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S U S TA I N A B L E P R O D U C T I O N A N D C O N S U M P T I O N 4 ( 2 0 1 5 ) 3 6 – 4 6

Contents lists available at ScienceDirect

Sustainable Production and Consumption

journal homepage: www.elsevier.com/locate/spc

Closed-loop fertility cycle: Realizing sustainability in

sanitation and agricultural production through the design

and implementation of nutrient recovery systems for

human urine

M. Ganesapillaia,b, Prithvi Simhac,d, A. Zabaniotoub,∗

a Mass Transfer Laboratory, Chemical Engineering Division, School of Mechanical and Building Sciences (SMBS), VIT University,

Vellore 632014, Indiab Chemical Engineering Department, Faculty of Engineering, Aristotle University of Thessaloniki, University Campus, GR 541 24,

Thessaloniki, Greecec Department of Environmental Sciences and Policy, Central European University, Nádor u. 9,

1051 Budapest, Hungaryd School of Earth, Atmospheric and Environmental Sciences (SEAES), The University of Manchester, Oxford Road, Manchester, M13 9PL,

United Kingdom

A B S T R A C T

As a solution to the drawbacks of modern sanitation systems and to shift towards a recycling society, source

separation of human wastes coupled with resource recovery could be seen as a potential solution. In this research,microwave activated coconut shells were utilized to recover urea from human urine. Batch adsorption studies

were carried out to determine the effect of initial concentration, adsorption temperature, microwave output power

and irradiation time on the urea uptake capacity of the tailored activated carbon. The shells pretreated with

microwave irradiation of 360 W, for 15 min (MACCS-360W-15) shown to be promising adsorbents with BET surface

area > 1000 m2 g −1. The sorption data were tested against different isotherm models and found to closely follow

Langmuir isotherm with a maximum monolayer sorption capacity of 312 mg g −1. Kinetic data over temperature

range of 30–60 ◦C was found to closely follow pseudo-first-order at all adsorbate concentrations. Gibbs free energy

(∆G◦), enthalpy (∆H◦) and entropy (∆S◦) indicated the spontaneity and physical nature of the sorption; sorption

experiments indicated a urea recovery of ∼95% from urine. Finally, the application of urea adsorbed carbon as a soil

conditioner in field trials resulted in significant improvement in the number of seed germination and plant biomass

(132%) with a substantial increase in the soil nitrogen and cation exchange capacity. Results also indicated that

nearly all the urea was desorbed within the soil during irrigation becoming readily available to the plants. This study

demonstrates a closed-loop sanitation cycle that channels nutrients from human beings back to agricultural fields.

Keywords: Urine; Urea; Activated carbon; Sustainable sanitation; Resource recovery

c⃝ 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +30 2310 996274; fax: +30 2310 996209.

E-mail address: [email protected] (A. Zabaniotou).Received 27 April 2015; Received in revised form 19 July 2015; Accepted 12 August 2015; Published online 20 August 2015.

http://dx.doi.org/10.1016/j.spc.2015.08.0042352-5509/ c⃝ 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.spc.2015.08.004


http://www.elsevier.com/locate/spc


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S U S TA I N A B L E P R O D U C T I O N A N D C O N S U M P T I O N 4 ( 2 0 1 5 ) 3 6 – 4 6 37

1. Introduction

The drawback of ‘modern’ sanitation systems is that it uses

freshwater (up to 50,000 liters per person annually), a scare

resource, as a transport medium and sink for waste dis-

posal (Esrey et al., 1998). Sanitation systems are based on the

premise that excreta are wastes that mandate disposal and

operate on the assumption that the environment can safelyassimilate them. Human appropriation of water for use in

sewage systems has deemed a significant portion of freshwa-

ter unusable, unavailable or raised the cost of down-the-pipe

wastewater treatment (Palaniappan et al., 2010; Gleick, 2014).

This continued improper handling of wastewater streams has

dramatically altered the fluxes of growth-limiting nutrients,

resulting in accelerated eutrophication of freshwater ecosys-

tems (Dupas et al., 2015; Humphrey et al., 2015). Moreover,

moving towards to agree upon and adopt a post-2015 devel-

opmental agenda, it is still evident that a substantial fraction

of our society still lacks access to adequate water and sani-

tation. Climate change adds another complexity to this dis-

cussion, as several projections point towards an increase inthe number of people living under the conditions of absolute

water scarcity (Haddeland et al., 2014; Martens, 2014).

