Low cost adsorbents have been extensively reported for use as a promising substitution for commercial adsorbents for pollutant removal in water and wastewater treatment. In this study, hydrochar produced from the hydrothermal carbonization (HTC) of faecal sludge (FS) (called HTC-hydrochar) was further chemically modified with KOH (called KOH-hydrochar) to improve its surface functional groups, which were suitable for copper (Cu) removal. The adsorption of Cu was conducted using the produced HTC-hydrochar and KOH-hydrochar as absorbents. Experimental results showed the KOH-hydrochar could adsorb Cu at the maximum adsorption capacity of 18.6 mg-Cu/g-hydrochar with Cu removal efficiency of 93%, relatively higher than the HTC-hydrochar and a commercial powdered activated carbon. The quantity of the surface functional groups of the adsorbents was more effective in Cu removal than the surface area. The Cu adsorption mechanism was found to follow the pseudo-second order and intra-particle diffusion models and fit well with Freundlich and Langmuir isotherms. Application of hydrothermal carbonization could be a novel candidate to convert FS into hydrochar which is pathogen free, and to employ the produced hydrochar as an adsorbent to remove Cu from industrial wastewaters.

INTRODUCTION

Many developing countries around the world are currently facing challenges from improper faecal sludge management (FSM), leading to health risks and contaminated water resources. Due to industrialization, several industries such as metal finishing, electroplating, plastics, pigments and mining can cause heavy metal contamination of water resources (Kolodynska et al. 2012). For example, copper (Cu) contamination in wastewater generated by electroplating and ornamental industries is about 20–100 mg/L, higher than the U.S. EPA effluent standard of 1.3 mg/L (Bilal et al. 2013). The toxicity and bioaccumulation impact of Cu on environmental ecology and human health are well documented (Keen et al. 2003; Solomon 2009).

Hydrothermal carbonization (HTC) is widely known as a thermochemical conversion process that can convert biomass in wet conditions at moderate temperatures (180–250 °C) into a carbonaceous material called ‘hydrochar’ (Libra et al. 2011). High moisture content biomass such as faecal sludge (FS), which is the accumulated sludge emptied from septic tanks, cesspools or pit latrines, can be converted into hydrochar (Fakkaew et al. 2015), which could be used as a solid fuel in combustion processes, adsorbents in water purification, soil conditioner in agriculture and energy storage for fuel cells and batteries (Titirici & Antonietti 2010; Libra et al. 2011; Koottatep et al. 2016).

Adsorption is one of the most effective and economic solutions for removing heavy metals from wastewater. Although activated carbon is typically used as an adsorbent, its main disadvantage is the high cost. Hydrochars containing high functional groups are promising candidates for use as cheap, sustainable and effective adsorbents for heavy metal and organic pollutant removal (Titirici 2013). Other adsorbents for heavy metal removal have been extensively studied and reported (Zhai et al. 2004; Kargi & Cikla 2006; Rozada et al. 2008; Varma et al. 2013), but there are only a few studies using hydrochar derived from sewage sludge, switchgrass, and peanut hull as an adsorbent for removing Cu and Cd from wastewaters (Regmi et al. 2012; Xue et al. 2012; Spataru 2014). Therefore, based on the FSM and heavy metal (Cu) problems mentioned earlier, the application of HTC could be a novel candidate to convert FS to hydrochar that is pathogen free and use the produced hydrochar as an adsorbent to remove Cu from industrial wastewaters.

The foremost objective of this study is to investigate Cu adsorption by using hydrochar produced from HTC of FS as an adsorbent. The specific objectives included: (1) to study the fundamental characteristics of the produced hydrochars, such as porosity, specific surface area, surface functional group and surface charge; (2) to investigate the Cu adsorption capacity and removal efficiency; and (3) to determine adsorption kinetics and isotherm models.

METHODS

Adsorbents

Adsorbents in this study included the hydrochar produced from HTC of FS (called HTC-hydrochar), hydrochar from chemical modification (called KOH-hydrochar) and powdered activated carbon (PAC) (AR Grade, S.D. Fine-Chem).

