Water Extractable Organic Carbon in Fresh and Treated Biochars

Yun Lin a, Paul Munroe a, Stephen Joseph a, Rita Henderson b, Artur. Ziolkowski c

a

School of Materials Science and Engineering,The University of New South Wales, Sydney, NSW 2052, AustraliaB School of Civil and Environmental Engineering,

The University of New South Wales, Sydney, NSW 2052, Australiac School of Environmental and Life Sciences, the University of Newcastle, NSW 2258, Australiap.munroe@unsw.edu.au

2

Introduction

Well known that the structure and chemistry of biochars depends on processing conditions, feedstock etc.

Water soluble organic substances, or water-extractable organic carbon (WEOC) adds to the pool of dissolved organic carbon (DOC) in the soil. This, presumably, plays a role in the behaviour of the biochar in the soil.

The aim of this work is to characterize the water soluble organic substances contained in a range of biochars.

3

Introduction

Further, chemical activation of biochars can affect behaviour. For example, phosphoric acid (H3PO4) and potassium hydroxide (KOH) treatment can modify surface chemistry and structure and, thus, promote enhanced interaction with the soil.

Liquid Chromatography - Organic Carbon Detection (LC-OCD) is an automated size-exclusion form of chromatography. It has the capability to measure low concentrations of organic carbon in water down to the ppm level.

Meanwhile, the soluble organic substances can be fractioned, which is based on their retention time during LC-OCD analysis, due to their molecular weight and interaction with material located in the separation column

4

Introduction

The aim is to examine water extractable organic carbon (WEOC) content in four primary biochars, produced from different feedstocks or under differing pyrolysis temperatures.

All biochars, subsequently, given chemical treatment with either H3PO4 or KOH.

Scanning electron microscopy (SEM) was also used to examine biochar microstructure.

The objective is to identify the effects of process conditions and chemical treatment on the concentration and composition of WEOC.

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Nomenclature of Samples

Feed stocks Pyrolysis temp. (°°°°C)Primary

biochars

KOH

treated

biochars

H3PO4

treated

biochars

Acacia

Saligna380 Saligna Saligna-K Saligna-P

Sawdust 450 SD450 SD450-K SD450-P

Sawdust 550 SD550 SD550-K SD550-P

Jarrah 600 SIM SIM-K SIM-P

These four biochars were treated with solutions of either H3PO4 or KOH. In each case 5 g of biochar was heated with 200ml of 1M of either H3PO4 or 0.1 M KOH for 1 h at 90ºC

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LC-OCD

UVD = Ultra Violet DetectorOCD = Organic Carbon DetectorOND = Organic Nitrogen Detector

10g of each sample was added to

100 ml distilled water at 50°C for 24 hr and regularly stirred, and then centrifuged/filtered to separate the solid and liquid. The column was used using a phosphate buffer. The chromatographic column is a weak cation exchange column containing polymethacrylate filler.

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LC-OCD

Spectrum records the fraction of each phase as a function of retention time. Curves are visible for OCD, UVD and OND samples

OCD is organic carbon detection.OND is organic nitrogen detection. UVD is Ultra-violet detection

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LC-OCD breaks water soluble carbon compounds in several broad groups:

Biopolymers – proteins and polysaccharides;

Humics – similar in structure and molecular weight to humic and fulvic acids;

Building blocks – oxidation products of humics;

Low molecular weight (LMW) acids and humics – LMW humics and small acids e.g. carboxylic;

LMW neutrals – uncharged small organics.

Hydrophobic organic carbon - the fraction of DOC remaining in the column, implying a strong hydrophobic interaction with the column material.

LC-OCD Fraction

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The proximate and ultimate analysis of the four primary biochars

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The proximate and ultimate analysis for the four primary biochars(before chemical treatment)

Chemical composition varied with both feedstock and pyrolysis temperature.

Saligna biochar has high ash content and the highest P and N content and more

Ca, K, Mg, and S, which is associated with the feedstock (acacia saligna).

SIM biochar contained the highest ash content and was rich in Na, which is

related to the high pyrolysis temperature and the feedstock composition of jarrah

wood.

For the sawdust biochars, a higher pyrolysis temperature resulted in a biochar

(SD550) with a higher ash and carbon content, lower volatile matter and a lower H and O percentage compared with SD450.

The highest oxygen content and cation exchange capacity (CEC) were noted in

SD450. This is related to the carboxylic group content in this biochar, as indicated

by its acidic pH value (~12).

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LC-OCD quantitative analysis of the WEOC before chemical treatment

The highest WEOC content was present in SD450. Both SD550 and SIM had low WEOC concentrations.