Consequently, as Esrey et al. (1998) points out, the tech-

nological developments that were once designed to solve the

sanitation problem have become part of the problem; in the

discussions of most issues, it has become an obligatory im-

perative to incorporate sustainable development as a crite-

rion irrespective of its vagueness or even triviality. However,

it is in the discourse surrounding the conceptualization of

modern sanitation systems, where the notion of sustainabil-

ity finds most appropriateness. This is emphasized in the

findings of the UN Task Force 7 which found inherent link-

ages between water and sanitation to other critical issues likefood security, environmental protection and women empow-

erment (Kvarnström et al., 2006). To this effect, the concept of

source separation has been put forward as an integrated solu-

tion. Though the notion of source-separating human wastes

is not new (Larsen and Gujer, 1996), of late, there has been re-

newed interest in promoting innovative processes designed

specifically to recover resources with economic value from

wastewater streams (Ganrot et al., 2007; Wilsenach et al.,

2007; Tilmans et al., 2014; Matassa et al., 2015; Ganesapillai

et al., 2015). The objective of such processes is to address the

limitations of conventional sewage systems that flush away

valuable resources, especially nutrients that could potentially

be used in agricultural production (Langergraber and Muelleg-ger, 2005). As Smit and Nasr (1992) argue, it is imperative that

new closed-loop sanitation systems should be envisioned

that promote a circular flow of nutrient and blur definitions of

‘wastes’ and ‘resources’. In order to usher in such a paradig-

matic change and a new philosophy in handling wastewater,

research effort must be directed towards devising processes

that demonstrate the potential benefits of resource recovery.

The human body retains a very small percentage of

nutrients that enter it; thus, at fairly constant rate, nutrients

enter into and leave the body ( Jönsson et al., 2004). Thus,

the human excrement represents a valuable source of plant-

building nutrients. One way to promote sustainability in

sanitation and subsequently, in water usage and agricultural

production could be to recover and channel such valuable

nutrients to agro-ecosystems which in essence, is the goal

of a ‘closed-loop fertility cycle’ (Langergraber and Muellegger,

2005). Human urine is one such wastewater stream that has

found large-scale application in supplementing agricultural

productivity as a liquid fertilizer (Stenström, 2004; Beler-

Baykal et al., 2011). Source separation of urine has been

made possible through the use of Urine Diversion Dry

Toilets (UDDTs) that utilize the anatomy of human body that

separately excretes urine and feces.

The use of UDDTs coupled with large-scale urine diversion

mainstreaming has made the use of urine in agriculture a

good venture with a number of projects successfully imple-

menting this approach (Lienert, 2013; Fam and Mitchell, 2012;

Wood et al., 2015). However, there are several documented

shortcomings in the direct application of urine as a liquid fer-

tilizer (Heinonen-Tanski et al., 2007) along with the cultural

and ethical prejudices against its use (Drangert, 1998; Jönsson

et al., 2004). To this effect, the conceptualization of a nutri-

ent recovery system that allows removal and recovery of re-

sources from urine for subsequent re-use on arable land could

be seen as a potential win–win scenario.

Currently, the loss of nutrients on farms is compensated

by the application of commercially produced, fossil fuel-

intensive synthetic fertilizers (Vinnerås, 2002). To move

towards the goal of ef ficiency in use of our limited resources,a change in the existing dynamics can be effected through the

design and implementation of resource recovery processes

that allow recycling of nutrients from humans to farmlands

where they act as soil supplements. In our previous work, a

process to this effect was demonstrated that recovers ∼70%

of urea from human urine through bio-sorption onto coconut

shells based activated carbon (Ganesapillai et al., 2014).

In the present investigation, an initiative to improve the

ef ficiency of urea recovery has been undertaken through

the incorporation of a wider range of process parameters.

To improve the ef ficacy of the sorbent, the effect of

activation parameters (microwave power and exposure time)

were investigated during the preparation of microwaveactivated carbonized coconut shells. Following the textural

characterization, the effect of various process parameters

was examined to evaluate the urea uptake capacity. Finally,

the effect of urea adsorbed carbon as a soil conditioner was

analyzed on the plant test species (Vigna mungo and Vigna

radiata).

2. Methods and materials

2.1. Adsorbate and adsorbent pre-treatment and prepara-

tion

Coconut shells obtained locally in Vellore, Tamil Nadu, India,

were washed with distilled water, dried at 105 ◦C for 24 h

and crushed to a size of 1–2 cm. The shells were then

subjected to different microwave output power (180–600 W) at

varying exposure times (5–20 min) in a domestic microwave

(CE104VD-Samsung, Malaysia). Subsequently, they were

carbonized at 500 ◦C for 2 h (heating rate of 24 ◦C min−1)

in an industrial high temperature furnace (T-14/HTF-1400—

Technico, India). The carbonized shells were then ground

using a mortar, sieved to 100 mesh (0.149 mm) and stored

in air tight bottles. The activated carbon thus prepared

was abbreviated as Microwave Activated Carbonized Coconut

Shells (MACCS). All chemicals (analytical grade) used in thestudy were purchased from Nice Chemicals private limited,

Cochin, India and used without further purification.