Hydrochar production

FS samples obtained from a municipal emptying truck which serviced Pathumtani municipality, a densely populated city located near Bangkok, Thailand, were used as feedstock to produce the hydrochar using a 1-L high-pressure stainless steel HTC reactor equipped with a pressure gauge, thermocouple, gas collection port, heater and cooling jacket, as illustrated in Figure 1. An electric heater equipped with a control panel (Figure 1(a)) was used to adjust the temperature and reaction time of the reactor. Each HTC experiment was performed with 350 mL of FS sample and the operating conditions were controlled at a temperature and reaction time of 200 °C and 5 h, respectively. At the end of each experiment, the reactor was cooled with water in a cooling jacket (Figure 1(b)) to quench the reactions. The carbonized FS remaining in the reactor was separated into solid (hydrochar) and liquid phases using vacuum filtration (Whatman filter paper, 1.2 μm). The produced hydrochar was subsequently dried in an oven at 105 °C for at least 12 h to remove the remaining moisture. The dried hydrochar (HTC-hydrochar) was either directly used as an adsorbent or processed further by chemical modification before being used for adsorption.
Figure 1

Schematic of HTC reactor: (a) heating system and (b) cooling jacket.

Figure 1

Schematic of HTC reactor: (a) heating system and (b) cooling jacket.

Chemical modification of hydrochar

The HTC-hydrochar was chemically modified by mixing 20 g of hydrochar with 3 mol/L KOH solution at a temperature of 60–70 °C for 1 h. The modified hydrochar (KOH-hydrochar) was filtered, washed with deionized water until the neutral pH condition was reached, and dried in an oven at a temperature of 105 °C for at least 12 h to remove the remaining moisture (Regmi et al. 2012; Spataru 2014; Sun et al. 2015; Yakout et al. 2015).

Adsorption experiments

Adsorption experiments were conducted in 50 mL centrifuge tubes, using the HTC-hydrochar, KOH-hydrochar and PAC as absorbents, at a temperature of about 27 °C. Each adsorbent was added into 30 mL synthetic wastewater containing 40 mg/L of Cu. To prevent precipitation, the pH of this solution was adjusted to be 5 using HCl and NaOH solutions. Each centrifuge tube of the adsorption sample was shaken at 200 rpm at the desired contact times (10–1,440 min) using an orbital shaker (Gemmy VRN200, Taiwan). After shaking, each adsorption sample was centrifuged at 5,000 rpm for 15 min, and the synthetic wastewater was filtered through a 0.45-μm nylon filter. The remaining Cu in the synthetic wastewater was measured using a flame atomic absorption spectrophotometer (iCE 3000 Series AA Spectrometer, Thermo Scientific, USA). Each adsorption experiment was replicated three times.

Analytical methods

Porosity analysis

Porosity characteristics of the adsorbents were analysed by nitrogen adsorption analysis at 77 K in a BELSORP-mini II volumetric adsorption analyser (BEL Japan Inc., Japan). The adsorbents were degassed for 2 h at 105 °C under vacuum condition to remove the residual moisture. The specific surface area was analysed by the Brunauer–Emmett–Teller (BET) method with adsorption isotherm data in a relative pressure (p/p0) range of 0.05–0.3. The pore size distribution analysis was analysed by the Barrett–Joyner–Halenda method with the adsorption and desorption branches of the isotherm.

Fourier transform infrared analysis

To analyse the surface functional group of the adsorbents, infrared spectra (4,000–400 cm−1) were recorded using an Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific, USA).

Surface charge

Surface charges of the HTC-hydrochar and KOH-hydrochar were analysed by the mass titration method (Schulthess & Sparks 1986). Ten mL of each hydrochar sample (10 g/L) were placed in 50 mL centrifuge tubes and varying amounts of 0.25 mol/L HCl and NaOH solutions (500, 400, 300, 200, 100, 50 and 10 μL) were added. The ionic strength of each solution was adjusted to be 0.01 mol/L by adding 0.1 mol/L NaCl electrolyte solution. At least one hydrochar sample was used as a control (without adding any acid or base solutions). Each sample was adjusted with triple-distillated water to a final volume of 30 m L, then shaken for 12 h using an orbital shaker and measured for pH. The surface charge (in meq/kg) of each sample was calculated according to the method of Schulthess & Sparks (1986).

