wheat straw-based hydrogels as soil conditioners?

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From agricultural by-products to value-added materials: wheat straw-based hydrogels as soil conditioners? Katja Heise, Maximilian Kirsten, Yvonne Schneider, Doris Jaros, Harald Keller, Harald Rohm, Karsten Kalbitz, and Steffen Fischer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00378 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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From agricultural by-products to value-added

2

materials: wheat straw-based hydrogels as soil

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conditioners?

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Katja Heise†*, Maximilian Kirsten‡, Yvonne Schneider#, Doris Jaros#, Harald Keller, Harald

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Rohm#, Karsten Kalbitz‡, Steffen Fischer†

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† Institute of Plant and Wood Chemistry, Technische Universität Dresden, Pienner

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Strasse 19, 01737 Tharandt, Germany

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‡ Institute of Soil Sciences and Site Ecology, Technische Universität Dresden, Pienner

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Strasse 19, 01737 Tharandt, Germany

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# Chair of Food Engineering, Technische Universität Dresden, Bergstrasse 120, 01069

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Dresden, Germany

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* Corresponding Author: Katja Heise, E-mail: [email protected]

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 BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany

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KEYWORDS Biomass valorization, Moisture sorption, Soil amendment, Water retention


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ABSTRACT

16

Herein, we present a simple synthetic approach to fabricate wheat straw-based

17

hydrogels, starting from the unfractionated and carboxymethylated lignocellulosic

18

matrix. Citric acid was used as a cheap and non-toxic crosslinker. The applied hydrogel

19

characterizations can be essentially distinguished into investigations on the synthetic

20

pathway and model-scale application-related tests. For the first part, three sample-

21

specific values were introduced: gel yield (%), swelling ratio (gwater/g) and gel stiffness (G´,

22

Pa). Optimized reaction conditions led to mechanically stable gels with a moderate

23

swelling ratio (up to 50 gwater/g). Moreover, dynamic vapor sorption analysis revealed

24

that these gels re-swell after complete drying. Finally, one selected hydrogel was

25

incorporated into two different model soil substrates, assessing its impact on the soils´

26

water retention. Our experiments showed that already low incorporation rates

27

(0.2 wt %) increased the water content of a sandy soil by 70 % (at pF 2.53). Overall, these

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results are promising and may lead to new soil amendments based on a sustainable

29

source and a simple synthesis.


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INTRODUCTION

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Inspired by Nature´s zero-waste principle, the valorization of lignocellulosic biomass

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wastes has an enormous economic and ecological potential, as they are produced in

33

excess, globally, and their recycling allows for circular production strategies in

34

agriculture and forestry.1–3

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To develop individual value-chains for cellulose, hemicelluloses and lignin, recent

36

biorefinery concepts prevalently target biomass fractionation using elaborate

37

pretreatments entailing large expenses for chemicals and energy.4 Alternatively, starting

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from unfractionated lignocelluloses in simplified operations may considerably reduce

39

processing efforts and increase the economic value of biomass residues for selected

40

applications.2

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In this regard, two product groups have been in the focus of applied research

42

recently:

lignocellulose-based

bioadsorbents

for

waste

43

lignocellulosic fiber-reinforcements for composite materials.5,6 A third and very

44

promising idea leads back to agriculture and considers applying lignocellulose-based

45

hydrogels to soils as miniature water reservoirs and nutrient carriers.7 These three

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concepts have one essential principle in common: they use chemical and/or physical

47

treatments to optimize inherent polymer characteristics of the biomass matrix toward

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the intended application.7–10


water

treatment

and

3

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Linear and crosslinked polymers have widely been investigated as soil amendments

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to prevent soil erosion or for improving the soils´ water holding capacity.11,12 In many

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cases, superabsorbent polymers (SAPs) are thereby based on crosslinked polyacrylate or

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polyacrylamide, which were found to absorb enormous amounts of water (10-10,000

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g/g) and improve the availability of water and fertilizers in soils.13 However, despite the

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successful use of SAPs in horticultural industry, their high costs and hampered

55

biodegradability have so far restricted an application on agricultural scale.13,14

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Lignocelluloses offer an ideal polymeric framework for the fabrication of hydrogels

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for soil usage owing to their inherent hydrophilic nature, biodegradability and

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modifiability. Moreover, compared to purely cellulosic substrates, residual lignin

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within the gel matrix might slow down biodegradation preserving the gels´ structural

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integrity over an adequate time period (e.g. one growth season).15,16 Nevertheless,

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synthetic concepts for lignocellulosic hydrogels are still scarce. In current strategies,

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lignocelluloses (e.g. cotton stalks, rice straw, sugarcane bagasse) basically serve as

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biopolymer backbone for the synthesis of acrylate-based graft copolymers.7,17–20 The

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obtained semisynthetic SAPs exhibited excellent water absorbencies (up to 350 g/g) and,

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therefore, improved soils´ water retention. Nevertheless, the use of acrylate-derived

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products may be questioned, as leaching and accumulation of potentially eco-toxic

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monomers in natural systems should be mandatorily avoided.21,22 Furthermore, a

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complete degradation of soil amendments should be guaranteed.


