Superabsorbent polymer for water management in forestry

doi:10.4236/as.2013.45B011

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Vol.4, No.5B, 57-60 (2013) Agricultural Sciences

doi:10.4236/as.2013.45B011

Superabsorbent polymer for water management in

forestry

Miguel Casquilho1*, Abel Rodrigues2, Fátima Rosa1

1Department of Chemical Engineering, Center for Chemical Processes, Instituto Superior Técnico, Technical University of Lisbon,

Lisbon, Portugal; *Corresponding Author: mcasquilho@ist.utl.pt

2Unidade de Produtos Florestais (Forest Products Unit), INIAV, Ministry of Agriculture, Oeiras, Portugal

Received 2013

ABSTRACT

The effectiveness of a super absorbent polymer

of sodium polyacrylate was studied, with em-

phasis on water management, i.e., absorption,

retention and desorption in the soil. The polymer

was applied in plots with a sandy soil near

Grândola (southern Portugal). Characterization

of the most relevant physical and chemical

properties of the polymer was made, namely, its

specific gravity, particle diameter, capacity of

water absorption and desorption. In the plots

with and without the polymer, soil moisture was

continuously monitored, and grassland biomass

samples were collected and weighed. The re-

sults reflected the effective role of the polymer

both in the improvement of the water regime in

the soil and in the substantial increment of

grassland productivi ty.

Keywords: Absorption; Desorption; Sodium

Polyacrylate; Superabsorbent Polymer; SAP

1. INTRODUCTION

A superabsorbent polymer (SAP) is a hydrogel that

absorbs water hundreds of times its own dry mass. Usu-

ally a SAP is produced by the polymerization of acrylic

acid, acrylic esters, acrylamide and other unsaturated

monomers. The carboxylic group along the polymer

chain facilitates the absorption of water, while the pres-

ence of crosslinking in the chain prevents their complete

solubilization ([1,4]). The superabsorbent materials are

of great interest due to their applications, namely in

medicine and in solving some ecological and biological

problems, in biomedicine, biotechnology, pharmaceutical,

veterinary, food industry, and, addressed in this study,

agriculture technologies. Specifically, they are used as

controlled release systems of drugs, pesticides, water or

other bioacti ve age nt s.

Great attention has been given to the application of

superabsorbent hydrogels in agriculture, soil improve-

ment and plant growth. Superabsorbent hydrogel parti-

cles distributed in the soil are capab le of absorbing water.

This should efficiently improve the managemen t of water,

through the water-holding capacity of the soil, and pro-

mote optimal plant growth ([3,4]). One of the polymers

that showed greatest results in this area was sodium

polyacrylate, used in this study, and applied in 4 plo ts in

the Grândola region, southern Portugal, a Mediterranean

area with hot summers, leading to water stress, corre-

sponding to the kind of environment where the effects of

SAPs should be more useful as attenuators of summer

water scarcity to plants.

2. MATERIALS AND METHODS

The SAP’s specific gravity was determined using a

calibrated pycnometer containing a sample of the SAP in

a non-solvent test liquid, n-heptane, with a total known

volume. The dry sample was weighed and placed in the

pycnometer, which then was filled with the test liquid

and weighed. The specific gravity was calculated as the

quotient of the dry mass to this volume.

A SAP’s capacity to absorb water is considered the

maximum mass that it can absorb when submerged in

water, leading to its “swelling”. Swelling experiments

were conducted to observe the swelling kinetics of the

polymer tested. The water uptake in sodium polyacrylate

was measured in the laboratory by the “tea-bag” method.

Each bag, with polymer particles, was placed in ap-

proximately 200 mL of water, and the weight of the

loaded bag was periodically measured until no weight

change was observed. The water uptake ratio, Q, was

determined as the water absorption amount divided by

the initial polymer weight, as in Eq.1, where W0 and Wt

are the weights of samples with water in the bag at the

initial time, 0, and at successive instants of time, t, re-

spectively.

Qt

 (1)

M. Casquilho et al. / Agricultural Sciences 4 (2013) 57-60

The water desorption kinetics from SAP particles was

also measured. After samples of the polymer in water

reached their equilibrium (maximum) swelling, they

were dispersed in samples of soil collected from the plots

where the polymer was applied. These samples were

periodically weighed till equilibrium. Desorption time

for soil without polymer particles was also measured.

The water desorption ratio (Q’) was determined as the

water desorption amount divided by the initial sample

weight, analogously given in Eq.2,









 (2)

where W’0 and W’t are weights of the samples at initial

time and t, respectively. In these absorption and de-

sorption experiments, the same measurements were

repeated three times for each sample. The water ab-

sorption and desorption ratios and rates were the av-

eraged results of the lab trials.

3. RESULTS AND DISCUSSION

The average specific gravity of the polymer particles,

determined by the method mentioned was 1.5 g﹒cm–3,

and the resulting particle diameter was about 438 µm.

Absorption capacity in mL of distilled water p er gram of

dry polymer is shown in Figure 1 for various diameters

tested. The absorption capacity varies from about 270 to

360 mL per gram of dry polymer, for particle sizes be-

tween 63 and 710 µm.

As shown in Figure 2, the water absorption rate in-

creased with decreasing particle size. Decreases in parti-

cle size led to increases in the surface area per unit of

polymer mass, resulting in more rapid water adsorption

on particle surfaces. The lower the particle size, the

faster is the absorption process. For greater particles, the

full absorption process takes more time than for a smaller

particle. For example, for particles with 63 µm it takes

about 7 minutes, while for a 425 µm particle, it takes

about 15 mi nu t es.

