Long‐term effects of sewage sludge–derived biochar on the accumulation and availability of trace elements in a tropical soil

Abstract

Thermal treatment by pyrolysis has been proposed as a sustainable alternative to enable the agricultural use of sewage sludge. The solid product obtained via pyrolysis of sewage sludge is called sewage sludge biochar and presents several advantages for its use as a fertilizer or soil conditioner. However, there are concerns about the accumulation and dynamics of trace elements in soil amended with sewage sludge biochar over the years. This study examined the effect of sewage sludge biochar, under field conditions for 5 yr, on the accumulation and availability of trace elements in a tropical soil. For this, 15 t ha–1 of sewage sludge biochar produced at 300 and 500 °C were applied in the first two growing seasons. Application was interrupted from the third to the fifth seasons to assess the residual effect of sewage sludge biochar in the soil. The total and available trace element concentrations were determined. The total contents of trace elements showed the following variation in the soil over the 5 yr (mg kg–1): Cd (16.8–20.0), Co (19.5–21.5), Cr (98.2–125.7), Cu (8.1–17.1), Mn (62.9–85.7), Ni (20.3–35.0), Pb (27.0–52.4), and Zn (20.3–35.8). There was no change in the availability of Cd, Cr, Ni, and Pb over the years. Additionally, a residual effect of the sewage sludge biochar was the increase in availability of trace elements that are considered essential (Cu, Mn, and Zn) and beneficial elements (Co) for plants. Therefore, in relation to contamination by trace elements, the pyrolysis of sewage sludge of domestic origin proved to be an adequate strategy to enable the safe use of this residue in tropical agriculture.

Pyrolysis of Sewage Sludge: Physical, Chemical, Morphological and Mineralogical Transformations

“…Compared with biochar from multiple feedstocks [19] and with other types of soil amendments [20], SSB has higher P levels, with great potential for use as a P fertilizer. Increases of Ca, Mg, S and micronutrients levels have also been reported in previous studies with SSB at 300ºC [8,17,21].…”

“…Pyrolysis also increased HMs levels (Table 1). Similar results also indicated higher HM content in SSB than in SS [21]. During the SS pyrolysis process, there may be an increase in the concentration of nonvolatile elements in the temperature range used, leading to higher HM contents [22].…”

“…The origin of the SS produced in Brasilia, Brazil, is predominately domestic with low HM concentration. Therefore, biochar produced from SS generated in this city has a low risk of soil contamination, as demonstrated in our previous works, including short-term [25] and long-term assessments [21].…”

“…During the SS pyrolysis process, there may be an increase in the concentration of nonvolatile elements in the temperature range used, leading to higher HM contents [22]. Therefore, because HMs have a higher boiling point than the pyrolysis temperature employed, they are concentrated in the final biochar [21]. Pyrolysis also increases the thermostability of HMs, where examples may include the various forms in which HMs can exist in SS, salts and hydroxides are generally converted to oxides or sulphides, which are more stable at elevated temperatures [22].…”

Biochar application ameliorated the nutrient content and fungal community structure in different yellow soil depths in the karst area of Southwest China

“…It was found that the application of biochar not only effectively increased the soil nutrient content but also improved soil pH and prevented soil acidification ( Dai et al., 2017 ; Shi et al., 2019 ). Chagas et al. (2020) indicated that the physical structure of the soil was effectively improved, which helped to increase soil porosity and reduce soil capacitance after years of continuous application of biochar.…”