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Polycarboxylic acids may be a very promising alternative in this regard, as they were

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found to effectively crosslink cellulose without the need for initiators or organic

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solvents.23–25 Among them, citric acid has gained increasing interest for preparing

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cellulose-based films or superabsorbent hydrogels, taking into account its low price,

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natural origin and non-toxicity.26,27 Moreover, implementations are simple, as the

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esterification of cellulose follows an easy heat-induced mechanism: citric acid

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dehydrates over two reactive anhydride stages and thereby crosslinks adjacent cellulose

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chains.28

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Inspired by this simple approach, we used citric acid to crosslink unfractionated and

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carboxymethylated wheat straw, which represents a class of highly available biomass

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residues with an estimated annual production of above 150 million tons in the

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European Union (FAO, 2017).29 The obtained gels were analyzed extensively focusing

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on the synthetic pathway and their potential application as soil conditioners.

82 83

MATERIALS AND METHODS

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Materials. Chopped wheat straw (WS; Agrargenossenschaft Rossau e.G. Germany)

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was ball milled (MM 400, Retsch® GmbH, Haan, Germany): 5 min, f = 20 rpm, median

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particle size 43 m. All chemicals were applied in analytical state. Deionized water was

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utilized for all experiments. For water retention tests, a sandy (from subsoil, soil depth

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> 40 cm, Duebener Heide, Germany) and a silty model soil substrate (from limestone


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quarry, soil depth = 300 cm, Ostrau, Germany) were collected, air-dried, sieved to < 2

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mm and stored at room temperature (RT). Soil characteristics are summarized in the SI

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(Table S1).

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Hydrogel preparation. Carboxymethyl (CM) intermediates were synthesized and

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purified as described previously.30 Milled WS was modified with sodium

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monochloroacetate (MCA) in isopropanol/NaOH (aq) as follows: (i) alkalization –

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1 hour, RT; (ii) carboxymethylation after MCA addition – 55 °C, 3 hours. Per mol

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anhydrous glucose units of cellulose (AGUcell, 48.9 % in wheat straw) 1.5, 3.0 and 6.0

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mol MCA were used for WS1, WS2, and WS3, resulting in carboxymethyl group

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contents of 1.75, 2.56 and 3.79 mmol/g, respectively. The product work-up was carried

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out by: decanting remaining medium, product dispersion in 100 mL water,

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neutralization with acetic acid, followed by disperser-assisted precipitation in 500 mL

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96-% ethanol and, finally, 5 repeated dispersion-precipitation steps in water/ethanol.

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Carboxymethyl group contents were determined by ICP-OES (Na determination,

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device: SPECTRO CIROSCCD, SPECTRO Analytical Instruments, Kleve, Germany)

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after dialysis against deionized water (membrane: SpectraPor® 3, MWCO: 3.5 kDa),

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lyophilization

106

HNO3/HF/HClO4.30,31

and

microwave-assisted

decomposition

of

sample

aliquots

in

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After carboxymethylation, CM-intermediates were crosslinked with citric acid

108

monohydrate. 2, 4, 8 or 16 wt % citric acid (w/w dry CM-intermediate) were dissolved


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in 45 mL deionized water and 5 g of dry CM-intermediate were suspended in the

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solution. The mixture was kept for 24 hours at RT to homogeneously distribute the

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crosslinker. Subsequently, the mixture was spread on a Petri dish (thin layer) and pre-

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dried at 60 °C for 24 hours, followed by crosslinking at temperatures of 120, 140 or

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160 °C for durations of 15, 30, 60 or 300 minutes. Afterwards, the product was gently

114

crushed, sieved to a defined particle size (150 to 800 µm) and suspended in deionized

115

water for 24 hours at RT. Finally, by-products and water-soluble fractions were

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removed by repeated water-washing and the purified gel particles were oven-dried at

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45 °C.

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Hydrogel characterization. Analytical characterizations of straw-based hydrogels

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were distinguished into investigations on the synthetic pathway, gel-moisture-

120

interactions and model-scale water retention tests in soil substrates.

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ATR-IR spectroscopy. Attenuated total reflection (ATR) FT IR spectroscopy was

122

performed with a Bruker Tensor 27 (Bruker Optik GmbH, Ettlingen, Germany), having

123

a RT-DLaTGS detector, KBr beam splitter and Platinum ATR (A225) diamond cell. ATR

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spectra were measured as follows: range = 4000 – 400 cm-1, resolution = 4 cm-1, 400

125

scans. Average spectra were formed (OPUS Ver. 6.5, Bruker) out of three

126

determinations.

