Efficacy of Woodchip Biochar and Brown Coal Waste as Stable Sorbents for Abatement of Bioavailable Cadmium, Lead and Zinc in Soil

Abstract

Organic sorbents alter physicochemical soil properties and mitigate heavy metal (HM) bioavailability. However, some sorbents are labile and, therefore, introduce the risk of HM release into soil after mineralisation. Before field application, new stable organic sorbents such as woodchip biochar (BIO) and brown coal waste (BCW) need to be tested and compared with standard organic amendments like farmyard manure (FYM). An incubated pot experiment was conducted to investigate the efficacy of FYM, BIO and BCW (added to soil in pots at 5 and 10% w/w) to alter soil physicochemical properties and mitigate bioavailability of Cd, Pb and Zn spiked in treatments at different doses (in mg kg−1); 0 (not spiked), 1 (1 Cd, 70 Pb, 100 Zn) and 2 (3 Cd, 500 Pb, 700 Zn), and incubated for 9 weeks. At the end of the experiment, the EDTA-extractable HM fractions, pH, cation exchange capacity (CEC) and specific surface area (SSA, to check trends) were determined in all treated soils. Results showed that FYM, BCW and BIO generally improved all soil properties (except reduced pH from BCW and apparent SSA reduction from FYM) and accounted for respective maximum abatements of Cd (50.2, 69.9 and 25.5%), Pb (34.2, 64.3 and 17.4%) and Zn (14.9, 17.7 and 11.8%) bioavailability in soil. FYM and BCW were more effective at 10% w/w especially in the low contaminated soil, whereas the highest efficacy for BIO was at 5% w/w and in the high contaminated soil. The efficacies of sorption by the organic sorbents varied for different HMs and were in the orders: BCW > FYM > BIO for Cd, FYM > BCW > BIO for Pb and BIO > BCW > FYM for Zn. Soil pH and CEC were strongly correlated with HM bioavailability in all treatments and implied that immobilisation of HMs occurred via complex formation, ion exchange and pH-dependent specific adsorption. All three sorbents were beneficial as soil amendments, and in terms of HM mitigation, BCW had the highest efficacy, followed by FYM and then BIO. Considering the documented high soil stability of BCW and BIO, these results are promising for further trialling at field scale.

Biochar and Its Broad Impacts in Soil Quality and Fertility, Nutrient Leaching and Crop Productivity: A Review

“…Omotade et al [43] applied biochar of maize cobs, peanut shells, cow dung, and poultry litter pyrolyzed at 300-600 • C for 3 h. Results from this research showed that the highest contents of N (0.62%) and K (16.2 mg g −1 ) were found in cow dung, P (66.4 mg g −1 ), Mg (0.28%), and sulfur (S; 0.28%) in maize cob, and Ca (4.21 mg g −1 ) in peanut shell. Thus, wood-based biochars are suitable when the final objective is carbon storage in soil [44,45], while biochars from animal manures and grasses are used when the objective is to increase the soil N, P, K, Ca, Mg, and S contents [45,46]. According to Ippolito et al [47], biochars produced from hardwoods supply 0.002, 2.2 and 17% of the total N, total P, and total K, respectively, while biochars of soft softwoods can supply 27% total P and 6% K [47].…”

Assessing Factors Controlling Structural Changes of Humic Acids in Soils Amended with Organic Materials to Improve Soil Functionality

“…The benefits of HAs in soil are far-ranging and are well reported in the literature, including improvement of soil structure (water holding capacity, aeration, and aggregation) and promotion of plant growth through biostimulation (enzymatic activation, microbial growth stimulation, and root elongation) [12,14,15]. This highlights the suitability of HA-rich organic amendments for both environmental and agricultural applications [1,10,16].…”

“…The amendment of agricultural and marginal soils with organic materials increases soil organic matter (SOM), enhancing soil resilience through improvements in soil physical, chemical, and biological properties [1][2][3]. Organic residues in soil are biologically transformed to their inorganic forms via mineralisation.…”

Brown Coal Waste in Agriculture and Environmental Protection: A Review

“…As a result, humic and fulvic acids acquire a negative charge and combine with positively charged ions (cations) to form organomineral compounds [47][48][49]. The formation of chelated complexes with cations of heavy metals contributes to soil detoxication and increases the availability of plant nutrients [50][51][52][53][54]. The effect of Rekulter on reducing the uptake of Zn, Pb, and Cd by plants presented in Table 3.…”