Human urine was obtained from randomly selected

twenty healthy young male volunteers of early twenties and



38 S U S TA I N A B L E P R O D U C T I O N A N D C O N S U M P T I O N 4 ( 2 0 1 5 ) 3 6 – 4 6

Table 1 – Characterization of major constituents andtheir concentration in the collected urine samples.

Constituents Concentration (mgL−1)

Urea 19 850

Creatinine 985

Chlorides 5 955

Sodium 3 185

Potassium 1 488Sulfates 805

Phosphates 689

Ammonium 518

Total solutes 33 475

with well-balanced diets. Urine was characterized to calculate

its major constituents and results have been summarized

in Table 1. The urine samples were collected in air-tight

containers and stored at −20 ◦C. Later, the samples were

thawed prior to the sorption experiments.

2.2. Adsorbent characterization

The N2 adsorption isotherms of the prepared activated car-

bon (outgassed at 300 ◦C for 2 h under constant N2 flow)

were measured at 77 K (Micromeritics ASAP 2020). The sur-

face area was calculated using the Brunauer Emmett Teller

(BET) method and pore volume was estimated using the liq-

uid volume of N2 adsorbed at a relative pressure of 0.98 (Yang

et al., 2015). The surface texture and the development of pores

in the activated carbon were analyzed using scanning elec-

tron microscopy (FE-SEM, SUPRA 55, Carl Zeiss) with a 20 kV

electron source. Fourier Transform Infra-Red spectroscopy (IR

Af finity-1, Shimadzu) was performed to detect the surface or-

ganic structures at 4 cm−1 of resolution between 4000 and

400 cm−

1. The pH of the adsorbent was measured as men-tioned by Cheng and Lehmann (2009).

2.3. Adsorption experiments

25 ml of urine in required urea concentration (25%–100%)

was prepared in Erlenmeyer flasks (250 ml). The effect of

microwave output power (100–600 W), microwave exposure

time (5–20 min) and sorption temperature (30–60 ◦C) on the

urea uptake capacity was examined for a fixed adsorbent

loading of 1.5 g, shaker speed of 100 rpm and adsorption

time of 0–360 min. At different time intervals, samples were

withdrawn from the flask, filtered through a 0.45 µm sy-

ringe filter and analyzed using UV–Vis Spectrophotometry(Shimadzu UV-1601, Japan) at 430 nm to get the amount of

unadsorbed urea and hence, to estimate the urea uptake ca-

pacity of MACCS (With et al., 1961). The urea uptake capacity

(qe, mgg −1) was evaluated as mentioned in Eq. (1).

qe =

(Co − Ct)V

W

(1)

where, Co is the initial liquid phase concentration (mgL−1); Ctis the liquid-phase concentrations of urea at time ‘t’ (mgL−1),

V is the volume of the solution (L) and W is the mass of dry

adsorbent, MACCS (g).

2.4. Crop studies with urea adsorbed carbon applicationas soil amendment

Crop trials were performed at the VIT University Research

Farm, Vellore, India (12◦55′12.79′′N–79◦7′59.9′′E; 216 AMSL)

with urea adsorbed carbon application as soil amendment.

The soil composed of 4.35% gravel, 92.85% sand and 2.8%

fines. The average sunshine duration of Vellore in the period

of the study between April and July 2014, was 12–13 h day−1

and the temperature fluctuated between 24 ◦C and 35 ◦C with

an average of 30 ◦C. The pH and electrical conductivity of the

soil was found to be 8.0 and 385 µS cm−1 respectively. The

average moisture retention of the soil (0–10 cm) was 26.4%and 14.5% (w/w) at 0.03 and 1.5 MPa suction, respectively. The

lentils, Vigna mungo and Vigna radiata, two of most important

pulses grown in the Indian subcontinent were chosen as the

plant test species. A composite soil samples was collected

from depth up to 0.2 m from the surface and sieved through

a 6 mm aperture. Trays (0.4 m × 0.4 m × 0.1 m; L ×W × H)

were filled with air-dried soil and studied with 8 replications

to ensure consistency in experimental observations.

The urea adsorbed activated carbon was utilized as the soil

supplement. The trays were arranged as six separate arrays,

each comprising of eight replicates of the six soil treatments,

on a grid containing 4 rows by 12 columns. Within each array,

the soil treatments were allocated by a randomized complete

block design, so that groups of six trays within each row

comprised one replicate. Twenty seeds were sowed in each

tray at a depth of 2 cm.