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Gel yield. The gravimetric gel yield (in %) after esterification with citric acid was

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determined in duplicate by suspending 0.1 g of dry and sieved (150-800 m) sample


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particles (before removing water-soluble fractions) in 10 mL deionized water for

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24 hours at RT. Water-soluble proportions were then removed by washing the swollen

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particles over a pre-dried (at 105 °C) and weighed glass fiber filter (WhatmanTM, GF6)

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with deionized water, followed by drying the loaded filter (105 °C) to mass constancy.

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The gel yield % is defined as: ((mtotal – mF) * 100 %)/ m0; with mtotal as total mass of

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recovered gel particles on the filter, mF as mass of the filter and m0 the initial dry mass

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(determined in duplicate by drying the samples to mass constancy at 105 °C).

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Swelling ratio. 0.1 g dry and sieved (150-800 m) gel particles were weighed into a

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heat-sealable teabag and were swollen in 700 mL deionized water for 24 hours at RT.

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The teabag was then removed from the water, unabsorbed water drained off for 10

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minutes, followed by weighing the swollen hydrogel within the bag. The swelling ratio

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(in gwater g-1, duplicate measurements) is defined as: (mtotal - mTB - WTB)/ mGEL; where mtotal

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is the total mass of swollen gel within the bag, mTB and WTB are mass and water uptake

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of the teabag, respectively, and mGEL the dry-mass related sample mass.

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Gel rheology. Rheological measurements were carried out in duplicate with sieved

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(150-800 m) and fully water-swollen (24 h/ RT) gel particles using an AR-G2 rheometer

145

(TA Instruments, New Castle, USA) with parallel plates (d = 25.0 mm) at 23 ± 1 °C.

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Dynamic strain sweep tests were conducted at  = 1.0 rad/s, in a strain range of 0.001 to

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1.0 (10 points per decade) to determine the linear viscoelastic (LVE) region. Dynamic

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frequency sweep tests ( = 0.1 to 100.0 rad/s, 10 points per decade) were performed at a


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strain of 0.002. The gel stiffness (G´ Pa) was taken from frequency sweep experiments

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at  = 1.0 rad/s.32

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Dynamic vapor sorption analysis (DVS). Gel-moisture-interactions were investigated for

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the following selected gel samples: WS1-CA4%, WS3-CA4% and WS3-CA8%, based on

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WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1), respectively;

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esterification: 140 °C, 30 min, 4 or 8 wt % citric acid (as indicated). The measurements

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were carried out in duplicate using a Q5000 SA dynamic vapor sorption analyzer (TA

156

Instruments, New Castle, USA). Approx. 5 mg sieved (150-800 m) and preconditioned

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(10 d over P2O5) xerogel particles were loaded into a quartz crucible. Relative humidity

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(RH) in the sample chamber (25 °C) was adjusted by mixing continuous streams of dry

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and moisture-saturated nitrogen with continuous flow. Initial mass of the dry sample

160

was determined at RH = 0 %. In the first adsorption cycle (Ad1), RH was increased to 90

161

% in 10 % increments. At each RH step, equilibrium mass was taken when the relative

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mass change was < 0.01 % for 5 min. The second cycle comprised of stepwise desorption

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to RH = 0 % (De1), the third cycle another adsorption to RH = 90 % (Ad2).

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Sorption isotherms were obtained by plotting equilibrium moisture contents of the

165

samples (Meq, mgwater g-1sample) against relative humidity (expressed as water activity aw

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-). Sorption hysteresis was calculated as absolute difference between desorption and

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first adsorption Meq. Deviations between the second (Ad2) and first adsorption Meq were

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calculated to examine structural alterations within the gel matrices. Moreover, for


9

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determining monolayer capacity (M0 mgwater g-1sample) of the samples, sorption data

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were fitted to the Guggenheim-Anderson-de Boer (GAB) model: cG, GAB monolayer

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sorption energy; kG, GAB multilayer sorption energy; aW, water activity = RH/100:33,34 Meq =

M0 × cG × kG × aw (1 ― kG × aw) (1 + (cG ― 1)kG × aw)

(1)

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Water-accessible surface area AG m2 g-1 was obtained by: N0, Avogadro constant

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(6.022 x 1023 molecules mol-1); am, area captured by a single water molecule at 25 °C (1.06

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x 10-19 m²); Msorb, molar mass of water (18 g mol-1):35 AG =

M0 × N0 × am Msorb

(2)

175 176

Water retention in model soil substrates. In model-scale, the impact of one selected

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hydrogel (WS3-CA4% – based on WS3 (3.79 mmol –CH2COONa g-1), esterification:

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4 wt % citric acid, 30 min, 140 °C) on the soils´ water retention was investigated. Two

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defined model soil substrates were used, representing a sandy and a silty soil

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(characteristics in SI, Table S1). Dry and sieved (150 - 800 m) gel granules were

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incorporated homogeneously into the soil substrates at rates of 0.2 or 0.7 wt % (w/w

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soil). Approx. 10.0 g of each soil/gel mixture were placed in aluminum cylinders

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(2.8 x 3.0 cm, diameter/height) and saturated with deionized water for 24 hours at RT.

