-RD and M3-RD. Fig. 2. FTIR spectrum of RAW, RD, M1-RD, M2-RD and M3-RD. Fig. 3. Effect of pH for Hg(II) adsorption by RAW, RD, M1-RD, M2-RD and M3-RD.

Fig. 4. Kinetic models fitting for the Hg(II) adsorption of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; C(Hg) = 1mg/L; S/L ratio = 10 mg/30mL; T = 30 °C).

Fig. 5. Isothermal adsorption of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; time = 120 min; S/L = 10 mg/30 mL; T = 30 °C; ).

Fig. 6. Adsorption thermodynamics of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; time = 120 min; S/L = 10 mg/30 mL; C(Hg) = 1mg/L).

Fig. 7. Pore size distribution of RD and M1-RD, M2-RD, M3-RD. Fig. 8. Selective adsorption of Hg(II) in actual WFGD wastewater by RD and M1-RD, M2-RD, M3-RD (pH = 5; time = 120 min; S/L = 10 mg/ 30 mL; T = 30 °C).


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Fig. 1. SEM images of RAW, RD, M1-RD, M2-RD and M3-RD.


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Fig. 2. FTIR spectrum of RAW, RD, M1-RD, M2-RD and M3-RD.


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Fig. 3. Effect of pH for Hg(II) adsorption by RAW, RD, M1-RD, M2-RD and M3-RD.


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Fig. 4. Kinetic models fitting for the Hg(II) adsorption of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; C(Hg) = 1mg/L; S/L ratio = 10 mg/30mL; T = 30 °C).


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Fig. 5. Isothermal adsorption of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; time = 120 min; S/L = 10 mg/30mL; T = 30 °C; ).


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Fig. 6. Adsorption thermodynamics of RAW, RD, M1-RD, M2-RD and M3-RD (pH = 5; time = 120 min; S/L = 10 mg/30mL; C(Hg) = 1mg/L).


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7. Pore size distribution of RD and M1-RD, M2-RD, M3-RD.


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Fig. 8. Selective adsorption of Hg(II) in actual WFGD wastewater by RD and M1-RD, M2-RD, M3-RD (pH = 5; time = 120 min; S/L = 10 mg/ 30 mL; T = 30 °C).


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List of tables Table 1 The functional group in raw and modified adsorbents

No. 1 2 3

Wavenumber (cm-1) 3276 1625 1520

4

1452

5

1224

Functional group -NH stretching vibration of amide C=C stretching vibration of amide II C=O stretching vibration in -COOH -OH bending in -COOH symmetrical stretching vibration -COOH bending vibration


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Table 2 Estimated parameters for adsorption kinetic models Models

Pseudo-first-order

Pseudo-second-order

RAW

RD

Hg2+ M1-RD

qe(mgg-1)

0.49 ± 0.01

0.65 ± 0.01

0.83 ± 0.02

0.80 ± 0.03

0.82 ± 0.01

k1 (min-1)

0.29 ± 0.04

0.16 ± 0.01

0.23 ± 0.03

0.17 ± 0.03

0.14 ± 0.01

R2

0.979 ± 0.006

0.994 ± 0.003

0.978 ± 0.002

0.952 ± 0.004

0.987 ± 0.001

qe(mgg-1)

0.51 ± 0.01

0.70 ± 0.01

0.88 ± 0.01

0.87 ± 0.02

0.89 ± 0.01

k2 (gmg-1 min-1)

1.24 ± 0.03

0.37 ± 0.03

0.48 ± 0.04

0.31 ± 0.05

0.24 ± 0.01

R2

0.984 ± 0.004

0.995 ± 0.002

0.998 ± 0.001

0.989 ± 0.009

0.999 ± 0.001

Parameters


M2-RD

M3-RD

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Table 3 Estimated parameters for Langmuir and Freundlich isotherm adsorption models

RAW

RD

Hg2+ M1-RD

Qm(mg/g)