All trays were regularly watered throughout the experi-

ments. After completion of the crop studies, the soil from

each tray was collected, air dried at 40 ◦C and crushed to

pass through a 2 mm sieve. The soil samples were analyzed

for total carbon, total nitrogen, and exchangeable cations; pH

was measured in a 0.01 M CaCl2 (1:5) extract according to the

method 4B2 of Rayment and Higginson (1992). Total carbon

and nitrogen were measured using the Dumas combustion

method at 900 ◦C with an oxygen flow rate of 125 ml min−1.

Prior to the crop trials, the unamended soil or ‘control’ was

also characterized for each of these parameters.

3. Results and discussion

3.1. Characteristics of microwave activated carbon

Coconut shells were activated at 360 W but for varied

exposure times (5–20 min). An irradiation time of 15 min at

360 W resulted in the highest surface area (1032 m2 g −1).

Thus, MACCS-360W-15 was chosen as the adsorbent and

utilized throughout the investigation. The internal surface

area development of the activated carbon as analyzed using the N2 adsorption and the Brunauer Emmet Teller (BET)

method arepresented in Table 2. It is evident from Table 2 that

both activation parameters, the microwave output power and

irradiation time had a significant effect on the development

of porosity and followed a similar trend. Through the analysis

of the trend on variation of output power it was evident that

360 W was ideal for activating the coconut shells.

The SEM micrograph of urea sorption onto the adsorbent is

presented in Fig. 1. The precursor, raw coconut shells showed

poor grain structure with little porosity (Fig. 1(a)). However,

following the microwave activation (360 W) and carbonization

(500 ◦C), high mesoporosity with pore sizes ranging from 10

to 60 µm was noticeable (Fig. 1(b)). Heat generation during ac-

tivation and subsequent carbonization may have caused the

conversion of cellulose, hemicellulose and lignin in the co-

conut shells through the removal of volatile organics, dehy-

dration, linkage breaking reactions, and structural ordering of




Table 2 – Textual characteristics and surface properties of the carbon samples.

Adsorbent Preparation conditions SBET (m2 g −1) V T (cm3 g −1) Moisture (%) Yield (%)

Effect of microwave output power (MW irradiation time of 10 min)

MACCS-100W MW irradiation at 100 W 596 0.1735 5.11 32.5





Effect of microwave irradiation time (MW output power of 360 W)

MACCS-360W-5 MW irradiation time of 05 min 678 0.1957 4.57 30.9




SBET —BET surface area; V T —Pore volume.

Fig. 1(a) – SEM image of raw coconut shells (20 kV,

51.04 KX).

Fig. 1(b) – SEM image of urea adsorbed MACCS-360W-15surface (20 kV, 14.68 KX).

the residual carbon (Hadi et al., 2015).The micrograph of urea

molecule following adsorption is as presented in Fig. 1(c). It is

evident that the high porosity and large internal surface area

was favorable for the urea sorption.

The FTIR spectra of un-adsorbed and urea adsorbed

MACCS-360W-10 and MACCS-360W-15 have been compared

(Figs. 2 and 3). The major IR peaks for both the curves seem to

match well, with dominant peaks being 3444 (O–H stretch),

1630 (C=C stretch), 1400 (C–C aromatic stretch), 1080 (C–N)

and 530 cm−1 (C–X bend). It is apparent that the application of

higher microwave output power and longer irradiation timeduring the pretreatment of the adsorbent caused significant

modifications in its surface functional groups. Moreover, the

peaks noticed at 3240.41, 2927.94 and 2854.65 cm−1 indicate

the presence of ammonium ions (possibly as a result of urea

Fig. 1(c) – SEM image of urea molecule adsorbed on

MACCS-360W-15 surface (20 kV, 82.47 KX).

hydrolysis) on the adsorbent surface (Fig. 3(b)). Similarly, mul-

tiple peaks in Fig. 2(b) over a broad range of 2800–3200 cm−1

(ammonium ions or N–H bend) is indicative of the urea sorp-

tion onto the adsorbent. In addition, overlapped peaks at

1020–1000 cm−1 also indicate a C–N stretch.

3.2. Effect of process parameters on urea uptake capacity

of activated carbons

3.2.1. Effect of microwave output power

The adsorption capacity versus time curve for urea

adsorption as influenced by microwave output power is

shown in Fig. 4. The sorption data clearly indicates a non-

linear rise in urea uptake by the adsorbent with increase in

microwave output power from 100 to 450 W. This increase inadsorption can be attributed to ef ficacy of microwave energy

to cause partial cell disconnection, collapse of cell structures

and violent removal of volatiles (Ganesapillai et al., 2009).

