Fertilizer and Soil Solubility of Secondary P SourcesâThe Estimation

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Environmental Processes

Fertilizer and soil solubility of secondary P sources – the estimation of their applicability to agricultural soils Šárka Václavková, Michal Šyc, Jaroslav Moško, Michael Poho#elý, and Karel Svoboda Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02105 • Publication Date (Web): 05 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Environmental Science & Technology

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Fertilizer and soil solubility of secondary P sources

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– the estimation of their applicability to agricultural

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soils

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Šárka Václavková‡*, Michal Šyc‡, Jaroslav Moško‡, Michael Pohořelý‡, Karel Svoboda‡

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Department of Environmental Engineering, Institute of Chemical Process Fundamentals of the

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CAS, v.v.i., Rozvojova 135, 165 02 Prague 6, Czech Republic

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Abstract

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The demand for phosphorus (P) sources is increasing with the growing world population,

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while objections to direct agricultural use of waste P sources, such as sewage sludge, are being

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raised. Therefore, the need arises to employ safe and efficient secondary P fertilizer sources,

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originating from P-rich wastes. These recycling sources are commonly tested in accordance with

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the current fertilizer rules, designed originally for conventional apatite-based P fertilizers. The

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behavior of sewage sludge ash, an inorganic recycling secondary P source, was investigated

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under soil-like conditions. Standardized soil P tests, including the soil buffering capacity test and

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the Olsen, the Mehlich3 and water extraction methods, were employed together with

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standardized fertilizer P-solubility tests by neutral ammonium citrate and 2 % citric acid

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extraction. In addition, total content and the overall soil mobility of selected metallic elements

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present in sewage sludge ash were investigated. The suitability of standardized soil tests for the

ACS Paragon Plus Environment

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evaluation of recycling P sources was shown. An apparent influence of Ca:Al content ratio on

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sewage sludge ash behavior under different soil-like conditions shows the inadequacy of the

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current fertilizer test and the necessity to understand soil-like behavior of secondary P sources,

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when considering these as possible agricultural P bearers (fertilizers).

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1. Introduction

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Phosphorus (P) plays a key role in mammalian energy metabolism as a part of ATP and ADP

25

molecules, contributes to genetic information transfer as a part of DNA and RNA molecules, and

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participates in the formation of tissues, such as bones or teeth. It is therefore an essential nutrient

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in the human diet, with the recommended intake of 700 mg/day for adults1. Mammals, including

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human beings, take in phosphorus via plant consumption, where plants rely on secure P intake to

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ensure energy production in photosynthesis2. The demand for P sources is increasing as a

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consequence of the growing world population and the increasing rate of meat products in the

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human diet3. To allow sustainable food production, nutrients’ amendment of agricultural soils

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becomes necessary, increasing the demand for P fertilizers.

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Sources of P for fertilizer production are limited. Over 85 % of the currently most common

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source of P, a phosphate rock– the apatite – is controlled and mined in countries like Morocco,

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China, Syria and Jordan4. About 8 % of apatite reserves is located in the USA and in South

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Africa, but the mined apatite production does not even cover their own needs. Recovery of P

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from waste materials is therefore becoming an important part of national resource strategies in

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developed countries, with the aim of applying secondary P sources directly in agriculture or

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using them for P fertilizer production. Most of the human consumed P ends up in wastewater and

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consequently in sewage sludge, waste products of current wastewater treatment technologies,

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where P is commonly precipitated by Al or Fe salts.


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Typical sewage sludge (SS) produced in central Europe is either thickened slurry with a total

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solid content of about 4-5 %, or dewatered mud with a total solid content of about 20-35 %,

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having a P content of up to approx. 40g/kg of SS dry matter5. Considering such P content and

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European SS production, exceeding 11 million tons dry mass annually6, SS produced in central

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Europe has theoretically the capacity to replace 40-50 % of mineral fertilizing P applied in

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agriculture yearly7,8. Thus, sewage sludge has the clear potential to significantly reduce the

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dependence of European countries on apatite import.