Adsorption kinetics

Kinetics of Cu adsorption using the KOH-hydrochar were determined by the pseudo-first order (Equation (1)), pseudo-second order (Equation (2)) and intra-particle diffusion models (Equation (3)), which were expressed as (Buhani et al. 2010; Kolodynska et al. 2012): 
formula
1
 
formula
2
 
formula
3
 
formula
4
where qe is the mass of Cu adsorbed per unit mass of hydrochar at equilibrium contact time (mg/g), qt is the mass of Cu adsorbed per unit mass of hydrochar at contact time t (mg/g), k1 is the pseudo-first order rate constant, k2 is the pseudo-second order rate constant, ki is the intra-particle diffusion rate constant, C is the intercept of the intra-particle diffusion model, C0 is the concentration of Cu in the initial solution (mg/L), Ct is the concentration of Cu in the solution (mg/L) at contact time t (min), V is the volume of solution (L) and m is the mass of hydrochar (g). To determine the Cu removal efficiency and adsorption kinetics, the adsorption experiments were performed using the KOH-hydrochar dose of 2 g/L for removing Cu from the synthetic wastewater containing 40 mg/L of Cu. The qt values at the various contact times (10–1,440 min) can be calculated using Equation (4).

Adsorption isotherms

Freundlich and Langmuir isotherms were determined from Equations (5) and (6) (Reynolds & Richards 1996; Tchobanoglous et al. 2003), respectively, using the KOH-hydrochar as an adsorbent: 
formula
5
 
formula
6
where Kf is the Freundlich capacity factor, Ce is the equilibrium concentration of Cu in solution after adsorption (mg/L), n is the Freundlich intensity parameter, qm is the mass of adsorbed Cu required to saturate completely a unit mass of hydrochar (mg/g) and b is the experimental constant. The linear forms of Freundlich and Langmuir isotherms are represented as Equations (7) and (8), respectively. 
formula
7
 
formula
8

Adsorption experiments were conducted by varying the adsorbent doses at 0.25, 0.5, 1 and 2 g/L, operating at pH 5, initial Cu concentration of 40 mg/L and contact time of 1,440 min.

RESULTS AND DISCUSSION

Adsorbent characteristics

Porosity

Results of the BET surface area, total pore volume and pore diameter of the initial dried FS, HTC-hydrochar, KOH-hydrochar and PAC samples are shown in Table 1. It is apparent that the HTC process enhanced the BET surface area and total pore volume of the hydrochars, resulting in lower pore diameters of the HTC-hydrochar and KOH-hydrochar samples than the initial dried FS samples. Mean pore diameters of both hydrochars were found in the range of mesopores (Tchobanoglous et al. 2003), which can adsorb large size molecules such as sugar and heavy metals and small size molecules such as micro-pollutants. However, the BET surface areas of the hydrochars were still much lower than commercially activated carbon such as PAC.

Table 1

Porosity characteristics of initial dried FS and adsorbents

Samples BET surface area (m2/g) Total pore volume (cm3/g) Mean pore diameter (nm) 
Initial dried FS 1.07 0.010 38.7 
HTC-hydrochar 4.03 0.035 1.72 
KOH-hydrochar 4.41 0.049 1.84 
PAC 993 0.468 0.84 
Samples BET surface area (m2/g) Total pore volume (cm3/g) Mean pore diameter (nm) 
Initial dried FS 1.07 0.010 38.7 
HTC-hydrochar 4.03 0.035 1.72 
KOH-hydrochar 4.41 0.049 1.84 
PAC 993 0.468 0.84 

FTIR analysis

Functional groups such as O-H (hydroxyl), C = O (carbonyl and aldehyde) and C-O (carboxyl and ether) were observed on the surface of the HTC-hydrochar and KOH-hydrochar samples, as shown in Figure 2. This indicated that these functional groups resulted from the HTC process and remained mostly intact during chemical modification with KOH, relatively higher than those functional groups on the PAC surface.
Figure 2

FTIR spectrums of HTC-hydrochar, KOH-hydrochar and PAC.

Figure 2

FTIR spectrums of HTC-hydrochar, KOH-hydrochar and PAC.

Surface charge

As the results from the mass titration method show (Figure 3), at pH 5, high positive surface charges can be observed for the KOH-hydrochar (about 1,200 meq/kg), which was higher than those of the HTC-hydrochar (about 600 meq/kg) and PAC (about 400 meq/kg). It can be hypothesized that chemical modification of the hydrochar by KOH resulted in the production of a higher quantity of the functional groups on the KOH-hydrochar surface, which could react with cations (or protonation reactions) that increased the positive surface charges, similar to results obtained by Ntalikwa (2007).
Figure 3

Surface charges of HTC-hydrochar, KOH-hydrochar and PAC.