Moreover this creates more porosity in the adsorbent, which

is the key factor in improving the sorption capacity ( Yang

et al., 2010). Increasing the microwave output power from 360

to 450 W did not produce a proportionate rise. Moreover, it

was observed that very high output power (600 W) lead to

poor urea uptake; this may be due to the collapse of ringed

structures in the ligno-cellulosic coconut shells (Ahmed and

Theydan, 2014). A substantial increase (∼70%) in the sorption

capacity (180 to 305 mgg −1) was observed as the microwave

output power was varied from 100 to 450 W.

3.2.2. Effect of microwave irradiation time

Following the experiments on the effect of microwave power

on sorption capacity, the MACCS-360W was chosen as the




4000 3000 2000 1500 1000 500

1/cm

%T

a

b

Fig. 2 – FT-IR image of coconut shells (a) un-adsorbed, (b)

adsorbed MACCS-360W-10.

4000 3000 2000 1500 1000 500

1/cm

%T

a

b

Fig. 3 – FT-IR image of coconut shells (a) un-adsorbed, (b)

adsorbed MACCS-360W-15.

Fig. 4 – Effect of microwave output power on the urea

uptake capacity of activated carbon (100% urine solution,

25 ml adsorbate, 1.5 g adsorbent loading, 30 ◦C ).

adsorbent to investigate the effect of irradiation time under

the process conditions: adsorbate concentration — 100%,

carbon loading — 1.5 g, 100 rpm at room temperature (30 ◦C).

Fig. 5 – Effect of microwave irradiation time on

MACCS-360W urea uptake capacity (100%: 25 ml, 1.5 g

adsorbent loading, 100 rpm and 30 ◦C ).

As seen in Fig. 5, the adsorption capacity of MACCS-360Wincreased by 57% (210–330 mgg −1) on increasing the exposure

time of the samples from 5 to 15 min. As corroborated

in Table 2, microwave pre-treatment caused significant

changes in textural properties of the carbon and resulted

into enhanced urea uptake. Microwave heating provided both

internal and volumetric heating, allowing removal of volatile

organics at a much lower activation temperature (500 ◦C)

( Jones et al., 2002). However, when the microwave irradiation

time is increased beyond 15 min, a lower adsorption capacity,

as noticed in the urea uptake and surface properties of the

MACCS-360W-20 sample was observed. This may be due to

violent internal heating, overexposure of the shell surface

to bulk heat generated during activation and intensifieddehydration (Gao et al., 2015).

3.2.3. Effect of initial concentration

For studying the effect of initial concentration (0%–100%)

on urea uptake capacity, MACCS-360W-15 (MACCS activated

at 360 W for 15 min) was chosen as the adsorbent with a

loading of 1.5 g, sorption temperature of 30 ◦C and shaker

speed of 100 rpm. The sorption attains equilibrium at around

120 min. It was observed that for all initial concentrations,

the urea removal from urine was greater than 90%. As

indicated by Fig. 6, increasing adsorbate concentration results

in increase in urea adsorption from 165 to 330 mgg −1. The

results were in good agreement with our earlier study whereinitial concentration was shown to be a substantial variable

affecting the urea uptake capacity (Ganesapillai et al., 2014).

3.2.4. Effect of temperature

The effect of temperature on the sorption process was inves-

tigated at incubator shaker temperatures of 30–60 ◦C under

the conditions: 1.5 g adsorbent loading of MACCS-360W-15,

100% adsorbate concentration and 100 rpm. It is evident from

Fig. 7 that increasing temperature has a negative effect on the

sorption. The urea uptake capacity decreases from 330 mgg −1

(30 ◦C) to 260 mgg −1 (60 ◦C). Since adsorption is a sponta-

neous process, an increasing uptake of organic molecules is

expected with a decrease in adsorption temperature as is

seen in Fig. 7 (Moreno-Castilla, 2004). Gupta et al. (2011) in

their study on azo dye adsorption using mesoporous acti-

vated carbon prepared from waste rubber tire report a similar




Table 3 – Isotherm model parameters, thermodynamic parameters and standard deviations at various temperatures.

Isotherm Parameter Temperature (◦C)30 40 50 60

Langmuir R2 0.9875 0.9872 0.9713 0.9938

qm (mgg −1) 312.50 294.12 277.78 270.27

KL (Lmg −1) 0.0205 0.0169 0.0133 0.0079

RL 0.0025 0.0030 0.0038 0.0063

Normalized deviation 0.1316 0.8397 0.3936 0.3055Normalized standard deviation 3.2216 3.5210 4.8831 3.3765

