Chemical Aging Changed Aggregation Kinetics and Transport of Biochar Colloids

Abstract

Little is known about aggregation and transport behaviors of aged biochar colloids in the terrestrial environment. This study investigated aggregation kinetics and transport of biochar colloids from aged (HNO3 treatment) and pristine pinewood biochars pyrolyzed at 300 and 600 °C (PB300 and PB600) in NaCl and CaCl2 solutions. In NaCl solutions, critical coagulation concentrations (CCCs) of aged PB300 and PB600 colloids (540 mM and 327 mM) were much greater than the CCCs of pristine biochar colloids (300 mM and 182 mM). This is likely due to substantial increase of negatively charged oxygen-containing functional groups (primarily carboxyl) on aged biochar surfaces. Intriguingly, in CaCl2 solutions the CCCs of the aged PB300 and PB600 colloids decreased to 25.2 mM and 32.1 mM from 58.6 mM and 41.7 mM for the pristine colloids, respectively. This probably resulted from greater surface charge neutralization and Ca2+ bridging for the aged biochar colloids. In salt solutions (e.g., 10 and 50 mM NaCl and 1 and 10 mM CaCl2), the aged biochar colloids showed higher mobility in porous media than the pristine biochar colloids. This study demonstrated that pristine and aged biochar colloids were stable in the solutions with environmentally relevant ionic strength, and the aging process might substantially increase their mobility in the subsurface.

Biochar particle aggregation in soil pore water: the influence of ionic strength and interactions with pyrene

“…The critical coagulation concentration for CaCl 2 has previously been determined for wheat straw biochar pyrolyzed at 600 C (0.038 M), 29 and for wood shaving biochar pyrolyzed at 400 C (0.075 M). 31 In good agreement with this, pyrene aggregation could not be enhanced in the 0.1 M CaCl 2 solution, because the IS of 0.1 M CaCl 2 exceeds previously reported critical coagulation concentrations 29,30,35 so the maximum attachment efficiency of the biochar particles has already been reached. The enhanced aggregation induced by the adsorbed pyrene could only occur below those concentrations in 0.01 M CaCl 2 solutions at which charge neutralization by pyrene acts together with the double layer compression by Ca 2+ .…”

“…The samples were shaken at 125 rpm for either 24 hours or 28 days. The 0.01 M CaCl 2 solution was used to represent soil pore water IS according to OECD guideline 106, 28 and the 0.1 M CaCl 2 solution was used for mechanistic comparisons, exceeding the critical coagulation concentration of CaCl 2 previously determined for wheat straw and pinewood biochar pyrolyzed at 600 C (0.038 M and 0.042 M, respectively) 29,30 and for wood shaving biochar pyrolyzed at 400 C (0.075 M). 31 Pyrene (stock solution in MeOH) was spiked to initial aqueous concentrations of 100 mg L À1 and 1500 mg L À1 .…”

Rice Seedling Growth Promotion by Biochar Varies With Genotypes and Application Dosages

“…Compared with micro-particles of biochar, these biochar nanoparticles present in the liquid phase had a larger surface area, smaller pore size, and higher negative zeta potential (Oleszczuk et al, 2016 ). Such nano-biochars could be formed during lignocellulose biomass degradation (Zhang M. et al, 2012 ; Yao et al, 2013 ; Oleszczuk et al, 2016 ; Wang et al, 2019 ). To readily enter the plant cell, biochar nanoparticles could be bound to organic compounds or coated with organic compounds (Cifuentes et al, 2010 ; Zhao et al, 2012 ).…”

“…Related to this, reduction of root hair density and length with a biochar extraction product containing humic-like substances was observed for Arabidopsis grown in P-sufficient or P-starved soil under sterile conditions (Graber et al, 2015 ). Moreover, nanoparticles isolated from biochar could function for plant or root development (Qu et al, 2016 ; Liu G. et al, 2018 ; Wang et al, 2019 ). Nanosized biochar particles were stable in the solution in the control of ionic strength, and their mobility could be increased significantly by biochar aging in soil (Wang et al, 2019 ).…”

“…Moreover, nanoparticles isolated from biochar could function for plant or root development (Qu et al, 2016 ; Liu G. et al, 2018 ; Wang et al, 2019 ). Nanosized biochar particles were stable in the solution in the control of ionic strength, and their mobility could be increased significantly by biochar aging in soil (Wang et al, 2019 ). The outer and inner surfaces of biochar particles could form an organic/mineral plaque layer, which could fragment or generate nanoparticles, with the help of microbes and fine roots in soil (Hagemann et al, 2017 ).…”