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Subsequently, the cylinders were placed on a ceramic plate and the pressure was

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reduced stepwise using a 100-centimeter water column. At each step (0, 10, 30, 40, 50,


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60, 80 and 100 cm, corresponding to a pF value of 0.00, 1.00, 1.48, 1.60, 1.69, 1.78, 1.90

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and 2.00) the samples were equilibrated for 24 hours, followed by recording their

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weight. The pF value is an expression of soil water tension (common logarithm of cm

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water column) describing the soil matrix potential (defined as energy required to move

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water against adsorptive and capillary forces). Afterwards, the samples were exposed to

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pF = 2.53 (water column: 338.8 cm) in a pressure chamber, again allowing an

192

equilibration for 24 hours. Finally, pF curves were obtained from average values out of

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three replications and were compared to results of pristine soil substrates.

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RESULTS AND DISCUSSION

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Hydrogel synthesis and gel properties. Figure 1 represents the synthetic pathway

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comprising carboxymethylation and citric acid crosslinking, and the resulting ATR-IR

198

spectra

199

carboxymethylation introduced a strong signal at 1591 cm-1 (–COO- asymmetric

200

stretching vibrations) into the biomass spectrum. The subsequent crosslinking reaction

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gave two new peaks of weak nature, attributable to newly formed ester bonds ((C=O):

202

1728 cm-1, (C-O): 1230 cm-1). Besides, a significant alteration of the lignocellulosic

203

matrix – particularly in the course of the carboxymethylation – became evident through

204

a considerable decline of hemicellulose-derived acetyl signals ((C=O): 1733 cm-1; (C-

205

O): 1228 cm-1). Our previous studies on the carboxymethylation of lignocelluloses

206

already showed that the strongly alcoholic-alkaline medium (pH > 12) caused a partial

207

extraction of hemicelluloses and lignin throughout the reaction.30 For instance,

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mercerization studies in isopropanol-NaOH mixtures showed that a typical

209

carboxymethylation conditions (55 °C, 3 hours) would reduce lignin and hemicellulose

210

contents to 15.1 % or 9.4 % from originally 22.8 % or 27.3 %, respectively. Nevertheless,

211

following the synthetic path, all lignocellulosic constituents may be involved in the

212

modification reactions as they bear accessible OH-groups.

of

the

modified

biomass

(band


assignments36).

Accordingly,

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Figure 1. (A) Schematic representation of the synthetic path, and (B) ATR-IR spectra of

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(top to bottom) unmodified wheat straw, carboxymethylated straw (WS2) and (c) citric

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acid-crosslinked WS2 (16 wt % citric acid, 140 °C, 30 min).

217

Synthesis parameters (covering: –CH2COONa content of CM-intermediates, reaction

218

temperature and duration, and crosslinker concentration) of citric acid esterifications

219

were evaluated by introducing three sample-specific values: gel yield (%), swelling ratio

220

(gwater g-1) and gel stiffness (G´, Pa). Exact data for each parameter set are summarized in

221

the SI (Table S2).


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Strain sweep experiments showed that the strain resistance of the gel network

223

decreased with an increasing content of carboxymethyl groups in the biomass (WS3 >

224

WS2 > WS1) (see SI, Figure S1). This implies a more pronounced network extension

225

through electrical repulsion of charged carboxymethyl groups – a typical observation

226

for polyelectrolyte hydrogels.37 Besides, reaction temperatures above 120 °C and citric

227

acid concentrations of > 2 wt % significantly enhanced the strain resistance of the gels; a

228

further increase in temperature and crosslinker amount, as well as long reaction

229

durations (300 min) rather affected the overall gel stiffness G´ in the same strain range

230

than the strain resistance of the network (SI, Figure S2). The frequency responses

231

revealed a dominant elastic behavior over the entire frequency range, indicating that the

232

polymer networks were sufficiently crosslinked (SI, Figure S3).

233 234

Figure 2. Crosslinking of the carboxymethyl intermediates WS1, WS2 and WS3 –

235

development of gel yield (◼), swelling ratio (○) and gel stiffness (◆, G´) as a function

236

of the carboxymethyl group content.


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Page 16 of 45

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Besides the impact on gel stiffness, carboxymethylation essentially determined yield

238

and swelling capacity of the hydrogels. Gel yield and stiffness decreased with

239

increasing carboxymethyl content, whereas the swelling capacity was significantly

240

enhanced (Figure 2). Following these trends, WS1-CA4% gave a high yield after

241

crosslinking (78 %) and rigid hydrogel particles (G´ = 1558 Pa), with a poor swelling

242

capacity (8 gwater g-1). Swelling increased from WS2-CA4% (38 gwater g-1) to WS3-CA4% (50

243

gwater g-1) with the higher content of ionic groups and corresponding to the increased

244

water-solubility of the carboxymethyl intermediates (WS2, WS3). However, the

245

crosslinking reaction required free and accessible hydroxyl groups. Therefore, yield and

246

gel stiffness decreased remarkably for the gel based on WS3 (45 %, 399 Pa).