Freundlich R2 0.7132 0.7739 0.7909 0.8132

N 0.8743 0.8909 0.9063 0.8583

K f (mgg −1) (g −1)n 103.78 86.139 67.422 53.211

Normalized deviation 18.870 18.890 18.890 18.890

Normalized standard deviation 19.996 19.546 19.546 19.546

Flory–Huggins R2 0.9378 0.9873 0.9863 0.9833

Ka 1.1E−05 1.3E−05 1.2E−05 1.4E−05

G◦ (kJ mol−1) −2.8651 −2.9123 −3.0325 −3.0771



Dubinin–Radushkevich R2 0.9614 0.9532 0.9253 0.9753

qs (mgg −1) 305.62 289.19 270.75 253.08

kad (mol2 kJ−1) 0.0139 0.0161 0.0192 0.0272Es (kJ mol−1) 5.9976 5.5728 5.1031 4.2875



Thermodynamic parameters H◦ (kJ mol−1) −6.4712

S◦ (kJ mol−1) −0.0721

Fig. 6 – Effect of initial concentration on urea sorption by

MACCS-360W-15 (100%: 25 ml, 1.5 g adsorbent loading,

100 rpm and 30 ◦C ).

trend, attributing this to the tendency of sorbate molecules

to escape from the solid phase to bulk phase with increase

in solution temperature. Moreover, urea being a volatile or-

ganic compound undergoes rapid hydrolysis to form carbon

di-oxide and ammonia with increase in temperature, which

is likely to escape to the atmosphere unless it reacts with wa-

ter to form ammonium ions (NH4+) (Thyagarajan and Sak-

thivadivu, 2014).

3.2.5. Adsorption dynamics

Analysis of isotherms describes how the adsorbate interacts

with the adsorbent and is critical in optimizing the use of the adsorbent (Kalavathy et al., 2009). The obtained sorption

data were fitted to Langmuir, Freundlich, Flory–Huggins and

Dubinin–Radushkevich isotherms. In order to better estimate

the fit to the isotherms the normalized deviation and

Fig. 7 – Effect of adsorption temperature on urea recovery

by MACCS-360W-15 (100%: 25 ml, 1.5 g adsorbent loading

and 100 rpm).

normalized standard deviations were evaluated (Singh et al.,

2008). The normalized deviation and normalized standard

deviation between predicted and experimental values for the

isotherm models were calculated using Eqs. (2) and (3).

Normalized Deviation =100

N

qe (exp) − qe (pred)

qe (exp)

(2)

Normalized Std Deviation

= 100

(qe (exp) − qe (pred)/qe (exp))2

N (3)

where, qe (exp) and qe (pred) represents the experimental and

predicted urea uptake capacity and N is the number of mea-

surements. Table 3 reports the values of these parameters.

For MACCS-360W-15, the Langmuir model displayed the min-

imum values of normalized deviation and normalized stan-

dard deviation.




Table 4 – Comparison of adsorption rate constant, and correlation coefficients for different kinetic models.

Temperature (◦C) Urea concentration (%) Pseudo-first-order Pseudo-second-order

R2 k1 (min) R2 k2 (mgg −1 min−1)

30 25.00 0.9772 0.0116 0.9439 1.3888

37.50 0.9953 0.0114 0.9007 1.4165

50.00 0.9955 0.0113 0.8993 1.6031

62.50 0.9971 0.0113 0.9015 1.5125

75.00 0.9941 0.0112 0.9649 0.6527

100.0 0.9891 0.0112 0.9937 0.1456

40 25.00 0.9771 0.0116 0.9839 0.8602

37.50 0.9949 0.0113 0.9924 0.4269

50.00 0.9959 0.0113 0.9919 0.5444

62.50 0.9966 0.0115 0.9856 0.1993

75.00 0.9932 0.0113 0.9782 0.2155

100.0 0.9765 0.0114 0.9681 0.3087

50 25.00 0.9766 0.0115 0.9321 0.1136

37.50 0.9938 0.0113 0.9676 0.1207

50.00 0.9954 0.0113 0.9898 0.1146

62.50 0.9936 0.0112 0.9816 0.1163

75.00 0.9946 0.0112 0.9865 0.1343

100.0 0.9973 0.0112 0.9546 0.1060

60 25.00 0.9737 0.0115 0.9064 0.0684

37.50 0.9936 0.0113 0.9587 0.0906

50.00 0.9953 0.0112 0.9798 0.0901

62.50 0.9977 0.0113 0.9736 0.0932

75.00 0.9967 0.0112 0.9645 0.0117

100.0 0.9958 0.0112 0.9536 0.0864

Fig. 8 – Langmuir isotherms for MACCS-360W-15 at

various temperatures.

A series of Langmuir isotherm lines were obtained for the

temperature range 30 ◦C–60 ◦C and are presented in Fig. 8.