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The common practice of SS direct field application is diminishing in the EU-28, due to the

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potential health and environmental risks associated with SS contamination by organics

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(including hormones, antibiotics, endocrine disruptors, and persistent organic pollutants), heavy

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metals, and pathogens9,10. Consequently, the importance of sewage sludge incineration is

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increasing, because all organics are decomposed during incineration, while sludge mass is

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reduced and phosphorus is concentrated in the resulting sewage sludge ash (SSA)11. Typical P

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concentration in SSA is between 50 and 130 g/kg SSA12–14, which is comparable to a low-grade

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apatite P content5. Minor and trace elements, including heavy metals, are commonly present in

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SSA as well15–17. A number of approaches (with different degrees of technological readiness) has

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currently been developed for P recovery from SS and SSA resulting from its monoincineration5.

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In this sense, SS and products of its post-treatment are changed from waste into secondary

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sources of fertilizer phosphorus (agricultural P).

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Crucial factors limiting the potential use of secondary P sources for agricultural purposes are

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purity, i.e. the presence and quantity of contaminants, and efficiency, i.e. the availability to

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plants of the provided P. Because rules and legislation addressing secondary nutrient sources are

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currently lacking, although technical proposals are being developed18, secondary P sources are


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commonly examined in accordance with the P fertilizer standards adopted for apatite-based

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fertilizers19–21.

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Legislation regulating the use of conventional fertilizers in Europe22–24 considers, by a practice

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well known, the behavior of apatite-based P compounds when applied to agricultural soil.

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Finally, soil conditions are a governing factor influencing the solubility of contaminants and

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nutrients in soil solution, as is their plant availability, as plants are able to utilize nutrients only

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when dissolved. Moreover, dissolved nutrients and contaminants are more mobile and may

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therefore be transported with soil water towards water bodies or distant soil environments25. To

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the authors’ knowledge, the behavior of secondary P sources, such as SSA under soil conditions,

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has not yet been well described. Recently, several studies consider the behavior of secondary P

75

sources in soil environments using pot experiments19,26,27. These tests are, however, extremely

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time- consuming, expensive and provide information only specific to the tested plants and crops.

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Here, we conducted a study evaluating the potential of using standardized soil tests to describe

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the behavior of new secondary P sources under soil-like conditions and thus to classify their

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applicability to agricultural soils. Furthermore, we determined the buffering capacity, cation

80

exchange capacity and P plant availability pH dependency of industrially produced SSAs.

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Moreover, the SSAs were examined in accordance with the current P fertilizer standards to

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enable continuity and comparison with other SSA surveys.

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2. Materials and methods

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2.1 Secondary P sources

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SSAs used within our presented study originated from three different mono-incineration

86

facilities in Europe using different combustion technologies. SSA1 was a bottom ash, produced

87

industrially by a grate furnace facility with a yearly capacity of 55 000 t SS dry matter (SSdm).


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SSA2 represented a fly ash, produced in a circulating fluidized bed incinerator with a yearly

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capacity of 84 000 t SSdm. SSA3 represented bottom ash, produced by a pilot-scale grate furnace

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incinerator with a yearly capacity of 125 t SSdm. Particle size distributions of representative SSA

91

samples were estimated by sieving of weighted, air-dried and cross-divided SSA samples, prior

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to further experiments.

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2.2 Selection of elements of interest in SSA

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During SS incineration, all organic matter decomposes, while stable heavy metals are expected

95

to be the main contaminants present in the resulting SSA. According to the current European

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fertilizer regulations22,28 and the technical proposals for new legislation18,23, it is mandatory to

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determine the total content of As, Cd, Cr, Ni, and Pb. As stated earlier, phosphorus is often

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removed from wastewater during its treatment, using chemical precipitation with Al or Fe salts,

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where precipitates concentrate in SS. Therefore, the total contents of Al and Fe were determined

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as well. In addition, the total contents of Ca and Mg, as naturally phosphate-forming elements,

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and the total contents of Cu and Zn, as the most common potentially toxic metal elements in

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municipal wastewaters and SS29, were also determined.