Figure 3

Surface charges of HTC-hydrochar, KOH-hydrochar and PAC.

Cu removal efficiency and kinetic models

Experimental results, shown in Figure 4, indicated that using KOH-hydrochar could effectively remove Cu from synthetic wastewater. A rapid adsorption can be observed during the first 10 min, followed by slow adsorption until equilibrium, suggesting two-step adsorption mechanisms such as: (1) film diffusion and (2) intra-particle diffusion and Cu adsorption. The KOH-hydrochar gave the highest Cu removal efficiency of 93%, while those of HTC-hydrochar and PAC were 38% and 18%, respectively. The maximum Cu adsorption capacity of the KOH-hydrochar was calculated to be 18.6 mg-Cu/g-hydrochar. These results suggested that the quantity of the surface functional groups of the adsorbents was more effective in Cu removal than the surface area. Likely, the low Cu removal efficiencies of PAC were observed because these experiments were not conducted at the optimum adsorption conditions of the PAC (pH 6.5, 500 mg/L of Cu and 10 g/L of adsorbent dose) suggested by Abudaia et al. (2013).
Figure 4

Cu removal efficiency of HTC-hydrochar, KOH-hydrochar and PAC as adsorbent in Cu adsorption at various contact times.

Figure 4

Cu removal efficiency of HTC-hydrochar, KOH-hydrochar and PAC as adsorbent in Cu adsorption at various contact times.

Due to the high Cu removal efficiency using KOH-hydrochar as an adsorbent, the Cu adsorption kinetics were determined. The kinetic parameters were obtained from the experiments and are listed in Table 2. It can be seen that the pseudo-second order and intra-particle diffusion models showed a high coefficient of determination (R2) which could adequately describe the kinetic of Cu adsorption by KOH-hydrochar.

Table 2

Kinetic models of Cu adsorption using modified hydrochar as absorbent

Pseudo-first order model Equation: y = 0.718–0.0016x 
 R2 = 0.6656 
 qe= 5.22 mg-Cu/g-hydrochar 
 k1= 0.0036 min−1 
Pseudo-second order model Equation: y = 0.0453x +0.6014 
R2 = 0.9997 
qe= 22.08 mg-Cu/g-hydrochar 
k2= 0.0034 g-hydrochar/mg-Cu.min 
Intra-particle diffusion model Equation: y = 0.1401x + 17.663 
R2 = 0.8225 
C= 17.66 mg-Cu/g-hydrochar 
ki= 0.1401 mg-Cu/g-hydrochar.min1/2 
Pseudo-first order model Equation: y = 0.718–0.0016x 
 R2 = 0.6656 
 qe= 5.22 mg-Cu/g-hydrochar 
 k1= 0.0036 min−1 
Pseudo-second order model Equation: y = 0.0453x +0.6014 
R2 = 0.9997 
qe= 22.08 mg-Cu/g-hydrochar 
k2= 0.0034 g-hydrochar/mg-Cu.min 
Intra-particle diffusion model Equation: y = 0.1401x + 17.663 
R2 = 0.8225 
C= 17.66 mg-Cu/g-hydrochar 
ki= 0.1401 mg-Cu/g-hydrochar.min1/2 

The pseudo-second order model indicates that the rate-limiting step may be chemical adsorption (Zhu et al. 2009). In this study, the adsorption of positively charged Cu ions (Cu2+) might occur via attraction to negatively charged surface functional groups such as O-H and C=O (FTIR analysis).

During the adsorption of Cu on the surface of KOH-hydrochar, the adsorption mechanism normally involves three steps: (1) film diffusion or boundary layer diffusion where the Cu2+ transfers from the bulk solution to the external surface of the KOH-hydrochar, which is the fastest step; (2) intra-particle diffusion, where the Cu2+ enters into the pores of the KOH-hydrochar by either pore diffusion or surface diffusion; and (3) Cu adsorption on the surface of the KOH-hydrochar (Kolodynska et al. 2012). The intra-particle diffusion model, as shown in Figure 5, illustrated a linear relationship not passing through the origin, which indicated that it was not the rate-limiting step for Cu adsorption by the KOH-hydrochar (Ali et al. 2016).
Figure 5

Intra-particle diffusion model of Cu adsorption using KOH-hydrochar as adsorbent.

Figure 5

Intra-particle diffusion model of Cu adsorption using KOH-hydrochar as adsorbent.