Figure 3 shows the water desorption kinetics of poly-

mer particles premixed with soil. The net amount of wa-

ter initially absorbed in polymer-dispersed soil was prac-

tically 2 times higher than that in pure soil. A slightly

higher initial water d esorption was observed for soils with

smaller polymer particles. The time of water evaporation

from the soil was significantly affected by the presence

of polymer particles. About 25 000 min (17 days) were

required for SAP-incorporated soils, but only 13 000 min

(9 days) for pure soil. The particle-size effect on the wa-

ter-releasing kinetics from SAP-loaded soils was negli-

gible compared with the water-releasing kinetics from

pure soil.

Figure 1. Absorption capacity of SAP particles with different

diameters (Dp).

Figure 2. Absorption capacity of SAP particles with different

diameters (Dp) as a function of time.

Figure 3. Water desorption kinetics of SAP particles premixed

with soil.

Field Work

The SAP was applied on 4 plots of 2500 m2 each, with

a sandy soil and a wavy terrain, according to the experi-

mental design on Figure 3. All the plots were sown with

M. Casquilho et al. / Agricultural Sciences 4 (2013) 57-60 59

a blend of gramineous and leguminous seeds, the latter

inoculated with specific Rhizobium. An NPK fertilization

was made in all the plots, except the blank, Q4. As

shown in Figure 4: plot Q1 received 100 kg of fertilizer,

4 kg of SAP, plus 6.25 kg of seed, as mentioned; plot Q2,

same as Q1 but 140 kg of fertilizer; and plot Q3, same as

Q1 but no SAP. To evaluate the effect of SAP biomass

productivity, grassland biomass samples were collected

and weighed. Three samples per quadrant were taken

from several locations, to randomize effects such as soil

slope or solar ex position. Speci mens were collected fro m

a surface area of ~1 m2, following a square shape. Then,

the biomass samples, screened from small branches, and

residues were dried in a kiln, at 85℃ for abou t 30 hours,

till constant weight.

The average results (per 1 m2) were: 73 g for Q1, 100

g for Q2; and 62 g for Q3. As can be seen, the plot with

greatest biomass production, Q2, is the one containing

SAP and an extra dose of fertilizer. The results show in-

creases of 17% from Q3 to Q1, and 38% from Q1 to Q2.

It is noted that the application of SAP plus fertilizer

alone produced an increase of (17%) on the growth of

vegetation, an increase which presents itself as a very

favorable result, taking into account the low levels of

precipitation in the region (about 70 mm) during the first

half of the year of 2012.

In order to obtain data about moisture and soil tem-

perature, as well as precipitation and relative humidity of

air, an automatic weather station was installed providing

continuous micrometeorological data during Sep-Oct

2012. The effectiveness of the SAP is verified through

the soil moisture data in the periods following the occu r-

rence of precipitation, depicted in Figur e 5.

Analyzing Figure 5, in the periods following the sec-

ond and third precipitation zone (the zone between the

5.th and 6.th days and between the 8.th and 9.th, respec-

tively) a trend may be identified (though slight) for an

increase in moisture content of the so il in ar eas wh e r e th e

polymer was applied. Especially after the 2.nd cycle,

between the days 16 and 24, an improvement in water

retention can be observed. It appears that as time passes

since the occurrence of precipitation, the effect of the

polymer in the soil moisture content is more notorious.

Q1 Fertilizer

NPK (0-20-17): 100 kg

Blend

(fertilizer, limestone)

SAP: 4 kg,

+ fertilizer and seed

Fertilizer

NPK (0-20-17): 140 kg

Blend

(fertilizer, limestone)

SAP: 4 kg,

+ fertilizer and seed

Fertilizer

NPK (0-20-17): 100 kg

Grassland: improved

Blend

(fertilizer, limestone)

Blank

Figure 4. Experimental design of the polymer application.

As would be expected from the short period during

which these data were collected, and given the frequent

occurrence of precipitation, the observed effects were

barely noticeable, showing an effective retention of water

by the applied SAP. The effect of the use of the polymer

is illustrated in Figure 6, in which the upper and lower

Figure 5. Precipitation (mm) and volumetric moisture (% v) of

the soil.

Figure 6. Height of vegetation grown

without (upper photo) and with (lower

photo) SAP.

M. Casquilho et al. / Agricultural Sciences 4 (2013) 57-60

photos show the height of vegetation, respectively, with-

out and with the application of the SAP.

4. CONCLUSIONS

A SAP of sodium polyacrilate showed to be an effec-

tive agent in increasing biomass productivity in grass-

lands, improving organic matter content and soil fertility

in a study in a region with Mediterranean type climate,

i.e., hot summers, in the South of Portugal. The amount

of biomass grown in the plots where SAP was applied

was significantly greater than in the plots without SAP.

The results of soil moisture retention by SAP proved

satisfactory, especially in periods after the occurrence of

precipitation. These effects, run only in two months, are

indicative of success, which should be quantified for a

whole hydrological year aiming to test its full potential

as soil water stress restrainers in vegetal environments.

Openly accessible at

5. ACKNOWLEDGEMENTS

A SAP of sodium polyacrilate showed to be an effective agent in in-

creasing biomass productivity in grasslands, improving organic matter

content and soil fertility in a study in a region with Mediterranean type

climate, i.e., hot summers, in the South of Portugal. The amount of

biomass grown in the plots where SAP was applied was significantly

greater than in the plots without SAP. The results of soil moisture re-

tention by SAP proved satisfactory, especially in periods after the oc-

currence of precipitation. These effects, run only in two months, are

indicative of success, which should be quantified for a whole hydro-

logical year aiming to test its full potential as soil water stress restrain-

ers in vegetal environments.

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http://www.m2polymer.com/html/history_of_superabsorb

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