Monolayer Langmuir adsorption capacity was found to be

312.5 mgg −1 at 30 ◦C. Though sorption data did not fit the

Freundlich model, high values of adsorption intensity (n) in-

dicated its favorability. The separation factor for the Langmuir

model (RL) was evaluated according to Eq. (4) and was found

to be within the favorable limits (Ho, 2003).

RL =1

1 + KLC0(4)

KL and Co are the Langmuir isotherm constant (Lmg −1)

and the initial concentration of urea in urine (mgL−1), re-

spectively. The equilibrium constant (KC) obtained from the

Flory–Huggins model was used to compute the standard

Gibbs free energy change (Eq. (5)).G◦ = RT ln

KC

. (5)

The negative value of G◦ confirms the feasibility and

spontaneity of the sorption (Ho, 2003). The mean sorption

energy Es (kJ mol−1) was computed from the Dubinin–

Radushkevich isotherm constant (mol2 kJ−1) (Eq. (6)).

Es =

1 2kad

. (6)

The mean free energy of sorption was <8 kJ mol−1,

indicative of the physical nature of the sorption by MACCS-

360W-15 (Bertoni et al., 2015).The standard enthalpy and

entropy changes during adsorption were evaluated using the

well-known Van’t Hoff equation (Eq. (7)). The plot of log (KC)

versus 1/T was linear; H◦ and S◦ were evaluated from the

slope and intercept of this plot.

Log (KC) =

S◦

2.303R

−

H◦

2.303RT

. (7)

3.3. Adsorption kinetics

To determine the rate controlling mechanism, the sorption

data were tested against the pseudo-first-order and pseudo-

second-order models. The rate constants were determinedfor different adsorbate concentrations and at different

temperatures. The (Lagergren, 1898) equation was used to

obtain the first order rate constant (Eq. (8)).

log qe − qt

= log

qe− k1t (8)

where, qe and qt are the amount of urea adsorbed (mgg −1) at

equilibrium and at time ‘t’ (min), and k1 is the rate constant

of adsorption (min−1). The plot of log (qe − qt) versus time

indicated a good fit to the experimental data for the first

order rate expression at all urea concentrations (Fig. 9). Forthe

second order rate constant, the following linearized equation

was used (Eq. (9)).

t

q =

1

k2q2e

+t

qe. (9)

The plot of t/q versus time indicated that the data does

not fit the model well. Table 4 enlists the kinetic parameters




T a b l e 5 –

C h e m i c a l a n a l y s e s o f s o i l a f t e r t r a y t r i a l s .

S o i l ( % w / w )

p H

A l

C a

K

M g

N a

C E C b

T o t . N

T o t . C

( C a C l

2 )

c m o l ( + )

k g − 1

c m o l ( + )

k g − 1

c m o l ( + )

k g − 1

c m o l ( + )

k g − 1

c

m o l ( + )

k g − 1

c m o l ( + )

k g − 1

( % )

( % )

C o n t r o l

6 . 2 3

1 . 9 3

5 . 5 6

2 . 0 5

6 . 2 3

0 . 7 6 1

1 4 . 6 0 0

0 . 1 8 2

2 . 0 3

C o n t r o l +

U A - M A C C S - 3 6 0 - 1 5 ( 0 . 5 % )

7 . 4 3

N D

1 1 . 4

4 . 7 1

6 . 2 5

3 . 2 6 1

2 5 . 6 7 1

0 . 2 5 2

2 . 4 1

C o n t r o l +

U A - M A C C S - 3 6 0 - 1 5 ( 1 % )

7 . 6 1

N D

1 1 . 7

5 . 1 7

6 . 2 5

4 . 8 5 2

2 7 . 9 9 7

0 . 3 2 4

2 . 7 8

C o n t r o l +

U A + M A C C S - 3 6 0 - 1 5 ( 1 . 5 % )

7 . 7 2

N D

1 2 . 1

5 . 6 3

6 . 2 8

5 . 7 8 4

2 9 . 7 1 5

0 . 3 9 5

3 . 1 5

C o n t r o l +

U A - M A C C S - 3 6 0 - 1 5 ( 2 % )

7 . 8 5

N D

1 2 . 3

5 . 9 6

6 . 2 9

7 . 7 2 2

3 2 . 2 9 9

0 . 4 6 6

3 . 5 5

C o n t r o l +

U A - M A C C S - 3 6 0 - 1 5 ( 2 . 5 % )

7 . 9 8

N D

1 2 . 5

6 . 2 3

6 . 3 1

8 . 4 3 1

3 3 . 4 8 8

0 . 5 3 6

3 . 8 9

N D a : N o t d

e t e c t e d ; C E C b : C a t i o n e x c h a n g e c a p a c i t y .




Table 6 – Biomass per plant, % biomass increase and number of germination for different soil treatments.