According to the results of adsorption by using PAC, due to the low positive surface charge (Figure 3), low Cu removal efficiencies were observed (Figure 4). These results suggested the adsorption mechanism of Cu on the surface of PAC were limited to the film and intra-particle diffusions, similar to the kinetic of adsorption suggested by Balakrishnan et al. (2010).

Adsorption isotherms

According to the literature, the Freundlich and Langmuir isotherms can well explain Cu adsorption by the activated carbon (Patnukao et al. 2008; Balakrishnan et al. 2010; Dowlatshahi et al. 2014). In this study, the experimental results for determining the Freundlich and Langmuir isotherms of Cu adsorption by KOH-hydrochar are shown in Figure 6, and the isotherm parameters are summarized in Table 3. Considering R2, both the Freundlich and Langmuir isotherms showed the linearized forms, which indicated these isotherms fit well with the experimental adsorption data. The maximum value of qm in the Freundlich isotherm, where Ce=C0= 40 mg/L, was calculated to be 34.2 mg-Cu/g-hydrochar, comparable to qm in the Langmuir isotherm (36.6 mg-Cu/g-hydrochar). According to the experimental results of Cu removal efficiency, the maximum capacity of the KOH-hydrochar was about 18.6 mg-Cu/g-hydrochar; therefore, the efficiency of this hydrochar for Cu adsorption could be further increased to achieve the qm values, such as by optimizing concentration of Cu, adsorbent dose, pH and contact time. As the n values in the range 2–10 represent a good adsorption characteristic (Ali et al. 2016), the large n value of 5.76 (Table 3) indicated that KOH-hydrochar is a good adsorbent for Cu adsorption.
Table 3

Summary of Freundlich and Langmuir isotherms

Freundlich Kf = 18.03 (mg-Cu/g-hydrochar)(L/mg-Cu)1/n 
n= 5.76 
R2 = 0.872 
Langmuir qm= 36.63 mg-Cu/g-hydrochar 
b= 0.31 L/mg-Cu 
R2 = 0.958 
Freundlich Kf = 18.03 (mg-Cu/g-hydrochar)(L/mg-Cu)1/n 
n= 5.76 
R2 = 0.872 
Langmuir qm= 36.63 mg-Cu/g-hydrochar 
b= 0.31 L/mg-Cu 
R2 = 0.958 
Figure 6

Freundlich and Langmuir isotherms of Cu adsorption using KOH-hydrochar as adsorbent.

Figure 6

Freundlich and Langmuir isotherms of Cu adsorption using KOH-hydrochar as adsorbent.

CONCLUSIONS

Experimental results obtained from this study showed the chemical modification of hydrochar by KOH, resulting in the production of a higher quantity of functional groups on the hydrochar surface. KOH-hydrochar has the maximum Cu adsorption capacity of 18.6 mg-Cu/g-hydrochar with Cu removal efficiency of 93%, which is relatively higher than HTC-hydrochar and commercial PAC. The quantity of the surface functional groups of the adsorbents was more effective in Cu removal than the surface area. Cu adsorption mechanisms could be described by the pseudo-second order and intra-particle diffusion models. The experimental adsorption data fit well with Freundlich and Langmuir isotherms, which indicated that the KOH-hydrochar is a good adsorbent for Cu adsorption. Application of the hydrochar as an absorbent for Cu removal could yield benefits for the treatment of both FS and Cu-containing wastewater.

ACKNOWLEDGEMENTS

This paper was based on the results of the project ‘Stimulating local innovation on sanitation for the urban poor in sub-Saharan Africa and Southeast Asia’ funded by the Bill & Melinda Gates Foundation.