247

Nevertheless, the product still appeared as mechanically stable and, thus, as promising

248

candidate for soil tests.

249

The impact of the reaction setting on gel yield, stiffness and swelling was evaluated

250

by esterifying WS2 applying various sets of reaction temperatures, durations and

251

crosslinker quantities (Figure 3). The actual amount of citric acid (Figure 3(A)) was

252

thereby naturally the most decisive factor: products crosslinked with 2 wt % citric acid

253

were of a very soft texture, whereas high crosslinker concentrations (8 or 16 wt %)

254

resulted in brittle gels and poor swelling. As depicted in Figure 3(B) crosslinking was

255

also reinforced through prolonged reaction durations. Considering both swelling and

256

structural stability of the hydrogels, 30 minutes were found to be the ideal curing time.


15

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257

A shorter duration (here: 15 min), however, insufficiently crosslinked the biopolymer

258

chains, resulting in a high proportion of water-soluble fraction. Elevating the reaction

259

temperature from 120 to 160 °C (Figure 3(C)) increased the product yield clearly (48 to

260

69 %), probably owing to an accelerated thermal degradation of citric acid to its reactive

261

anhydride at temperatures above 153 °C.38 The stiffness of these gels, however,

262

remained within almost the same range.


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Figure 3. Esterification of the carboxymethyl intermediate WS2 (2.56 mmol –

265

CH2COONa g-1): gel yield (◼), swelling ratio in deionized water (○) and gel stiffness


17

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(◆, G´) as functions of (A) citric acid concentration, (B) reaction duration and (C)

267

temperature.

268

Overall, the ideal reaction setting was found to be: 140 °C, 30 minutes, 4 wt % citric

269

acid. Compared to citric acid gels based on pure carboxymethyl cellulose,27 swelling of

270

the straw gels was considerably lower indicating to remaining lignin-carbohydrate

271

interconnections. Also, gels obtained from lignocelluloses by graft-copolymerization, as

272

in studies of El-Saied et al. (e.g. 2000 and 2016)17,18, had a much higher water-uptake

273

(350 g/g), however, likely originating from the synthetic vinyl-based polymer part.

274

Moreover, it is questionable whether too high swelling is beneficial for the integrity of

275

the soils´ unique pore structure.

276

Gel-water-interactions. Dynamic vapor sorption (DVS) analysis delivers a valuable

277

insight into gel-water-interactions (e.g. accessibility of hydrophilic sites, swelling

278

behavior) by studying the response of an inititally dry sample (xerogel) to humidity

279

changes. In our study, DVS gave answers on two central questions: (i) How did the

280

synthetic path affect the interaction between lignocellulosic matrix and water? and (ii) Do the

281

gels retain their structural integrity upon the contact with water and in a sequenz of sorption-

282

desorption cycles? The later point thus gives vital information for a potential real

283

application, considering complete drying throughout the period of usage. To evaluate

284

both aspects, samples with different carboxymethyl contents and/or different network

285

densities were analyzed.


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Page 20 of 45

286

Figure 4 shows adsorption and desorption isotherms of the gels. Throughout Ad1 and

287

Ad2, all samples clearly displayed an increasing Meq with the rising aw. The sigmoid

288

curve shape – having a significant upswing at high aw – corresponds to type II isotherms

289

(Brunauer´s

290

physisorption.33,40 The water uptake at aw ≤ 0.2 is due to monolayer adsorption41, which

291

was fairly low for all samples. This indicates a poor accessibility of hydrophilic sites and

292

weak adsorbent-adsorbate interactions at low vapor pressures.40 Intermediate

293

humidities (0.3 ≤ aw ≤ 0.7) provoked a more convex isotherm shape. In this stage

294

multilayer sorption proceeded after the monolayer was saturated with water41, and the

295

deviation between the samples´ sorption isotherms started to grow remarkably.

classification39)

indicating

unrestricted

monolayer-multilayer

296 297

Figure 4. Sorption isotherms of the first and second adsorption (Ad1/Ad2) and the

298

intermediate desorption (De1) of gels based on WS1 (1.75 mmol –CH2COONa g-1) or


19

Page 21 of 45 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


299

WS3 (3.79 mmol –CH2COONa g-1). (crosslinking conditions: 140 °C, 30 min, 4 or 8 wt %

300

citric acid)

301

At high water activities (aw 0.8-0.9), Meq of the straw gels increased rapidly, probably

302

following moisture penetration into the sample particles, which led to swelling and

303

plasticization of the rigid polymer network and a liberation of further hydrophilic

304

sites.41,42 The curve upswing in this stage can be clearly linked to the degree of

305

carboxymethylation (WS3 > WS1), with respect to ascent and final Meq. Furthermore,

306

comparing isotherms of WS3-CA4% and WS3-CA8% reveals that the higher crosslinking

307

density of the latter gel has not reduced water sorption until aw 0.9. On the contrary, the

308

moisture uptake of WS3-CA8% was slightly enhanced, though equlibrium swelling was

309

much lower (WS3-CA4%: 50 g/g, WS3-CA8%:12 g/g). This finding can be likely attributed

310

to the DVS methodology, in which a thermodynamic equlibrium of the samples´

311

swelling process was not accomplished.

