Nitrate sorption and desorption in biochars from fast pyrolysis

Highlights

• Chemical activation has effect on surface functional groups of biochars.

• Chemical activation has effect on surface charge of biochars.

• Chemical activation influences the nitrate sorption and desorption.

• Desorption is higher in chemical activated biochars.

• Herbaceous and woody biochars differ in their sorption and desorption potential.

Abstract

Increasing the nitrate (${\text{NO}}_3^{-}$) sorption capacity of Midwestern US soils has the potential to reduce nitrate leaching to ground water and reduce the extent of the hypoxia zone in the Gulf of Mexico. The objective of this study was to determine the sorption and desorption capacity of non-activated and chemically activated biochars from microwave pyrolysis using selected biomass feedstocks of corn stover (Zea mays L.), Ponderosa pine wood chips (Pinus ponderosa Lawson and C. Lawson), and switchgrass (Panicum virgatum L.). Surface characteristics such as surface area and net surface charge have shown significant effects on nitrate sorption and desorption in biochars. Freundlich isotherms performed well to fit the nitrate sorption data (R² > 0.95) of biochars when compared to Langmuir isotherms. Nitrate sorption and desorption was significantly influenced by solution pH and presence of highly negative charged potential ions such as phosphate (${\text{PO}}_4^{3-}$) and sulfate (${\text{SO}}_4^{2-}$) in aqueous solution. Chemical activation with concentrated HCl had significant effect on surface characteristics of biochars and enhanced the nitrate sorption capacity. The first order model fit the nitrate desorption kinetics of biochars with a high coefficient of determination (R² > 0.95) and low standard error (SE).

Introduction

In Midwestern US agriculture, ammonium (${\text{NH}}_4^{+}$) based fertilizers are routinely applied to meet the nitrogen (N) requirement of agricultural crops. Nitrogen fertilizers generally undergo chemical transformations such as ammonification, nitrification, de-nitrification, nitrogen fixation, and immobilization in soil [1], [2]. Nitrification is one of the quickest reaction pathways. It results in the formation of nitrate ion, which is plant available but also highly mobile due to its weak affinity to form surface complexes and therefore prone to leaching [3], [4], [5] or is subject to denitrification. Increasing nitrate concentrations in surface and groundwater resources across the world have been attributed to agricultural and industrial inputs [6], [7]. Several nitrate removal technologies have been developed such as anion exchange [8], biological denitrification [9], chemical denitrification [10], [11], catalytic denitrification [12], reverse osmosis [13], and electrodialyis [14]. These available physico-chemical and biological technologies may be expensive, generate additional by-products and toxic wastes, and have limited applicability in large scale remediation scenarios [15]. For instance, chemical denitrification (reduction) using reductants such as zero-valent iron (ZVI) have been extensively used to reduce nitrate (${\text{NO}}_3^{-}$) in ground water. But this process requires reduced pH and produces ammonia as by-product [16]. Biological denitrification is slow and inefficient to remove nitrate from highly concentrated systems, and also requires additional organic substrates as electron donors [17]. Reverse osmosis and ion exchange resins are not economical for large scale nitrate removal. Ion exchange resins are not selective and also retain other anions (such as ${\text{SO}}_4^{2-}$ and ${\text{HCO}}_3^{-}$) which cause the disequlibrium in the ionic composition of soil and water systems and also release chloride (Cl⁻) ions during the exchange process [18]. Among these nitrate removal technologies, adsorption has been found less expensive and more effective in removal of nitrate from water systems. Adsorption technologies have evolved as highly efficient remediation tools with lesser energy input to produce sorbent materials; adsorption is easy to apply at large scales [15].

Control of nitrate mobility at its source would eliminate the need for remediation. Various sorbents have been identified which have potential to reduce of nitrate ion mobility in soil and water ecosystems [19], [20]. The success of adsorption techniques depends on the selection of appropriate sorbents, their removal efficacy, and economic feasibility. Sorbents (either naturally available or synthetically produced) to remove nitrate from water and waste water systems include functionalized mesoporous siliceous and non-siliceous materials [21], [22], [23], metal organic frame works [24], mesoporous carbon materials [25], red mud [26], agricultural waste [27], [28], polypropylene-g-N,N-dimethylaminoethyl methacrylate [29], poly (dimethylaminoehtyl methacrylate/2-hydroxy ethyl methacrylate) [30], poly (dimethyl diallyl ammonium chloride)/polyacrylamide [31], sepiolite [19], bentonite [32], [33], slag [19], activated carbon [19], [34], chitin and chitosan derivatives [35], [36], and M^II-Al-Cl layered double hydroxides [37], [38]. In recent years, the depletion of petroleum oil reserves and growing demand for energy across the world has initiated the development of sustainable alternative energy technologies based on renewable biomass. Biochar is the by-product of biomass conversion using pyrolysis to produce bio-oil and syngas. The scientific community has found opportunities to develop these biochar materials as climate change mitigation tools due to their ability to store carbon in soil for long periods. Biochar based materials have the potential to sorb anionic nutrients such as nitrogen (N) and phosphorus (P) from aqueous solutions [39]. Several studies have described that biochars can also supply nutrients to crop plants and reduce the leaching of nutrients in soil [40], [41]. Unlike other existing nitrate removal sorbents, biochar can be produced by a simple, cost effective process and its application to agricultural lands may improve soil quality and health.

The sorption capacity of highly carbon rich biochars have been found to be excellent due to their unique surface characteristics such as high surface area, pore volume, and surface functional groups [42]. The surface area and porosity of biochars can be enhanced by either physical or chemical activation methods [43]. The activation of biochars from agricultural products has been increased in recent years due to its use in environmental pollution control [44], [45]. Among the activation methods, chemical activation is more economical with higher yield at lower temperature and less production of burn-off char [46]. Slowing the rate at which of nitrate is leached from surface to subsurface soils could improve water quality and reduce greenhouse gas generation. The sorption capacity of soils is critical along with the water flow to determine the leaching rate of nitrate in soil. Cation transport is slowed by sorption on soils with negative charge, while anion transport is slowed by sorption on positively charged soil particles [47]. Most Midwestern United States soils have relatively low anion sorption capacity. For example, the anion exchange capacity of Kranzburg silty clay loam soil (fine-silty, mixed, superactive, frigid, Aeric Calciaquoll) was ranged from −0.25 to 0.27 cmol_c kg⁻¹. This relatively minor anion sorption capacity must be compared with the soil cation exchange capacities that ranged from 27.9 to 33.127 cmol_c kg⁻¹ [47].

Batch experiments are often used to quantify the ionic retention of biochar and other sorbents [48]. The determination of sorption coefficients (K_d) and sorption maxima (q_max) of these sorbents help in developing them as remediation tools to control contaminant transport in soil and water eco-systems. In this context, it is important to investigate the interaction of biochar-based sorbents from agricultural products with highly mobile anionic nutrient such as nitrate is essential. Therefore this study was designed with the following objectives: (1) determine the surface characteristics of non-activated and activated biochars produced from microwave pyrolysis using corn stover (Zea mays L.), Ponderosa pine (Pinus ponderosa Lawson and C. Lawson) wood residue, and switchgrass (Panicum virgatum L.); and (2) determine the nitrate sorption anddesorption potential of non-activated and activated biochars.