REFERENCES

REFERENCES
Abudaia
J. A.
Sulyman
M. O.
Elazaby
K. Y.
Ben-Ali
S. M.
2013
Adsorption of Pb (II) and Cu (II) from aqueous solution onto activated carbon prepared from dates stones
.
International Journal of Environmental Science and Development
4
(
2
),
191
195
.
Balakrishnan
V.
Arivoli
S.
Begum
A. S.
Ahamed
A. J.
2010
Studies on the adsorption mechanism of Cu(II) ions by a new activated carbon
.
Journal of Chemical and Pharmaceutical Research
2
(
6
),
176
190
.
Bilal
M.
Shah
J. A.
Ashfaq
T.
Gardazi
S. M. H.
Tahir
A. A.
Pervez
A.
Haroon
H.
Mahmood
Q.
2013
Waste biomass adsorbents for copper removal from industrial wastewater-a review
.
Journal of Hazardous Materials
263
,
322
333
.
Dowlatshahi
S.
Torbati
A. R. H.
Loloei
M.
2014
Adsorption of copper, lead and cadmium from aqueous solutions by activated carbon prepared from saffron leaves
.
Environmental Health Engineering and Management Journal
1
(
1
),
37
44
.
Fakkaew
K.
Koottatep
T.
Pussayanavin
T.
Polprasert
C.
2015
Hydrochar production by hydrothermal carbonization of faecal sludge
.
Journal of Water, Sanitation and Hygiene for Development
5
(
3
),
439
447
.
Keen
C. L.
McArdle
H. J.
Ward
E. M.
2003
A Review: The Impact of Copper on Human Health
.
International Copper Association
,
New York
,
USA
.
Kolodynska
D.
Wnetrzak
R.
Leahy
J. J.
Hayes
M. H. B.
Kwapinski
W.
Hubicki
Z.
2012
Kinetic and adsorptive characterization of biochar in metal ions removal
.
Chemical Engineering Journal
197
,
295
305
.
Koottatep
T.
Fakkaew
K.
Tajai
N.
Pradeep
S. V.
Polprasert
C.
2016
Sludge stabilization and energy recovery by hydrothermal carbonization process
.
Renewable Energy
99
,
978
985
.
Libra
J. A.
Ro
K. S.
Kammann
C.
Funke
A.
Berge
N. D.
Neubauer
Y.
Titirici
M. M.
Fuhner
C.
Bens
O.
Kern
J.
Emmerich
K. H.
2011
Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis
.
Biofuels
2
(
1
),
89
124
.
Ntalikwa
J. W.
2007
Determination of surface charge density of α-alumina by acid-base titration
.
Bulletin of the Chemical Society of Ethiopia
21
(
1
),
117
128
.
Patnukao
P.
Kongsuwan
A.
Pavasant
P.
2008
Batch studies of adsorption of copper and lead on activated carbon from Eucalyptus camaldulensis Dehn. bark
.
Journal of Environmental Sciences
20
,
1028
1034
.
Regmi
P.
Moscoso
J. L. G.
Kumar
S.
Cao
X.
Mao
J.
Schafran
G.
2012
Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process
.
Journal of Environmental Management
109
,
61
69
.
Reynolds
T. D.
Richards
P. A.
1996
Unit Operations and Processes in Environmental Engineering
,
2nd edn
.
PWA Publishing
,
USA
.
Rozada
F.
Otero
M.
Morán
A.
García
A. I.
2008
Adsorption of heavy metals onto sewage sludge-derived materials
.
Bioresource Technology
99
,
6332
6338
.
Schulthess
C. P.
Sparks
D. L.
1986
Back titration technique for proton isotherm modeling of oxide surfaces
.
Soil Science Society of America Journal
50
,
1406
1411
.
Solomon
F.
2009
Impacts of copper on aquatic ecosystems and human health. Available at
:
Spataru
A.
2014
The use of Hydrochar as a low Cost Adsorbent for Heavy Metal and Phosphate Removal from Wastewater
.
Master thesis
,
UNESCO – IHE Institute for Water Education
,
Delft
,
the Netherlands
.
Tchobanoglous
G.
Burton
F. L.
Stensel
H. D.
2003
Wastewater Engineering: Treatment and Reuse
,
4th edn
.
McGraw-Hill
,
New York
,
USA
.
Titirici
M. M.
2013
Sustainable Carbon Materials from Hydrothermal Processes
.
John Wiley & Sons
,
Chichester
,
UK
.
Varma
G.
Singh
R. K.
Sahu
V.
2013
A comparative study on the removal of heavy metals by adsorption using fly ash and sludge: a review
.
International Journal of Application or Innovation in Engineering and Management
2
(
7
),
45
56
.
Yakout
S. M.
Daifullah
A. E. H. M.
El-Reefy
S. A.
2015
Pore structure characterization of chemically modified biochar derived from rice straw
.
Environmental Engineering and Management Journal
14
(
2
),
473
480
.
Zhai
Y.
Wei
X.
Zeng
G.
Zhang
D.
Chu
K.
2004
Study of adsorbent derived from sewage sludge for the removal of Cd2+, Ni2+ in aqueous solutions
.
Separation and Purification Technology
38
,
191
196
.