17 ± 1

25 ± 2

63 ± 3

42 ± 2

31 ± 4

KL(Lmg-1)

0.12 ± 0.01

0.03 ± 0.02

0.12 ± 0.02

0.16 ± 0.01

0.21 ± 0.01

R2

0.995 ± 0.001

0.992 ± 0.009

0.994 ± 0.002

0.997 ± 0.002

0.994 ± 0.004

1.0 ± 0.1

2.0 ± 0.2

6.5 ± 0.2

5.6 ± 0.2

5.3 ± 0.2

1/n

0.80 ± 0.03

0.67 ± 0.03

0.84 ± 0.04

0.78 ± 0.04

0.70 ± 0.03

R2

0.997 ± 0.009

0.993 ± 0.003

0.996 ± 0.006

0.995 ± 0.004

0.996 ± 0.007

Models

Parameters

Langmuir

Freundlich KF(Lng-1mg1-n)


M2-RD

M3-RD

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Table 4 The comparison of different adsorbents on Hg(II) adsorption. Capacity Materials

Species

pH

Authors (mg/g)

Soybean stalk based biochar

biomass

7.0

0.7

Kong et al.50

Guava bark

biomass

9.0

3.4

Lohani et al.51

Modified microalgae residuals biomass

5.0

62.5

This work

Chlamydomonas reinhardtii

biomass

6.0

72.2

M. Yakup Arıca et al.52

Immobilized Bacillus cereus

biomass

7.0

104.1

Arvind Sinha et al.53

Pectate beads

chemical

3.3

340.0

S. Cataldo et al.54

PVA/IL beads

chemical

5.8

49.9

Y. Zhang et al.55

Modified activated carbons

carbon

4.8

41.0

Jenny H. et al.56

Multi-walled carbon nanotube

carbon

6.0

84.7

M. Hadavifar et al.57


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Table 5 Estimated parameters for adsorption thermodynamics biomass

RAW

RD

M1-RD

M2-RD

M3-RD

Temperature (K)

△G0 (KJ mol-1)

303

-0.07 ± 0.05

313

-0.76 ± 0.03

323

-1.45 ± 0.01

303

-1.81 ± 0.02

313

-2.36 ± 0.05

323

-2.83 ± 0.02

303

-4.65 ± 0.07

313

-5.38 ± 0.02

323

-6.81 ± 0.01

303

-4.43 ± 0.08

313

-5.4 ± 0.1

323

-6.4 ± 0.2

303

-4.7 ± 0.1

313

-5.7 ± 0.2

323

-6.4 ± 0.2

△H0 (KJ mol-1)

△S0 (J mol-1 K-1)

Adsorption type

20.8 ± 0.1

68.9 ± 0.1

endothermic spontaneous

13.6 ± 0.8

51 ± 2


28 ± 3

106 ± 1


25.0 ± 0.4

97 ± 1


21 ± 2

85 ± 5



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Table 6 Particle size and porous characterization

RAW RD M1-RD M2-RD M3-RD

Average particle size (µm) 2.17 2.03 0.10 0.09 0.11

Surface Area (m2/g) 2.6 3.0 58.2 49.9 42.8

Pore Volume (cm3/g) 4.71E-3 1.70E-3 4.43E-2 4.12E-2 3.99E-2


Pore Size (nm) 18.0 10.4 4.5 5.9 7.1

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Table 7 The elemental characterization of WFGD wastewater ·Element Mg Al Si P S Cl K Ca Ti Mn Fe Ni Zn Se Br Rb Sr Cd I Hg U

Concentration (ppm) 33.465 0.035 3.188 0.006 404.053 1375.293 6.324 64.055 0.235 2.459 0.022 0.030 0.031 0.060 9.544 0.010 0.321 0.048 0.756 0.017 0.054


adsorption by monocarboxylic acid-modified microalgae residuals in

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