Alkali Extraction to Detoxify Rice Husk-Derived Silica and Increase Its Biocompatibility

Abstract

As interest in natural biogenic silica nanoparticles (NPs) from rice husks grows, it is important to know how their preparation (i.e., combustion, acid leaching, or alkali extraction) impacts their biocompatibility. Acid leaching and alkali extraction both generated highly pure silica NPs (>99.1% SiO2 vs 93.1% with combustion). However, toxicity tests with six different human and mouse cell lines found alkali-extracted silica NPs were the least harmful; for example, the concentration leading to 50% loss in viability (LC50) was typically around 500, 500–2000, and >2000 mg/L for the combustion, acid-leached, and alkali-extracted silica NPs, respectively. To mimic the presence of carbon during calcination (900 °C for 6 h), polyethylene glycol (PEG) was incorporated in the alkali-extracted silica NPs prior to this step. This significantly increased their toxicities; that is, the LC50 dropped to between 500 and 2000 mg/L and was exacerbated further when calcination was performed at a lower temperature and time (550 °C for 2 h): LC50 values dropped to between 62.5 to 1000 mg/L. These results show that the biocompatibility of rice husk-derived silica NPs is negatively affected by the presence of residual carbon during calcination, but also that it can be significantly improved via alkali extraction.

Progress on Separation and Hydrothermal Carbonization of Rice Husk Toward Environmental Applications

“…In this case, removing the coexisting impurities and increasing the silica content will yield higher-quality materials thus research is being conducted on the removal of impurities by acid leaching using hydrochloric acid, sulfuric acid, and carboxylic acids, including citric acid, acetic acid, oxalic acid, and gluconic acid. [40][41][42][43][44][45] Various studies have assessed alkali treatment as an effective process for the leaching and removal of inorganic components. Alkali treatment is also a common chemical separation method for silica and carbon.…”

“…[51] The selective separation of carbon and silica is necessary to improve the biomass value. Silica, the main ash component, dissolves under strongly alkaline conditions, Wet chemical treatment Acid treatment (e.g., HCl) Hydrolysis of lignocellulose, removal impurities (metal, salts) [44][45][46] Alkali treatment (e.g., NaOH) Dissolution of ash precursors (silica) [44][45][46] Ionic liquid Extraction of lignocellulose and silica by dissolution of lignocellulose into ionic liquid [47,48] Thermal treatment Dry heat treatment Decomposition of organic substance [49] Hydrothermal leaching Dissolution of ash precursors, carbonizing organic substance [50] such as potassium hydroxide and sodium hydroxide, thereby improving the biomass quality. [52] Treatment with a sodium hydroxide solution with a pH of over 10 solubilizes amorphous silica in RHs.…”

A Sustainable Molten Salt Leaching-Electrodeposition Route Toward Converting Rice Husks into Functional Silicon Nanowires and Porous Carbon

“…Biomass refers to various organic materials formed through photosynthesis, including all forms of plants, animals, and microorganisms, which can be directly combusted to generate thermal energy or transformed into various forms of biofuels. , As the earliest renewable resource utilized by humanity, biomass has played a significant role in the progression of human history. In response to the current stage of global environmental problems, it is of great significance to realize the green and efficient utilization of biomass …”

Rose bengal-decorated rice husk-derived silica nanoparticles enhanced singlet oxygen generation for antimicrobial photodynamic inactivation

“…Thus, the natural resources-based RHs have gained considerable interest in energy/ecological fields since various chemical products can be derived from RH biomass, including active carbon, silica, syngas, and biofuel [ 26 – 28 ]. RH-derived SiOx NPs are amorphous and biocompatible; therefore, they are suitable raw materials for biomedical applications [ 29 – 31 ]. However, to our knowledge, there is no study on the encapsulation of photosensitizers into RH-derived SiOx NPs for aPDT.…”