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2.3 Total content of phosphorus and elements of interest

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The total contents of P and the elements of interest were determined according to ISO 11466,

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1997 methodology by microwave-assisted acid digestion with aqua regia (HNO3, HCl), a

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previously confirmed suitable method for the total amount of P in secondary P sources1. In

107

addition, Si content was determined by microwave-assisted digestion with HNO3, HCl, HF and

108

H2O2, followed by complexation with H3BO3 according to Method 305230. Digestion was carried

109

out in a solid to liquid (s:l) ratio of 1:60, at a temperature of approx. 200 °C in a microwave


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reactor (Anton Paar Multivawe 3000). All reagents used for the SSA analysis were purchased in

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analytical grade.

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2.4 Examination of SSA as fertilizer product

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The amount of plant-available P was estimated in tested SSAs in addition to the total P and

114

contaminants’ content. Water-extractable P indicated the amount of immediate plant-available P

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and was estimated in a s:l ratio of 1:100, according to European Regulation relating to fertilizers

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method No. 3.1.622. Neutral ammonium citrate (NAC) and 2 % citric acid (2 %CA) provided

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information about mid- to long-term plant-available P. NAC- extractable P was estimated in s:l

118

ratio of 1:100, according to method No. 3.1.422, P soluble in 2 %CA was determined according to

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the standard method 3.1.3 in s:l ratio of 1:10022.

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2.5 SSA behavior under soil-like conditions

121

2.5.1 Solubility of phosphorus and the elements of interest under soil-like conditions

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The behavior of secondary P sources under soil-like conditions was examined as their

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solubility in extraction agents simulating various soil solutions. The most basic test used was the

124

water extraction31 of soil material process, as described above22. Directly available P and

125

immediate release of heavy metals was estimated by this test. When describing material behavior

126

under soil-like conditions, basic soil type categorization should be considered. Soil pH is the

127

most informative and useful single soil characteristic, as soil properties, including plant-

128

availability of nutrients, microbial activity, base saturation and soil structure, depend on pH25.

129

Mostly calcareous, neutral or slightly-to-moderately acidic soils are used for crop production in

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developed countries (basic soil groups shown in Table S1). To describe solubility, thus potential

131

plant-availability of P under alkaline soil conditions, the Olsen procedure was used (extraction

132

with 0.5M NaHCO3 in s:l ratio 1:20)32,33. Olsen’s test is probably the most common method in


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the world for soil analysis of P, suitable to extract P from calcareous, alkaline and neutral soils.

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To describe solubility, thus potential P plant-availability, under acid and neutral soil conditions,

135

Mehlich3 extraction was used (extraction with a mixture of CH3COOH, NH4NO3, NH4F, HNO3

136

and EDTA in s:l ratio 1:10)32. The Mehlich3 extraction test is standardized in many countries,

137

also for testing the solubility of the following compounds in acidic to neutral soils: Ca, Cu, Fe,

138

K, Mg, Mn, Na, and Zn. For this reason, we used this test to describe solubility, thus potential

139

mobility of other selected elements of interest under acidic to neutral soil-like conditions.

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2.5.2 SSA pH, buffering and cation exchange capacity

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SSA pH was measured in water and 1M KCl extracts (s:l ratio 1:25) by pH electrode (WTW

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pH/Cond 340i).The relative ability of SSAs to store nutrients and to attract, retain and exchange

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cation elements was calculated as the cation exchange capacity (CEC). The CEC, expressed in

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meq/100 g (equal to cmol(+)/kg) was calculated from the levels of cations of Ca, Mg, Na and K in

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the Mehlich3 extraction solution34. The buffering capacity of SSAs, as a measure of the ability of

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their ability to resist pH decreases, was estimated as an amount of H+ ions necessary to change

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pH of SSA solution. 10±0.01g of SSA was placed in plastic vials and amended by 0.1 L of

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boiled distilled water, containing a graded quantity of 0.1 M HCl: 10, 9, 8, 7, 6, 5, 4, 3, 2 and 1

149

ml. Samples were stored for 72 hours in a dark, cold place (fridge) with occasional mixing.

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Thereafter, the mixed solution pH was measured.

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2.6 Elemental analysis

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Concentrations of elements in all SSA leachates (summary is shown in Table 1) were

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determined by means of inductively coupled plasma optical emission spectroscopy (ICP-OES).

