Efficient aqueous molybdenum removal using commercial Douglas fir biochar and its iron oxide hybrids.

Molybdenum (Mo) is a naturally-occurring trace element in drinking water. Most commonly, molybdate anions (MoO42-) are in well water and breast milk. In addition, it is used in medical image testing. Recently, the EPA classified Mo as a potential contaminant, as exposure can lead to health effects such as gout, hyperuricemia, and even lung cancer. We have assessed the sorptive removal of aqueous molybdate using Douglas fir biochar (DFBC) and a hybrid DFBC/Fe3O4 composite containing chemically-coprecipitated iron oxide (Fe3O4). Adsorption was studied at various: pH values, equilibrium times (5 min-24 h), initial Mo concentrations (2.5-1000 mg/L), and temperatures (5, 25, and 40 °C) using batch sorption and fixed-bed column equilibrium methods. Langmuir capacities for DFBC and DFBC/Fe3O4 (at pH 3, 2 hrs equilibrium) were within 459.3-487.9 mg/g and 288-572 mg/g, respectively. These adsorbents and their Mo-laden counterparts were characterized by elemental analysis, BET, PZC, SEM, TEM, EDS, XRD, and XPS. MoO42- adsorption on DFBC is thought to be governed primarily via electrostatic attraction. Adsorption by DFBC/Fe3O4 is primarily governed by chemisorption onto magnetite surface hydroxyl groups, while electrostatics prevail in the DFBC-exposed phase. Stoichiometric precipitation of iron molybdates triggered by iron dissolution was also considered. The data suggest that DFBC and DFBC/Fe3O4 are promising candidates for molybdate sorption.

High capacity aqueous phosphate reclamation using Fe/Mg-layered double hydroxide (LDH) dispersed on biochar.

Phosphate is a primary plant nutrient, serving integral role in environmental stability. Excessive phosphate in water causes eutrophication; hence, phosphate ions need to be harvested from soil nutrient levels and water and used efficiently. Fe-Mg (1:2) layered double hydroxides (LDH) were chemically co-precipitated and widely dispersed on a cheap, commercial Douglas fir biochar (695 m2/g surface area and 0.26 cm3/g pore volume) byproduct from syn gas production. This hybrid multiphase LDH dispersed on biochar (LDHBC) robustly adsorbed (~5h equilibrium) phosphate from aqueous solutions in exceptional sorption capacities and no pH dependence between pH 1-11. High phosphate Langmuir sorption capacities were found for both LDH (154 to 241 mg/g) and LDH-modified biochar (117 to 1589 mg/g). LDHBC was able to provide excellent sorption performance in the presence of nine competitive anion contaminants (CO32-, AsO43-, SeO42-, NO3-, Cr2O72-, Cl-, F-, SO42-, and MoO42-) and also upon remediating natural eutrophic water samples. Regeneration was demonstrated by stripping with aqueous 1 M NaOH. No dramatic performance drop was observed over 3 sorption-stripping cycles for low concentrations (5 ppm). The adsorbents and phosphate-laden adsorbents were characterized using Elemental analysis, BET, PZC, TGA, DSC, XRD, SEM, TEM, and XPS. The primary sorption mechanism is ion-exchange from low to moderate concentrations (10-500 ppm). Chemisorption and stoichiometric phosphate compound formation were also considered at higher phosphate concentrations (>500 ppm) and at 40 °C. This work advances the state of the art for environmentally friendly phosphate reclamation. These phosphate-laden adsorbents also have potential to be used as a slow-release phosphate fertilizer.

Biochar Adsorbents with Enhanced Hydrophobicity for Oil Spill Removal.

Oil spills cause massive loss of aquatic life. Oil spill cleanup can be very expensive, have secondary environmental impacts, or be difficult to implement. This study employed five different adsorbents: (1) commercially available byproduct Douglas fir biochar (BC) (SA ∼ 695 m2/g, pore volume ∼ 0.26 cm3/g, and pore diameter ∼ 13-19.5 Å); (2) BC modified with lauric acid (LBC); (3) iron oxide-modified biochar (MBC); (4) LBC modified with iron oxide (LMBC); and (5) MBC modified with lauric acid (MLBC) for oil recovery. Transmission, engine, machine, and crude oils were used to simulate oil spills and perform adsorption experiments. All five adsorbents adsorbed large quantities of each oil in fresh and simulated seawater with only a slight pH dependence, fast kinetics (sorptive equilibrium reached before 15 min), and high regression fits to the pseudo-second-order kinetic model. The Sips isotherm model oil sorption capacities for these sorbents were in the range ∼3-11 g oil/1 g adsorbent. Lauric acid-decorated (60-2 wt %) biochars gave higher oil adsorption capacities than the undecorated biochar. Lauric acid enhances biochar hydrophobicity and its water contact angle and reduces water influx into biochar's porosity preventing it from sinking in water for 3 weeks. These features were observed even at 2% wt of lauric acid (sinks only after 2 weeks). Magnetization by magnetite nanoparticle deposition onto BC and LBC allows the recovery of the exhausted adsorbent by a magnetic field as an alternative to filtration. Oil sorption was endothermic. Recycling was demonstrated after toluene stripping. The oil-laden adsorbents' heating values were obtained, suggesting an alternative use of these spent adsorbents as a low-cost fuel after recovery, avoiding waste disposal costs. The initial and oil-laden adsorbents were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer-Emmet-Teller surface area, contact angle, thermogravimetric analyses, differential scanning calorimetry, vibrating sample magnetometry, elemental analysis, and X-ray photoelectron spectroscopy.

