Effectiveness and mechanisms of mercury sorption by biochars from invasive Brazilian pepper at different temperatures

Xiaoling Dong1, Lena Q Ma1*, Yingjia Zhu1, Yuncong Li1, and Binhe Gu2

1. Soil and Water Science department1 University of Florida, Gainesville, FL 326112. South Florida Water Management District, West Palm Beach, FL 33411

Characteristics and mechanisms of mercury sorption by biochars from Brazilian pepper (BP; Schinus terebinthifolius) through pyrolysis at different temperature (300, 450, and 600˚C) were investigated. The surface characteristics of BP biochars before and after Hg(II) sorption were examined with scanning electron microscopy-equipped with energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The sorption process can be described by the pseudo-second order equation and Langmuir equation. The kinetic data suggested that Hg sorption was rapid initially fast and reaching equilibrium after 24 h. All biochars were effective in Hg sorption, with maximum sorption capacity being 28.6-41.0 mg/g and the sorption capacity decreasing as temperature increased. FTIR and modification results showed that carboxylic and amine groups were responsible for Hg sorption . Release of Ca2+, K+ and Na+ cations confirmed the role of ion exchange for Hg(II) removal by BP biochars. Our results suggest that BP biochars can be used as an alternative sorbent to remove Hg from wastewater.

Hg: one of the most toxic heavy metal in the environment. Brazilian pepper: the most aggressive, evergreen shrub-like tree in Florida. Now it covers more than 700,000 acres in south and central Florida, as well as many of the islands on the east and west coasts of the state. Biochar: refers to charcoal produced from carbon-rich biomass, shows great affinity for heavy metals.

Investigate the effectiveness of BP biochars in removing Hg from water via sorption/desorption studies. Evaluate the effects of various parameters including pH, initial Hg concentration, and contact time on its effectiveness.Determine the mechanisms governing Hg removal by BP biochars.

Table 1. Properties of Brazilian pepper biochar derived from different pyrolysis temperatures (300-600C).

Fig.2. Effect of reaction time on Hg(II) removal by BP biochars after reacting with 10 mg/L Hg(II) for 24 h at pH 6.0.

Biochar preparation and characteristicsBrazilian pepper (BP) biochar was made at specified temperatures 300, 450, and 600˚C in a muffle furnace. The surface morphology, point zero charge and metal composition were measured. Hg(II) sorption experimentBatch kinetic study, isotherm study , effect of pH were performed with sorbent dose of 2 g/L at room temperature.Fourier transform infrared (FTIR) analysis and Modification experimentFTIR analysis was conducted to identify the function groups involved in Hg(II) removal. Modification experiment was performed to confirm the role of functional groups involved in Hg(II) removal . Ca 2+, Na+, and K+ after Hg(II) sorptionThe concentration of cations released after Hg(II) sorption were analyzed to identify weather ion exchange participate in Hg(II) removal.

Fig.4. Fourier transform infrared spectra of BP biochars (a, b, c) before and after reaction with 10 mg/L Hg(II) for 24 h at pH 6.0.

Fig.3. Influence of pH on Hg(II) removal by BP biochars after reacting with 20 mg/L Hg(II) for 24 h.  

Fig.5. Ca2+, Na+, and K+ concentrations (a, b, c) in solution before and after Hg(II) sorption onto BP biochars .

The Hg(II) sorption by BP biochars was strongly pH-dependent. The higher the pH, the higher the removal efficiency. And the higher the pyrolysis temperature, the lower the removal efficient.

Hg(II) sorption by biochar can be described by the pseudo-second-order model and Langmuir model reasonably well, with sorption capacity being 28.6-41.0 mg/g .

FTIR analysis suggested that amine and carboxylic groups were involved in Hg(II) removal, and the modification experiment confirmed their participation.

Hg(II) removal was probably via ion exchange and complexation mechanisms.

Abstract

Introduction

Objectives

Materials and Methods

Results

Conclusions

AcknowledgementThis research is supported in part by TSTAR and University of Florida.

Hg(II) was sorbed onto BP biochars, and after 24 h sorption process reached equilibrium and the sorption capacity decreased with the pyrolysis temperature (Fig.2.). The efficiency of Hg(II) removal increased with the increase of pH (Fig.3). FTIR analysis suggested that amine and carboxylic groups were involved in Hg(II)

removal, and the modification experiment confirmed this result (Fig.3&Table 2). Ca2+ ,K+ and Na+ cations were released during Hg(II) removal by BP biochars and those cations concentration increased with the increase of Hg(II) concentration (Fig.5&Table 1).

Reference1. S.K. Das, A.R. Das, A.K. Guha, A Study on the Adsorption Mechanism of Mercury on

Aspergillus versicolor Biomass, Environmental Science & Technology, 41 (2007) 8281-8287.

2. J.-H. Yuan, R.-K. Xu, H. Zhang, The forms of alkalis in the biochar produced from crop residues at different temperatures, Bioresource Technology, 102 (2011) 3488-3497.

3. C. Jeon, K. Ha Park, Adsorption and desorption characteristics of mercury(II) ions using aminated chitosan bead, Water Research, 39 (2005) 3938-3944.

Mercury and cations analysisThe concentrations of mercury were analyzed by hydride generation atomic fluorescence spectrometry (HG-AFS). The concentrations of Ca2+, Na+, and K+ cations in the filtrate were measured by ICP-AES.

0

2

4

6

8

0 6 12 18 24 30 36 42 48

Hg Co

ncen

tratio

n (m

g/L)

time (h)

BP300

BP450

BP600

0

20

40

60

80

100

2 3 4 5 6 7 8

Hg re

mov

al (%

)

pH

BP300

BP450

BP600

(a)

(b)

(c)

Metals BP300

(mg/g)

BP450

(mg/g)

BP600

(mg/g)

Ca 2.56±0.14 12.3±0.45 19.3±0.47

K 0.66±0.03 1.80±0.06 2.21±0.06

Na 1.28±0.08 5.22±0.16 7.28±0.16

Mg 0.42±0.03 1.38±0.05 1.37±0.03

Al 0.09±0.001 0.13±0.003 0.10±0.007

Fe 0.09±0.005 0.24±0.003 0.39±0.008

Point zero charge 6.65 8.35 9.91

Particle size ≤ 106 μm

Table 2. Effect of modification on Hg(II) removal by BP biochars after reacting with 50 mg/L Hg(II) for 24 h at pH 6.0.

Hg concentration in the solution

BP300

(mg/L)

BP450

(mg/L)

BP600

(mg/L)

Original biochar 13.5 23 29.4

Formaldehyde fixation (Amine group ) 16.07 26.82 32.1

Methanol modification (Carboxylic group) 16.59 27.95 34.2

0

1

2

3

4

5

H2O 1ppm 10ppm 20ppm 50ppm 100ppm 150ppm 200ppm

K con

centra

tion (m

g/L)

Hg treatments

BP300

BP450

BP600

0

2

4

6

8

10

12

14

16

H2O 1ppm 10ppm 20ppm 50ppm 100ppm 150ppm 200ppm

Na co

ncentr

ation (m

g/L)

Hg concentration (mg/L)

BP300

BP450

BP600

0

5

10

15

20

25

30

35

H2O 1ppm 10ppm 20ppm 50ppm 100ppm 150ppm 200ppm

Ca con

centra

tion (m

g/L)

Hg treatments

BP300

BP450

BP600

(a) (b) (c)