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All leachates were acidified with nitric acid prior to analysis. All results of elemental analysis are


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presented as a portion of dry matter, i.e. mg/kgSSA or mass percentage (%) of dry matter, unless

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otherwise stated. Table 1 Summary of methods used to describe SSAs and their behavior as fertilizer product and their behavior under soil-like conditions Method

Label

Reagents

Microwave-assisted

TEC

HCl, HNO3

TEC-Si

HCl, HNO3, HF, H2O2, H3BO3

WE

Distillated water

acid digestion

Water extraction Neutral ammonium citrate extraction 2 % citric acid extraction

NAC

2 %CA

Purpose Total element content except Si Total Si content Immediately available P and release of heavy metals

Neutral ammonium citrate

Mid- to long-term fertilizer

solution

available P

2 % citric acid solution

Mid- to long-term fertilizer available P P plant-availability in

Olsen test

O

0.5M NaHCO3

calcareous, alkaline and neutral soils P plant-availability in acid

Mehlich3 extraction

CH3COOH, NH4NO3, NH4F,

ME3

HNO3, EDTA

to neutral soils, solubility (mobility) of Ca, Cu, Fe, K, Mg, Mn, Na and Zn in acid to neutral soils

SSA buffering capacity Cation exchange capacity pH

157

SSA buffering

HCl solution with boiled

SSA ability to resist pH

capacity

distillated water

decreases Ability to store nutrients and

CEC

Mehlich3 solution

to retain and exchange cation elements

pH

1M KCl/distillated water

pH

3. Results and discussion

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Different technological set-ups of incinerators led to different particle size distributions in

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resulting SSAs35. While the majority of grate furnace originating SSA1 and SSA3 particles are


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bottom ash particles with a diameter of over 0.2 mm, over 90 % of SSA2 fly ash particles are of a

161

diameter below 0.2 mm as shown in Figure 1.

162 163

Figure 1 Tested SSAs’ cumulative particle size distribution

164

3.1 SSA composition

165

SSAs samples originate from different technologies and thus it may be expected that these

166

originate from different SS (and wastewater). This study focuses on the description of behavior

167

of the resulting SSAs and thus the incinerator technological set-up influence on the elements in

168

focus are not within the scope of this study.

169

The P contents of all tested SSAs were in accordance with current findings on SSA as a

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possible secondary P source12–14, reaching up to 102 g/kgSSA. For comparison, typical P

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concentration in raw phosphate rocks from North Africa and Asia is 130 ± 40 g/kgROCK5.

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Contents of Al and Fe indicated that sludges used in the SSA2- and SSA3-forming incinerators

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come mainly from wastewater treatment plants, using dominantly Fe salts for P precipitation.

174

The sum of Al and Fe content is similar in industrially obtained ashes, while the ash from an

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experimental device contains a higher quantity of Al and Fe. For more details, see Table 2. Table 2 Composition of tested SSAs – the content of phosphorus and elements of interest and Si


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(TEC) and their Standard Deviations (SD; n=3) achieved, using microwave-assisted acid digestion SSA1

SSA2

SSA3

TEC

SD

TEC

SD

TEC

SD

[mg/kgSSA]

[mg/kgSSA]

[mg/kgSSA]

[mg/kgSSA]

[mg/kgSSA]

[mg/kgSSA]

Al

52 700

198

20598

243

31944

577

As

6

2

19

2

20

2

Ca

151140

4262

122356

43

80264

4278

Cd

1

0

2

0

2

0

Cr

87

0

76

8

217

19

Cu

545

6

2200

2

528

13

Fe

97737

444

153958

870

178082

2916

Mg

19838

4

11603

148

12227

148

Ni

44

1

48

2

108

3

P

101467

7

86904

304

81563

1688

Pb

50

0

100

2

45

1

Zn

1509

30

2512

7

1963

124

Si

86392

3523

105970

4026

46273

4421

176 177

SSA contains no organic matter, hence SSA might be judged as mineral matrices, when

178

considering SSA as a sole P fertilizer, soil amendment or a source of P for fertilizer production.