312

Deviations between adsorption and desorption processes are characterized by

313

hysteresis loops, implying (irreversable) structural alterations within the sorbents´

314

domains following their interaction with water.40 Sorption hysteresis occurred

315

throughout the entire humidity range for each sample. The extent and exact position of

316

hysteresis maxima within the humidity range, however, obviously depended on

317

individual sample characteristics, coinciding, in particular, with the crosslinking

318

density. Gels crosslinked with 4 wt % citric acid showed a maximum of hysteresis at


20


Page 22 of 45

319

intermediate aw. WS3-CA8% evidently retained high amounts of moisture particularly at

320

low water activities (aw 0.1-0.4), pointing on an entrapment of water within the gel

321

network. This may be related to the affinity of water to fill pores, which increases with

322

the crosslinking density.42 However, it is further conceivable that citric acid

323

esterification increased the interaction with water molecules due to the introduction of

324

further OH-groups.

325

Deviations between Ad1 and Ad2, including shifted starting points and a reduced

326

moisture-absorbency at aw = 0.9, were most likely induced by structural alterations

327

during the first experiment cycle. Particularly, in the case WS3-CA8%, Ad2 started from

328

a significantly higher moisture level, which again indicates water entrapment.

329

Moreover, the fact that Meq,

330

humidity range implies that water penetrated more easily in Ad2. This effect may be

331

linked to the formation of new sorption sites with the irreversable cleavage of weaker

332

bonds (e.g. hydrogen bonds) during network extension in Ad1.42,43 High contents of

333

carboxymethyl groups would increase this effect as they promote swelling.

334

Furthermore, for WS1-CA4%, the final moisture load (at aw = 0.9) was slightly reduced,

335

which can be attributed to gel shrinkage during desorption entailing an irreversable

336

formation of hydrogen bonds (hornification).44,45 The icreasing carboxylate content from

337

WS1 to WS3, however, obviously mitigated this effect.

Ad2

exceeded Meq,

Ad1

for each sample in the intermediate


21

Page 23 of 45 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


338

To interpret gel-moisture interactions, sorption isotherms were fitted to the GAB

339

equation (SI, Table S3) followed by calculation of monolayer capacity and water-

340

accessible surface area (Figure 5). As indicated by the higher M0 and AG for WS3-based

341

gels, carboxymethylation increased the accessibility of sorption sites. The subsequent

342

crosslinking step enhanced the interaction with water, likely owing to the additional

343

introduction of OH-groups. As discussed previously, higher monolayer capacities

344

during De1 imply a hysteresis of sorption domains (i.a. entrapment of moisture).

345

346 347

Figure 5. Monolayer capacity M0 (blue bars/values) and water-accessible specific surface

348

area AG (◻, black values) obtained by GAB model fitting of sorption isotherms of first

349

and second adsorption (Ad1/Ad2) and intermediate desorption (De1). (Gels based on

350

WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1), crosslinking

351

reaction: 140 °C, 30 min, 4 or 8 wt % citric acid)


22


Page 24 of 45

352

Although it was shown before that WS3-CA8% retained water at low vapor pressures

353

during De1, the GAB model fit gave significantly decreased M0 and AG for the

354

desorption cycle. Both parameters further decreased in Ad2. The same phenomenon can

355

be observed for WS1-CA4%. In both cases, sorption sites obviously became unavailable

356

for water binding, which might be explained by an irreversable formation of hydrogen

357

bonds during De1. In contrast, WS3-CA4% gave a steady increase in monolayer moisture

358

levels and water-accessible surface area.

359

Overall, the development of M0 and AG underlines the impact of each synthesis step

360

on water-matrix-interactions. Sorption processes probably also correlate with the

361

disintegration of the lignocellulosic matrix and decrystallization of the cellulose fraction

362

accompanying carboxymethylation. The presence of charged carboxylate groups promoted

363

network extension, which accelerated moisture penetration and clearly mitigated the

364

irreversable aggregation of biopolymer chains during drying. This ensures a re-

365

swellability of the gel network, for example, in a real soil application. Particularly, WS3-

366

CA4% showed a promising structural integrity upon the contact with moisture

367

suggesting its further evaluation in a model-scale soil application.

368

Water retention in soil substrates. Using two different model soil substrates, water

369

retention tests were conducted to give a more application-related assessment of the gel

370

performance. Based on promising previous results, the straw gel WS3-CA4% was

371

exemplarily selected. Furthermore, the model soils chosen – classified as a sandy and a


23

Page 25 of 45 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


372

silty substrate – provided us information about the practical gel application (covering

373

particle sizes, exchangeable cations, etc. – SI, Table S1). However, in Nature, mixtures of

374

both soil substrates may rather be found. Increasing the storing capacity of plant

375

available water in sandy soils, for instance, may enable agricultural cultivation on

376

formerly unusable areas and could be, therefore, an important contribution to securing

377

food supply. Silty soils, on the other hand, as valuable agricultural grounds, may

378

benefit from an improved water storing capacity, particularly, in view of climate change

379

scenarios.