Treatment Biomass per plant (g) % Biomass increase GerminationsProof of concept

Residualeffect

Proof of concept

Residual effect (out of 20 seedssowed)

Absolute control 2.23 2.75 – – 11

Control + UA-MACCS-360-15 (0.5%) 3.16 3.83 41.71 39.27 13

Control + UA-MACCS-360-15 (1%) 4.06 4.86 82.06 76.72 15

Control + UA-MACCS-360-15 (1.5%) 4.31 5.12 93.27 86.18 15Control + UA-MACCS-360-15 (2%) 4.89 5.63 119.2 104.7 16

Control + UA-MACCS-360-15 (2.5%) 5.18 5.95 132.2 116.3 17

Fig. 9 – Pseudo-first order kinetic fit for different initial

concentrations onto MACCS-360W-15 at 30 ◦C.

evaluated. The first order rate constant was found to be

0.01132 min−1.

3.4. Application of urea adsorbed activated carbon in crop

studies

Urea adsorbed carbon (UA-MACCS-360W-15) addition to the

soil influenced major soil properties (Table 5). The soil pH

increased from 6.23 to ∼8 due to amendment. Soil amend-

ment significantly reduced the exchangeable Al from nearly

2 cmol (+) kg −1 to below detection levels. All treatments con-

siderably increased exchangeable Ca levels in the soil (5.5 to

∼12 cmol (+) kg −1). Moreover, in comparison to the control,

the cation exchange capacity improved in all treatments. To-

tal soil carbon also was elevated by nearly 2% during the 2.5%

UA-MACCS-360W-15 treatment. However, the most notewor-

thy effect in all treatments was the increase in the soil nitro-

gen and electrical conductivity, and this can be attributed to

desorption of urea in the soil. Measurements of total soil ni-

trogen indicated that nearly 90% of urea was desorbed from

the MACCS-360W-15 surface.

The effectiveness of a soil conditioner can be measured

in terms of growth of the plant and by the amount of dry

matter accumulated in different plant organs (Abrahamson

and Caswell, 1982). Analysis of plant growth and biomass

revealed that soil conditioning by the urea adsorbed carbon

had notable effect on the plant growth (Table 6). The addition

of 2.5% (w/w) of carbon had the most significant effect with

a biomass increase of more than twice the control (132%)

and maximum number of seed germinations (17/20). In the

residual effects study, the biomass obtained for the control

increased to 2.75 g from 2.23 g, which was probably due

to increase in soil nitrogen content due to nitrogen fixative

properties of the test plant species. Consequently there was

a reduction in percentage biomass increase for all treatments

in comparison with the proof of concept study.

4. Conclusions

Recycling in the present day resource-scarce scenario is

paramount. The use of human urine in agriculture through

urine diversion and its subsequent utilization as a nutrientsourcefor enhancing productivity has a two-fold advantage; it

reduces health risks to humans by diverting urine from water

systems where it is a pollutant, while recycling nutrients back

quickly into food systems. The cycle of source separation of

human urine to its subsequent rechanneling to cultivated

fields using high surface area activated carbon prepared from

renewable agricultural wastes could thus play an important

role in realizing sustainability in agriculture.The present investigation elucidated the potential of

activated carbon prepared from coconut shells towards the

recovery of nutrients from human urine. Finally, this study

demonstrates the feasibility of employing biomass based

(coconut shells) activated carbons towards the successful

recovery of urea. Through the analysis of various processparameters, the biosorption was optimized to ensure almost

complete recovery of urea from urine. This approach to

sanitation exhibits that resource recovery from human urine

could minimize the dependency of farmers on industrially

manufactured fertilizers, reduce water usage when such

systems are coupled with UDDTs while providing adequate

sanitation.Microwave irradiation had a profound effect on the tex-

tural characteristics of the tailored carbon and subsequently

affected its urea uptake capacity. Microwave irradiation at

360 W for 15 min provided ideal conditions for porosity

development in the carbon with surface area in excess of

1000 m2 g −1. Sorption experiments indicated a urea recov-

ery from urine of nearly 95%. Based on the normalized de-

viations and correlations coef ficients, the Langmuir isotherm

and pseudo-first-order kinetic model were found to represent

the sorption data satisfactorily. Thermodynamic parameters

indicated the favorability and spontaneity of the sorption.

Application of urea adsorbed carbon as soil conditioner re-

vealed a plant biomass increase of 132% in comparison to the

control and improved seed germinations. In addition, soil

amendment took place with positive changes in soil pH, and

soil nutrient content.

Acknowledgments

The authors greatly acknowledge all the volunteers for

generously donating urine. We also wish to acknowledge VIT

University, India, for providing the resources intended for

conducting the experiments.




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