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In addition, SSA is rich in Mg and Ca, which might reduce the need for soil liming when applied

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in agriculture.

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3.2 SSA as fertilizer material

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Each country sets its own rules and demands for fertilizer quality, efficiency and composition,

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by considering the local geology, resources availability and crop production structure desired.

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According to these rules, the quality of fertilizers is understood as the maximum allowed

185

concentration of contaminants and desired P content and its plant availability (bioavailability). In

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the EU-28, the proposed European Fertilizer Ordinance23 sets the rules for the maximum content


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of contaminants, which are now set on national basis24 (Table 3). As our study focuses on the

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fate of P and heavy metals after sewage sludge incineration, SSAs were tested for the total

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amount of heavy metals, i.e. As, Cd, Cr, Ni and Pb. The amount of Hg was not estimated, as Hg

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presumably evaporates during incineration. Tested secondary P sources do fulfill the proposed

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European Fertilizer Ordinance, but may not sufficiently fulfill current limits given by national

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rules as shown for Pb in all SSAs and Cr in the case of experimental SSA3 (Tables 2, 3). Table 3 Demands on fertilizers: rules for undesired elements’ maximum content in mineral fertilizer with more than 5 %wt. of P2O5 Contaminant [maximum allowed concentration] As [mg/kgfertilizerdrymatter]

Current (EU-CZmodification)

Future (EU)23

24

20

60 60 – at the day of regulation application 40 – 3 years after the date of regulation

Cd [mg/kgP2O5]

50

application 20 – 12 years after the date of regulation application

Hg [mg/kgfertilizerdrymatter] VI

Cr/Cr [mg/kgfertilizerdrymatter]

1

2

150/ -

- /2

Ni [mg/kgfertilizerdrymatter]

-

120

Pb [mg/kgfertilizerdrymatter]

15

150

-

12

-

50

Biuret (C2H5N3O2) [mg/kgfertilizerdrymatter] Perchlorate (ClO4-) [mg/kgfertilizerdrymatter]

The column Current: Limits to the content of heavy metals such as cadmium and other contaminants set by national legislation in the EU - the Czech degree of determination of fertilizer requirements24 for inorganic fertilizers containing more than 5 %wt. of P2O5. The column EU-Future: Limits for inorganic fertilizers containing more than 5 %wt. of P2O5 listed in Annex I of the Proposal of the new European Fertilizer Ordinance23.

193


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According to the rules for fertilizers, information about P fertilizer solubility in either 2 %

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citric acid (2 %CA), neutral ammonium citrate (NAC), water or formic acid is required,

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depending on the actual type of fertilizer. This information then serves as an indicator of

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fertilizer efficiency, which is a function of the P plant-availability19. The P extraction in NAC

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was previously found to be a suitable method for SSA by Kruger and Adam1. However, these

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authors suggest additional parameters for the determination of fertilizers’ efficiency of secondary

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P sources, as its precise structure is as yet undescribed. A precise description of SSA matrices is

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complicated, because of the heterogeneous composition of incinerated SS. Its composition

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depends on the resource area, wastewater treatment technology used and the season36,37. To

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achieve a more precise description of SSA behavior as fertilizer-like material, we widened SSA

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fertilizer quality testing by solubility tests with 2 %CA (Figure 2) and water extraction, where the

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water solubility shows the immediate P release (availability), (Figure 3).


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Figure 2 SSA as fertilizer material: a) NAC-P shows P fertilizer solubility of used SSAs in NAC

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solution as a portion of total P content, b) 2 %CA-P shows fertilizer solubility of used SSAs in

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2 %CA as a portion of total P content, c) pHKCl shows measured pHKCl of used SSAs and d)

210

Ca:Al shows differences in Ca:Al content ratio of used SSAs.

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Between 35 % (SSA2) and 63 % (SSA3) of total P was dissolved in NAC, while alternative

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testing of plant P availability by dissolution in 2 %CA (Figure 2) showed P solubility in a range

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from 15 % (SSA3) to 40 % (SSA2), being in accordance with previous studies on SSA NAC-P

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availability1, and P solubility in 2 %CA20. NAC-P and 2 %CA-P solubilities are in relatively


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wide ranges of 28 % for NAC-P and 25 % for 2 %CA-P, similarly to the aforementioned studies.