Microwave and open vessel digestion methods for biochar.

Digestion of biomass derived carbonaceous materials such as biochar (BC) can be challenging due to their high chemical recalcitrance and vast variations in composition. Reports on the development of specific sample digestion methods for such materials remain inadequate and thus require considerable attention. Nine different carbonaceous materials; slow-pyrolyzed tea-waste and king coconut BC produced at 300 °C, 500 °C and 700 °C, sludge waste BC produced at 700 °C, wet fast-pyrolyzed Douglas-Fir BC and steam activated coconut shell BC have been tested to evaluate a relatively fast and convenient open-vessel digestion method using seven digestion reagents including nitric acid (NA), fuming nitric acid (FNA), sulfuric acid (SA), NA/SA, FNA/SA, NA/H2O2 and SA/H2O2 mixtures. From the tested digestion reagents, SA/H2O2 mixture dissolved low temperature produced BC (LTBC) within 2 h with occasional shaking and no external heating. Except peroxide mixtures, the other reagents were used to evaluate microwave digestion (MWD) efficiency. Nitric acid mixture was capable of only completely digesting LTBC in the MWD procedure whereas FNA, NA/SA and FNA/SA mixtures resulted in the successful dissolution of all tested carbonaceous materials. Amongst them, FNA provided the least matrix effect in the quantification of the four metals tested using flame atomic absorption spectrophotometry. Tested recoveries for FNA were satisfactory as well. It was concluded that FNA is a preferable reagent for microwave digestion of BC.

Removal of Arsenic(III) from water using magnetite precipitated onto Douglas fir biochar.

Magnetic Fe3O4/Douglas fir biochar composites (MBC) were prepared with a 29.2% wt. Fe3O4 loading and used to treat As(III)-contaminated water. Toxicity of As(III) (inorganic) is significantly greater than As(V) and more difficult to remove from water. Removal efficiency was optimized verses pH, contact time and initial concentration. Column sorption and regeneration were also studied. Adsorption kinetics data best fitted the pseudo second order model (R2 > 0.99). Adsorption was analyzed with three isotherm models at 20, 25 and 40 °C. The Sips isotherm showed the best fit at 25 °C with a 5.49 mg/g adsorption capacity, which is comparable with other adsorbents. MBC gave faster kinetics (~1-1.5 h) at pH 7 and ambient temperature than previous adsorbents. The Gibbs free energy (ΔG) of this spontaneous As(III) adsorption was -35 kJ/mol and ΔH = 70 kJ/mol was endothermic. Experiments were performed on industrial and laboratory wastewater samples in the presence of other co-existing contaminants (pharmaceutical residues, heavy metals ions and oxi-anions). The composite reduced the arsenic concentrations below the WHO's safe limit of 0.2 mg/L for waste water discharge. X-ray photoelectron spectroscopy (XPS) studies found As(III) and less toxic As(V) on Fe3O4 surfaces indicating adsorbed (or adsorbing) As(III) oxidation occurred upon contact with O2 and possibly dissolved Fe(III) or upon drying under oxic conditions. Under anoxic conditions magnetite to maghemite transformation drives the oxidation. A pH-dependent surface chemisorption mechanism was proposed governing adsorption aided by XPS studies vs pH.

The influence of three acid modifications on the physicochemical characteristics of tea-waste biochar pyrolyzed at different temperatures: a comparative study.

Tea-waste is an abundant feedstock for producing biochar (BC) which is considered to be a cost effective carbonaceous adsorbent useful for water remediation and soil amendment purposes. In the present study, tea-waste BC (TWBC) produced at three different temperatures were subjected to nitric, sulfuric and hydrochloric acid modifications (abbreviated as NM, SM and HM respectively). Characteristics of the raw and modified BC such as ultimate and proximate analyses, surface morphology, surface acidity and functionality, point of zero charge, cation exchange capacity (CEC) and thermal stability were compared to evaluate the influence of pyrolysis temperature and of modifications incorporated. The amount of carboxylic and phenolic surface functionalities on TWBC was seen to decrease by 93.44% and 81.06% respectively when the pyrolysis temperature was increased from 300 to 700 °C. Additionally, the yield of BC was seen to decrease by 46% upon the latter temperature increment. The elemental analysis results provided justification for high-temperature BC being more hydrophobic as was observed by the 61% increase in H/C ratio which is an indication of augmented aromatization. The CEC was the highest for the low-temperature BC and was seen to further increase by NM which is attributed to the 81.89% increase in carboxylic functionalities. The surface area was seen to significantly increase for BC700 upon NM (∼27 times). The SM led to pore wall destruction which was observed in scanning electron microscopy images. Findings would enable the rational use of these particular modifications in relevant remediation and soil amendment applications.