Physicochemical properties and morphology of biochars as affected by feedstock sources and pyrolysis temperatures

Abstract

Feedstock sources and pyrolysis temperatures affect the physicochemical and morphological properties of biochars. We evaluated biochars derived from switchgrass (SGB) and poultry litter (PLB) pyrolyzed at 350 °C (SGB350, PLB350) and 700 °C (SGB700, PLB700) to identify their potential ability in improving soil health. Except for SGB350, the pH of biochars was high (> 10.0) and can be used as an amendment in acid soils. PLB700 had higher mineral content and nutrient availability due to its higher ash content (tenfold higher) and electrical conductivity. Surface functional groups responsible for metal retention were evidenced in all biochars. Cation exchange capacity (CEC), specific surface area (SSA), and microporosity more than doubled by increasing pyrolysis temperature from 350 to 700 °C. The pH-buffering capacity measured through acid titration curve was better than that calculated with acid/alkali additions. Biochars pyrolyzed at 700 °C have much higher pH, CEC, SSA, and stronger buffering capacity, and thus are more promising to improve soil health and reduce contaminant bioavailability.

The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs) Phytoavailability

“…A significant decrease in the transfer factor values (TF) of PTMs (Zn, Pb, and Cd) from ryegrass roots to ryegrass shoots when evaluating PLB and SGB additions to a multi-metal contaminated soil was found by [22], and that the PLB was more efficient in such reduction than SGB. This was probably a consequence of their higher pH, CEC, specific surface area (SSA), and stronger buffering capacity as reported by [23], which resulted in the higher efficiency of PLB in decreasing PTMs uptake, as highlighted by the higher decrease of the bioconcentration factor (BCF = [PTMs in shoots/PTMs concentration in soil]) as PLB application rates increased. Figure 5 (and Figure 3) briefly summarizes such findings and highlights the PTMs immobilization as a function of two feedstocks derived-biochar application rates.…”

“…The role of biochar in improving soil pH, organic carbon (OC), and CEC was also highlighted by [16]. Moreover, biochar can immobilize PTMs (immobilization is the reduction of the potential migration of PTMs to plants, or reduction of phytoavailability) such as cadmium (Cd), lead (Pb), and zinc (Zn) and thereby to reduce the phytoavailability of PTMs (concentration of PTMs in plant parts, or contents of PTMs in soils available to plants) to plants in contaminated soils, notably because it raises the soil pH [18,22] and increases CEC and OC [23]. Many studies also found biochar application promotes the ability to remove organic contaminants [24,25].…”

Influence of Biochar Derived Nitrogen on Cadmium Removal by Ryegrass in a Contaminated Soil

“…Switchgrass-and poultry litter-derived biochars are referred to as SGB and PLB, respectively. The biochars coarse materials were ground with a mortar and pastel gently before sieved through 1 mm for further physicochemical analyses [24] and through 0.25 mm for the potting experiment. A full characterization of SGB and PLB including all physicochemical properties such as moisture, ash content, particle-size distribution, elemental composition, surface functional groups, chemical attributes, specific surface area, cation exchange capacity (CEC), and morphology can be found in [24].…”

“…The biochars coarse materials were ground with a mortar and pastel gently before sieved through 1 mm for further physicochemical analyses [24] and through 0.25 mm for the potting experiment. A full characterization of SGB and PLB including all physicochemical properties such as moisture, ash content, particle-size distribution, elemental composition, surface functional groups, chemical attributes, specific surface area, cation exchange capacity (CEC), and morphology can be found in [24]. For this study, it is useful to emphasize the ash, total carbon (TC), total nitrogen (TN), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) contents, as presented in Table 1.…”

“…The Cd accumulation in ryegrass plant parts decreased with biochar application rates regardless the feedstock from which the biochar was produced (Table 2). However, it must be pointed out that such reduction is not only a consequence of the reduced N removal and ryegrass yields at 4% of biochar amendment but also due to a direct effect of the biochar properties with the potential to immobilize Cd in the soil, amongst them surface functional groups, alkalinity, organic carbon (OC), and CEC [24]. Table 3 shows the ANCOVA of linear regressions between ryegrass yield and N accumulation and between N and Cd accumulations plotted in Figure 1.…”