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While Adams’20 aim was to increase the 2 %CA-P solubility from about 25-40% to up to 100 %

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by SSA thermochemical treatment, Duboc et al.19 addressed the question of reconsidering the

218

validity of standard P fertilizer tests as a measure of plant-available P in recycling fertilizers.

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Both studies were focused on the fate of P only, which might not be sufficient information, as

220

indicated in the following chapters. The suitability of standard P fertilizer tests for recycling

221

fertilizers was previously discussed. Egle et al.7 suggested the pot or field test as more

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meaningful, as these simulate the real scenario based on a mixture of soil, fertilizer and plant.

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These are, however, time-consuming, expensive and provide information specific to the tested

224

crop only. Cheaper and faster alternatives are solubility tests, or the DGT technique38.

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Considering the accuracy of solubility tests, Kratz et al.39 found an acceptable correlation

226

between short-term plant P uptake and the solubility of different P fertilizers in water, but a weak

227

correlation between long-term plant P uptake and P solubility in NAC or 2 %CA in the case of

228

recycling fertilizers. Steckenmesser et al.27 found NAC-P and 2 %CA-P solubilities

229

overestimating real plant availabilities of sewage sludge-based P sources. Duboc et al.19 raised

230

the importance of soil P testing for different recycling fertilizers, in accordance with our

231

hypothesis that standardized soil tests will give more precise information about SSA behavior as

232

soil amendment or P fertilizer material.

233

3.3 SSA as soil-like material

234

The availability of SSA-P in soil is given firstly by the P speciation formed during sewage

235

sludge incineration, and secondly by the behavior of such P speciation under specific soil

236

conditions. The most informative and useful single soil characteristic is soil pH, because soil

237

properties, including plant availability of nutrients, microbial activity, base saturation and soil


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structure, are dependent on pH25. Phosphorus in soil (as well as in SSA) is mostly present in the

239

form of inorganic phosphates and thus is considered the most available to plants at neutral to

240

slightly acidic soil pH. Phosphates have the strong tendency to form ion pairs and complex

241

species with available metallic cations: when pH exceeds 7 (calcareous soils), P is bound in Ca-

242

phosphates (and to a lesser extent in Mg-phosphates), under slightly acidic conditions

243

(pH below 6) P is bound in Al-phosphates (and to a lesser extent Fe-phosphates), which become

244

unavailable to plants40,41.

245

NAC and 2 %CA P availability tests of conventional fertilizers use the well-known (in nature)

246

formation of organic P complexes, where organic ligands (in this case citrates) are present.

247

Phosphorus bound in citrate complexes stays in solution and is therefore potentially available to

248

plants. The citrate-P formation will be influenced by SSA pH, by the pH of the extracting

249

solution (or soil solution, in the case of SSA’s agricultural application), and by the amount and

250

ratio of the present (and dissolved) Ca and Al. Within the present study, the behavior of three

251

different SSAs was studied:

252

The SSA2 is the ash with the highest Ca:Al ratio (Table 2/Figure 2), the highest pH (Table 4)

253

and the highest buffering capacity in neutral environments (Figure S1). This most alkaline

254

(calcareous in soil sciences) material, showed the lowest NAC-P solubility and by far the highest

255

2 %CA-P solubility. The SSA2’s high Ca content keeps the pH of the extracting NAC solution in

256

a neutral to slightly alkaline range, allowing Ca-P complexation, which reduces the efficiency of

257

desired citrate-P complexation. Under acidic conditions of 2 %CA solution, the influence of Ca

258

is inhibited, which, together with a low potential of Al-P complexation (caused by relatively low

259

SSA2’s Al content), increases the efficiency of the desired citrate-P complexation.


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260

Conversely, the highest NAC-P solubility (citrate-P complexation) and the lowest 2 %CA-P

261

solubility (citrate-P complexation) was seen with the SSA3, where the SSA3 represents an ash

262

with the lowest pH (among studied) and the lowest Ca:Al ratio. At the same time, the SSA3 is

263

more resistant to pH changes in acid environments than the SSA2 (Figure S1).

