Assessing the potential of sewage sludge-derived biochar as a novel phosphorus fertilizer: Influence of extractant solutions and pyrolysis temperatures

Highlights

• P solubility and availability of sewage sludge-derived biochar (SSB) were determined.

• Relationships between extractable P and physicochemical properties of SSB were evaluated.

• Pyrolysis temperature differently affected the SSB P solubility by several extractants.

• P in different extractants should be considered in the recommendations for SSB as a P fertilizer.

Abstract

Sewage sludge-derived biochar (SSB) is a phosphorus (P) source with potential to replace soluble P fertilizers. However, SSB presents a diversity of P compounds, mainly in mineral forms with different degrees of chemical stability. This hinders the prediction of P bioavailability. In the present study we evaluated P solubility and bioavailability using different chemical extractants. Additionally, the relationships between extractable P and physicochemical properties were evaluated for SSB obtained over a wide range of temperatures (200 °C; 300 °C; 500 °C and 600 °C). Available phosphorus content was extracted using 2% citric acid (P-CA), neutral ammonium citrate + water (P-NAC) and Mehlich 1 solution (0.0125 mol L⁻¹ H₂SO₄ + 0.050 mol L⁻¹ HCl). Physicochemical properties and extractable P were strongly affected by pyrolysis temperature. Higher pyrolysis temperature resulted in increased pH, BET surface area, pore volume, ash, fixed carbon, Ca, Mg and Zn contents, as well as formation of stable Ca minerals (calcite and oxalate). The total P content increased with pyrolysis temperature (≥300 °C). Nevertheless, the solubility of biochar-P in the extractants presented different trends with temperature. The P-NAC content reached a maximum (79% of TP) at 300 °C and then declined at higher temperatures. Only at 600 °C P-CA and available P were affected by the temperature, where the P-CA increased and available P decreased. Therefore, it is recommended that the P solubility in different extractants should be considered when using SSB as an alternative to inorganic P fertilizers.

Introduction

Phosphorus (P) is a crucial element for global food security, with the demand for P from fossil resources growing rapidly in the last 100 years (White and Cordell, 2017). As phosphate rock (PR) resources are depleting, new management tools for environmentally friendly P fertilizers are needed (Glaser and Lehr, 2019). Sewage sludge (SS) and biosolids are considered secondary P resources with potential for mitigating the demand for mineral P sources (Torri et al., 2017).

The presence of pathogens, organic and inorganic pollutants has limited the agricultural use of SS in several countries around the world (Mateo-Sagasta et al., 2015). Among the alternatives to overcome these limitations, thermal treatment has advantages as compared to other types of SS disposal (Racek et al., 2019). The solid product of SS pyrolysis, rich in carbon, is called SS biochar (SSB) and can be used as a soil amendment (Sousa and Figueiredo, 2016).

Phosphorus is the nutrient present in the highest concentrations in SSB (Hossain et al., 2011, Faria et al., 2018, Figueiredo et al., 2018, Adhikari et al., 2019). Consequently, there has been an interest in the scientific community to evaluate SSB as a P fertilizer (Steckenmesser et al., 2017). Phosphorus is present in biochar in several organic and inorganic fractions with different degrees of stability (Li et al., 2019). The concentration and chemical forms of P in SSB are influenced by pyrolysis temperature, heating rate and residence time (Adhikari et al., 2019).

Unlike other feedstock, SS is essentially heterogeneous, comprised by a mixture of organic and inorganic materials with a high ash content (Torri et al., 2017). Distinct types of SS (based on P precipitation/removal), heterogeneity of the SS and the changes promoted by pyrolysis impair the adoption of suitable methods for characterization of plant-available nutrients, particularly P (Steckenmesser et al., 2017). Despite containing multiple P compounds with different chemical stability (Steckenmesser et al., 2017), when utilized as a soil amendment SSB has increased the yield of several short (around one month) and long (around five months) cycle crops (Sousa and Figueiredo, 2016, Faria et al., 2018). Several studies have shown that biochar can serve as a P reservoir for soils, and that a fraction of this P is in available forms for plants (Zhang et al., 2016). However, the extent of biochar effects on soil P availability varies significantly with the biochar type, mainly feedstock and pyrolysis temperature (Zhang et al., 2016, Uzoma et al., 2011), and P extractant used (Rose et al., 2019). For instance, the concentration of water-extractable P was found to decrease with temperature (Zhang et al., 2015). In general, the mechanisms by which biochar can influence the components of the soil P cycle include: biochar as a direct source of soluble P salts and exchangeable P, biochar as a modifier of soil pH and ameliorator of P complexing metals (Al³⁺, Fe^3+,2+, Ca²⁺), and biochar as a promoter of microbial activity and P mineralization (Deluca et al., 2009).

The available P fraction in biochar has been evaluated by using extractants commonly applied to soils, based on P solubility in water and solutions of acids and salts (Steckenmesser et al., 2017). Available P in biochar is generally extracted with water, sodium bicarbonate (NaHCO₃) or acids (e.g. sulfuric acid, formic acid, Bray solution and citric acid) (Zhang et al., 2016, Rose et al., 2019).

Considering that P in SSB is predominantly in inorganic forms (Adhikari et al., 2019), simple analytical techniques typically applied to quantify P in mineral and organomineral fertilizers can be useful to advance the understanding of SSB as a source of P for plants. Additionally, in the specific case of SSB the use of citric acid (CA) and neutral ammonium citrate (NAC), typically used to evaluate P solubility in mineral fertilizers (Braithwaite, 1987), may be suitable to quantify soluble P since SSB is an essentially organomineral material. Therefore, considering that: i) SSB is predominantly composed of inorganic P (Adhikari et al., 2019) and presents a wide variety of minerals (Zhang et al., 2015); ii) in addition to concentration, the mineral form affects the availability of elements (Clemente et al., 2018); and iii) all these characteristics are influenced by temperature, it is concluded that the extraction methods based on P solubility in acid or salt solutions must be studied over a wide temperature range. Temperatures ranging from torrefaction (200 to 300 °C) to pyrolysis (500 to 600 °C) where chosen. Despite not being assessed in the present study, at 400 °C biochars have shown intermediate values between 300 and 500 °C for proximate, ultimate, and physicochemical analysis (Hossain et al., 2011, Zhao et al., 2017).

Furthermore, pyrolysis temperature affects multiple biochar physicochemical characteristics which are related to the extractant efficiency, such as pH, electrical surface charges and mineral form. Therefore, information from multiple extraction methods is needed to better understand the chemical composition of biochars (Clemente et al., 2018) and its relationship with the available P forms in SSB. There is a lack of studies on the relationship between extractants solutions for P and physicochemical properties of SSBs. To the best of our knowledge, the present work is the first to explore the effect of different extractants on P solubility in SSB prepared using a wide temperature range.