380

Figure 6 demonstrates how the amount of water retained in the sandy or silty soil and

381

in soil/gel mixtures developed as function of the applied matrix potential (pF). This

382

relationship represents important soil-moisture characteristics, e.g. the plant water

383

availability depending on water content. For both model soils, the incorporation WS3-

384

CA4% significantly increased the water retention throughout the pF range, with a direct

385

correlation to gel incorporation rate and, additionally, soil characteristics. The starting

386

point of the assay (pF = 0) enables the examination of unaffected soil-gel-interactions,

387

without external impairments. At that point (as well as throughout the pF-range), the

388

silty substrate inherently retained more water, owing to its smaller grain sizes and

389

consequently higher volume of medium sized pores compared to the sand (SI, Table

390

S1). However, swelling of the gel was obviously less restricted in the sandy soil,

391

resulting in a more remarkable improvement of the sands´ water retention. The


24


Page 26 of 45

392

deviation between the water content retained within the different soil/gel mixtures

393

became more evident with the increasing matrix potential and incorporation time. From

394

pF ≥ 1.60 (experiment duration of ≥ 96 hours) the water retention of silt/gel mixtures

395

dropped down, which is especially significant for the higher gel incorporation rate of

396

0.7 wt %. In contrast, curve progressions of gel/sand mixtures at both rates remained

397

almost linear throughout the pF-range. Within the matrix potential range, the region

398

between pF 1.80 and 2.53 is of particularly interest for agronomic applications as a

399

control of irrigation.46 The amount of water in this pF region is thereby defined as field

400

capacity and represents the maximum of plant available water.46 The incorporation of

401

WS3-CA4% thus significantly improved the water retention of the sand at both rates

402

within this specific range (e.g. at pF 2.53: 0.2 or 0.7 wt % gel increased water retention by

403

70 or 300 %, respectively). However, the effect on the silty substrate at pF 2.53 in our

404

lab-scale survey was much smaller, with an increase of circa 12 or 20 %, by

405

incorporating 0.2 or 0.7 wt % dry gel granules, respectively.

406


25

Page 27 of 45 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


407 408

Figure 6. Water retention in soil substrates – water retained in a (A) sandy or (B) silty

409

soil or gel/soil mixtures as function of matrix potential (0 ≤ pF ≤ 2.53) and gel

410

incorporation rate (w/w soil). (hydrogel: WS3-CA4%; blue error bars: standard deviation,

411

n = 3)

412

This significant deviation between the two soil substrates thereby most probably

413

corresponds to both the different soil characteristics and the ionic character of the

414

lignocellulosic hydrogel. Cations – particularly of a divalent nature as Ca2+ – released


26


Page 28 of 45

415

from the soil substrate may have strongly impaired the swelling of the gel particles over

416

time by physically crosslinking the carboxylated polymer chains. This aspect can be

417

demonstrated by a very simple experiment, swelling WS3-CA4% in aqueous saline

418

solutions (SI, Figure S5). Accordingly, the addition of cations to the surrounding

419

medium strongly limited the water absorbency of the hydrogel under equilibrium

420

swelling conditions. Another reason for diminished swelling of WS3-CA4% in the silty

421

soil might have been its fine texture and, therefore, strengthened capillary forces within

422

soil pores. With the increasing matrix potential, the induced capillarity might thus have

423

entailed a drainage of the gel particles and an alignment of the water potential in gel

424

and surrounding soil substrate. Also, the soil pH (sand: 4.07, silt: 6.98) might have

425

affected the swelling properties of the gel. Especially, a soil pH below the pKA of

426

carboxylic groups (4.6)47 would cause, over time, a collapse of the gel network due to

427

the induced ion exchange. Nevertheless, throughout our short-time lab-scale

428

experiments, WS3-CA4% had a higher swelling capacity in the more acidic sandy

429

substrate.

430

According to our experiments, the incorporation of WS3-CA4% showed promising

431

potential for the sand within the examined pF range. However, to thoroughly illuminate

432

the effect of the gel on the water accessibility in soils, it would be essential to extent

433

water retention tests to pF = 4.2. Beyond a matrix potential of 4.2 (permanent wilting

434

point) water would be unavailable for plants.46,48


27

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435

Finally, a crucial point in further studies should be to translate the model-scale into

436

real applications, involving optimized synthetic upscaled procedures and field

437

experiments that offer realistic conditions. Also, the high loss of biomass due to

438

extraction/dissolution throughout the synthetic path (> 60 % for, e.g., WS3-CA4%)

439

demands for further process optimization and/or strategies to recover and valorize lost

440

material fraction. For the actual soil application, the structural integrity of the gel over

441

an adequate time period (e.g. one growth season) and its final biodegradation should be

442

central points of assessment. For the upscale to an economic volume, citric acid

443

crosslinking, as a very simple water-based, heat-induced reaction would allow for

444

continuous processing (e.g. on belt furnaces).