264

The SSA1 with the pH and the Ca:Al ratio between the SSA2 and the SSA3 showed similar

265

NAC-P solubility as the SSA2, which might be associated with the high Ca content, but also

266

2 %CA-P solubility similar to the SSA3, which might be associated with the highest Al content

267

lowering the resulting Ca:Al ratio. An apparent influence of a Ca:Al ratio on P behavior is

268

commonly seen in soils and thus enables the employment of soil characterization methods, when

269

describing the SSA material as a possible agricultural P bearer. Table 4 SSA material pH (measured by soil pH technique) and the change in extraction solution pH caused by interaction with SSA SSA characteristics

NAC extraction

2%CA extraction

water extraction

pHKCl

pHH2O

pHin

pHfin

∆

pHin

pHfin

∆

pHin

pHfin

∆

SSA1

8.85

8.69

7.00

6.31

-0.69

2.23

2.39

0.16

6.25

8.16

1.91

SSA2

11.14

11.18

7.00

7.15

0.15

2.23

2.49

0.26

6.25

10.84

4.59

SSA3

7.90

7.63

7.00

6.72

-0.23

2.23

2.51

0.28

6.25

6.9

0.65

pHin is the initial pH of extraction solutions and pHfin is a pH of the same solution after SSA extraction

270 271

The soil buffering capacity, together with soil pH and the cation exchange capacity (CEC),

272

describes soil ability or resistance to changes. In this system, pH is determined by the proportion

273

of acidic (H+, Al3+) and basic cations (Ca2+, Mg2+, K+, Na+). CEC is reported as an indicator of

274

soil’s ability to attract, retain and exchange cation elements. This value is calculated from the

275

concentration levels of cations of Ca2+, Mg2+, K+, Na+ dissolved in Mehlich3 solution (which is a

276

solution commonly used to estimate mobility of elements in acidic and neutral soils42–44). The

277

higher the CEC value, the higher the soil ability to hold cations and the greater capacity of soil to


16

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278

hold nutrients. In other words, the lower the CEC, the faster the soil pH will decrease with

279

time42. Among the studied SSAs, SSA1 shows the greatest ability to resist pH changes, having

280

the largest CEC (170 meq/100g). SSA2, with almost half the CEC (96meq/100g), is able to resist

281

pH changes only in neutral to slightly acidic environments, while SSA3 (CEC of 77 meq/100g)

282

shows the lowest ability to resist pH changes. This attitude of the studied SSAs’ pH changes was

283

also confirmed by the buffering capacity (Figure S1), confirming that soil characterization

284

methods are useful when studying SSA. The described resistance to changes correlates with the

285

already discussed influence of Ca:Al content on SSA P solubility in NAC and 2 %CA.

286

3.3.1 Solubility of SSA phosphorus and elements of interest under soil-like conditions

287

Not only each country, but each region and even each field has its specific soil conditions,

288

which are determined by the local geology, hydrology, climate, biota and anthropogenic impact.

289

Our antecedents used the three-field agronomy system in order to prevent starving the soil of

290

nutrients, as each crop exchanges different substances with its surroundings. Later, when the

291

population grew and the natural field capacity did not serve adequately to feed the population,

292

soil amendment by nutrients became necessary. Today, when the behavior of apatite-based

293

compounds is well described, only knowledge about soil parameters is sufficient to decide on the

294

amount and quality of fertilizer needed to be applied. Based on the local soil pH, its buffering

295

capacity or CEC and the available soil P level, the necessary P fertilizer addition may be

296

estimated. In the USA and Europe, soil-available P levels are estimated either by the Bray or

297

Olsen procedure in neutral and calcareous soils, where Bray is commonly used when soil pH is

298

less than 7.445. In acid to neutral soils, available P is commonly estimated by the Mehlich3

299

procedure, which is also suitable for other elements including B, Ca, Cu, Fe, K, Mg, Mn Na, and

300

Zn46. As described earlier, SSAs contain a notable amount of metallic components and their


17


Page 18 of 28

301

potential mobility in the soil environment might be a limiting factor when considering SSAs as

302

agricultural P bearer. To describe the potential mobility of SSA components in the soil

303

environment, Mehlich3 was chosen as the most suitable method for SSA P availability.