There was no hydrophobic dissolved organic carbon fraction detected in the primary biochars.

The dominant fraction in both Saligna and SD450 was LMW-neutrals.

LMW-acids was the main fraction in both SD550 and SIM. Moreover, the aromaticity of the humics fraction was similar in Saligna and SD450, which were higher than that measured in SD550 and SIM.

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Relative percentage of the fractions separated in the WEOC

Saligna contains 40% - 45% LMW-neutrals.

When the pyrolysis temperature was

increased, a LMW-acids fraction appeared in

SD450 together with a slight decrease in the percentage of the building blocks fraction. The

percentage of LMW-neutrals was similar to that

of Saligna, at 40% - 45%.

At a pyrolysis temperature of 550°C (SD550) the LMW-acids content reached 42%, mainly through the consumption of the biopolymer,

humics, and building blocks fractions.

At 600°C (SIM), almost no biopolymer fraction was present in SIM. LMW-acid is the principal fraction in this biochar, where a content of ~63% was measured. The percentage of LMW-neutrals in SIM decreased to ~ 25%.

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LC-OCD of WEOC after chemical treatment

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LC-OCD of WEOC after chemical treatment

For the KOH and H3PO4 treated biochars these chemical treatments greatly increased the WEOC content.

This is possibly due hydrolysis of the ester groups formed by cross-linking between lignin and cellulose during the pyrolysis process.

The KOH treatment showed a greater increase of WEOC in Saligna-Kand SIM-P compared with samples subjected to the H3PO4 treatment.

It is possible that the KOH not only accelerates the hydrolysis reaction, but also improves the dissolution of phenols /humics with both a higher aromaticity and molecular weight.

However, the concentration difference of WEOC between KOH and H3PO4

treatment from either SD450 or SD550 was much less pronounced.

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LC-OCD of WEOC after chemical treatment

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LC-OCD of WEOC after chemical treatment

Hydrophobic organic carbon was only detectable in the KOH-treated SIM biochar (SIM-K) and was the dominant fraction (~68%). This was also the principal contributor to the increase of WEOC in SIM-K, compared to SIM. This is probably associated with some phenolic tar-like component.

In SD550-K, both building blocks and LMW-neutral were at contents of more than 40%, and little biopolymer was separated.

In SD450-K, the highest humics concentration was found. The highest LMW-acid content was noted in Saligna-K. These changes indicated that both feedstock and pyrolysis conditions also control the composition of weakly-bonded components or groups (labile carbon and volatile matter), which contribute to the increase of WEOC.

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SEM Analysis

SEM images of the SD550 biochar before and after chemical treatmenta) SD550; b) P-SD550; c) K-SD550

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SEM Analysis

SEM analysis showed that the primary biochar possesses a porous structure and smooth wall surfaces.

H3PO4 treatment roughens/etches the biochar surface and leads to the formation of nano-pores. Likely that H3PO4 treatment dissociated the weakly-bonded components (e.g. labile carbon and volatile matter) on the biochar surface, which is likely to contribute to an increase of WEOC in the H3PO4-treated biochars. Energy dispersive x-ray spectroscopy analysis (EDS) shows P was localised on the surface.

For the KOH-treated biochar it is difficult to identify any enhancement of porosity, but fine, uniformly distributed grains are visible on the biochar surface. EDS showed K is present uniformly on the biochar surface

KOH-dissociated organic carbon/WEOC was adsorbed onto the biochar during the dehydration process, because of the interaction between K+

cations and nucleophilic groups (such as phenolic and carboxylic groups) included in the dissociated WEOC.

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Summary

Variations in feedstock and pyrolysis temperature effects WEOC content and composition.

High ash contents from feedstocks can catalyse thermochemical reactions at relatively low temperature and produce biochars with a relatively high carbon content.

Lower pyrolysis temperatures favour the formation of WEOC with LMW-neutrals and building blocks and the produced biochars contain more weakly-bonded carbon (labile carbon and volatile matter), which could be dissociated by post-chemical treatment (chemical activation).

Higher pyrolysis temperature (›450°C) may cause secondary reactions during pyrolysis and result in a lower WEOC content.

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Summary

Chemical treatment by either H3PO4 or KOH increases WEOC content, by catalysing hydrolysis reaction on biochar surfaces.

Moreover, either P or K was found to be incorporated on to the surfaces of the treated biochars.

KOH treatment also promoted the dissociation of phenols and humics with relatively high aromaticity and molecular weight in biochars by ionization.