445 446

CONCLUSIONS

447

Herein we presented a simple synthetic approach to fabricate wheat straw-based

448

hydrogels starting from the unfractionated and carboxymethylated lignocellulosic

449

matrix. Citric acid was used as a cheap and non-toxic low-molecular-weight crosslinker.

450

Optimized reaction conditions led to mechanically stable hydrogels with moderate

451

swelling capacities of up to 50 gwater/g. Gel swelling and rheology were thereby

452

essentially determined by both the content of ionic carboxylate groups and the

453

crosslinking density. Moreover, DVS analysis revealed that the straw-based gels re-

454

swell after complete drying, implying a promising durability in terms of a potential soil


28


Page 30 of 45

455

application. Finally, incorporating one selected hydrogel into different model soil

456

substrates showed that already low gel rates (here: 0.2 wt %) may effectively increase

457

the water holding capacity of, especially, sandy soils.


29

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458


ASSOCIATED CONTENT

459

Supporting Information. A supporting information file (PDF) is available free of

460

charge containing: soil substrates´ characteristics; additional results for gel rheology,

461

crosslinking conditions vs. gel properties, DVS analysis, and swelling experiments.

462 463

AUTHOR INFORMATION

464

Corresponding Author

465

* [email protected]

466

Present Addresses

467

 Department

468

00076 Aalto, Espoo, Finland

469

Author Contributions

470

The manuscript was written through contributions of all authors. All authors have

471

given approval to the final version of the manuscript.

472

Funding Sources

473

The authors gratefully acknowledge the financial support from BASF SE (Germany) and

474

from the Graduate Academy, Technische Universität Dresden (Excellence Initiative of

475

the German federation and the federal states).

of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, FIN-


30


476

Notes

477

The authors declare no competing financial interests.

478

ACKNOWLEDGMENT

Page 32 of 45

479

The authors would like to thank Dr. Arndt Weiske (Institute of Plant and Wood

480

Chemistry, TU Dresden), Dr. Thomas Klinger and Gisela Ciesielske for ICP-OES

481

measurements and the assistance with water retention tests.

482

ABBREVIATIONS

483

Ad1(2), first (second) adsorption; CA, citric acid; De1, intermediate desorption; DI

484

water, deionized water; GAB, Guggenheim-Anderson-de Boer; ICP-OES, inductively

485

coupled plasma optical emission spectroscopy; IPA, isopropyl alcohol; RT, room

486

temperature; WS, wheat straw

487

REFERENCES

488

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489 490

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623 624

For Table of Contents Use Only

625 626

SYNOPSIS

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Herein, lignocellulosic biomass wastes – a sustainable polymer source – are used to

628

fabricate hydrogels as miniature water reservoirs for agricultural applications.


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Figure 1. (A) Schematic representation of the synthetic path, and (B) ATR-IR spectra of (top to bottom) unmodified wheat straw, carboxymethylated straw (WS2) and (c) citric acid-crosslinked WS2 (16 wt % citric acid, 140 °C, 30 min). 272x208mm (300 x 300 DPI)


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Figure 2. Crosslinking of the carboxymethyl intermediates WS1, WS2 and WS3 – development of gel yield (◼), swelling ratio (○) and gel stiffness (◆, G´) as a function of the carboxymethyl group content. 160x94mm (300 x 300 DPI)



Figure 3. Esterification of the carboxymethyl intermediate WS2 (2.56 mmol –CH2COONa g-1): gel yield (◼), swelling ratio in deionized water (○) and gel stiffness (◆, G´) as functions of (A) citric acid concentration, (B) reaction duration and (C) temperature. 160x279mm (300 x 300 DPI)


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Figure 4. Sorption isotherms of the first and second adsorption (Ad1/Ad2) and the intermediate desorption (De1) of gels based on WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1). (crosslinking conditions: 140 °C, 30 min, 4 or 8 wt % citric acid) 175x80mm (300 x 300 DPI)



Figure 5. Monolayer capacity M0 (blue bars/values) and water-accessible specific surface area AG (◻, black values) obtained by GAB model fitting of sorption isotherms of first and second adsorption (Ad1/Ad2) and intermediate desorption (De1). (Gels based on WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol – CH2COONa g-1), crosslinking reaction: 140 °C, 30 min, 4 or 8 wt % citric acid) 119x70mm (300 x 300 DPI)


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Figure 6. Water retention in soil substrates – water retained in a (A) sandy or (B) silty soil or gel/soil mixtures as function of matrix potential (0 ≤ pF ≤ 2.53) and gel incorporation rate (w/w soil). (hydrogel: WS3-CA4%; blue error bars: standard deviation, n = 3) 144x209mm (300 x 300 DPI)


wheat straw-based hydrogels as soil conditioners?

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