304

Moreover, the possible immediate release of P and metals into the environment were estimated,

305

using the standardized water extraction test, which is also required when testing mineral P

306

fertilizers.

307 308

Figure 3 SSA as soil-like material: SSA-P soil solubility in acidic environment by Mehlich3

309

solution as a portion of total P content (columns without fill); immediate SSA-P release into the

310

water as a portion of total P content (fully colored columns) and SSA-P soil solubility in alkaline

311

environments by Olsen’s solution as a portion of total P content (columns with striped pattern)

312

Due to the very limited solubility of P at pH > 7, only a limited amount of SSA-P, less than

313

0.5 %, was found to be soluble, thus available, by the Olsen test (Figure 3). The Mehlich3 test,

314

used as a measure of long-term P availability in acidic soils, is based on the formation of organic

315

P-chelates and the formation of Al-F complexes. Similarly to plant availability tests of P from

316

fertilizers, the highest P availability under acidic soil conditions was shown for SSA2 and the

317

lowest for SSA3. The achieved soil availabilities are more than ten times lower than those shown


18

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318

by the fertilizer test for all SSAs studied. A weak correlation between P availability of recycling

319

fertilizers achieved by standardized fertilizer tests and P availability under soil conditions were

320

also seen in other studies, as discussed earlier. Water extraction tests, which are common for

321

testing both fertilizers and soils, showed the very low immediate availability of P for all SSAs

322

used. In the water solution, there is no complexing agent forcing P to leave its original bounds at

323

pH of about 6.2, and only a part of P corresponding to dihydrogen phosphates may enter the

324

solution. These are, however, not likely to occur in SSA. Levels of P released by standardized

325

soil tests clearly show that, to reach a reasonable substitution of apatite-based fertilizers by SSA,

326

additional post-treatment is needed. The same applies when SSAs are tested by traditional

327

fertilizer techniques, which shows a more optimistic P recycling potential than what is seen in

328

SSA behavior under soil-like conditions.

329

No significant immediate release of potentially harmful metals was seen by standardized soil

330

tests. On the contrary, SSA may immediately enrich the soil by Ca and Mg, reducing the need for

331

its liming (Figure 4).

332

Considering the overall mobility (water and Mehlich3 extracted portion) of other (than P)

333

elements from SSAs, a significant amount of Ca and Mg would enter the soil solution together

334

with some Zn (up to 280 mg/kgSSA i.e. 11 % of Zn content) and Cu (up to 110 mg/kgSSA i.e. 5 %).

335

Notable release was also detected for Al, As and Ni, but the Mehlich3 test is not standardized for

336

studying the mobility of these compounds. Zn and Cu did not show any immediate release from

337

SSA. This is most probably associated with the pH of the water extraction solution, being 6.2 (at

338

the beginning) and between 6.9 and 10.8 (at the end of extraction), as mobility of heavy metals in

339

soil decreases with an increasing pH25,39, unless this system is affected by organic matter

340

complexation, e.g. with humic acids.


19


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341 342

Figure 4 SSA as soil-like material: SSA elements’ soil mobility in acidic environment by acidic

343

Mehlich3 solution, shown as the amount of elements’ mg released from kg of ash (columns

344

without fill); and immediate SSA elements’ release into water solution, shown as the amount of

345

elements’ mg released from kg of ash (fully colored columns)

346 347

AUTHOR INFORMATION

348

Corresponding Author

349

*[email protected]

350

Authors’ contributions

351

The manuscript was written with contributions by all authors. All authors have given approval

352

for the final version of the manuscript. ‡These authors contributed equally.

353

ACKNOWLEDGMENTS

354

This work was conducted within the Waste-to-Energy Competence Centre (project no.

355

TE02000236) and within the project Possibilities of using sewage sludge as a secondary source


20

Page 21 of 28


356

of phosphorus in the Czech Republic (project no. TJ01000074), both with support from the

357

Technology Agency of the Czech Republic.

358

ASSOCIATED CONTENT

359

Supporting Information Available

360 361

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