Perspectives on the use of biochar in the valorization of mining wastes from sulfide minerals flotation: Recovery of metals and effects on toxicity.

This study aims to evaluate the use of two biochars obtained by pyrolysis of sugarcane-bagasse and compare it with commercial activated carbons as catalysts for the recovery of metals from one mining waste from sulfide minerals flotation (MW). It is also intended to determine the influence of carbon materials on the toxicity of the final residues. Leaching tests were performed in 250 mL erlenmeyer flasks using plates with magnetic stirrers during 24 h, at 90 °C and a stirring speed of 350 rpm. For each test, 5 g of MW were mixed with carbon material in two ratios of MW/carbon material (1/0.1 and 1/0.2 wt/wt) and 100 mL of leaching agent (H2SO4 solution at pH = 0.8-0.9 and 5 gL-1 of Fe3+). The experimental results showed that the addition of biochar and activated carbon enhances the recovery of Cu and Zn. The use of commercial activated carbons in ratios of 1/0.1 and 1/0.2 MW/carbon material leads to the extraction of more than 91 % of Cu and 97 % of Zn, after 24 h of leaching. For biochars, the highest recovery values of Cu (82.9 %) and Zn (98.1 %) were achieved with biochar prepared at 750 °C and used in the ratio of 1/0.2. However, the addition of carbon materials does not improve the recovery of Co. The presence of carbon materials decreased the electrical conductivity and pH of the final residue. The leaching of samples MW + W35 (1/0.1) and MW + BC550 (1/0.1) leads to a germination index higher than 90 %. For two biochars, all samples showed non-phytotoxicity.

Remediation of mining soils by combining Brassica napus growth and amendment with chars from manure waste.

Mining activities lead to important physical, chemical and biological effects on soil properties, generating severe impacts in the establishment and maintenance of vegetation. Assisted phytoremediation can be considered an environmentally friendly approach for soil remediation. In this study, two mining soils (PORT and GAM) were treated with 10%, by mass, of the following amendments: manure biochars prepared at 450 °C (BMW450) and 600 °C (BMW600), hydrochars prepared by hydrothermal carbonization (HTC) of manure at 190 °C (HWM190) and 240 °C (HMW240) and manure waste (MW). Brassica napus was used as a phytoextraction species. After 45 days of plant growth, soil samples were widely characterized, including microbial biomass carbon, enzymatic activity and metal content. In addition, plant biomass production, bioconcentration factor, translocation factor and metal uptake were determined. Experimental results showed that addition of biochars improved the As uptake by Brassica napus in both soils but just in the roots increasing bioconcentration factor between 22.1 and 39.5% for GAM soil and between 28.6 and 53.4% for PORT soil. Brassica napus cannot be considered as Zn accumulator in GAM soil samples and in the case of PORT samples, only the addition of BMW600 and HMW240 enhanced the phytoextraction process of Zn on the roots. Soil enzyme activity improved in hydrochar amended soils.

The effect of post-pyrolysis treatment on waste biomass derived hydrochar.

Hydrochars are materials with a promising future, as their high carbon content and porosity renders them suitable for uses including peat substitutes, soil remediation and carbon adsorbent precursors. Combining hydrothermal carbonization and pyrolysis offers the prospect to provide advanced materials with a higher porosity and carbon content. This approach would mitigate drawbacks associated to hydrochars, including phytotoxicity. This research studied the influence of pyrolysis temperature and heating time on the resulting properties of chars made from hydrothermal carbonization of biomass wastes at 200 °C for 4 h and compared them to biochars that had not received any prior hydrothermal carbonization. Interestingly, hydrochar followed by pyrolysis was able to result in phytostimulation, while, when only pyrolysis was carried out, phytotoxicity was eliminated, but no phytostimulant effect was observed. In addition, the results indicated that the higher and longer the pyrolysis temperature (from 350 to 550 °C) and duration time (from 1 to 5 h), respectively, the more microporosity was generated, while phytotoxicity was reduced. In addition, aromaticity and thermal stability significantly increased with pyrolysis treatment. Consequently, hydrochars improve their properties and offer more potential for environmental applications after a pyrolysis post-treatment.

Influence of pyrolysis parameters on phosphorus fractions of biosolids derived biochar.

Transforming biosolids into biochar, through pyrolysis, could result in more sustainable waste management. Influence of pyrolysis conditions (temperature, heating rate and residence time) on physico-chemical properties of biosolids (collected at Mount Martha Water Recycling Plant, Melbourne), phosphorus fractions and phosphorus forms was investigated. Twelve different biochar samples were produced at 400, 500 and 600 °C, at two heating rates (5 and 20 °C/min) and at two residence times (30 and 120 min). Biochar yield, pH, electrical conductivity (EC), elements (C, H and N) and BET surface area were analysed. Sequential extraction of P in biosolids and resultant biochars was done using Hedley method. Characterization was completed with SEM images and results from 31P liquid state NMR. Increased temperatures would not only increase the alkalinity, decrease EC and increase the adsorption capacity by increasing the surface area but also convert the readily available P to a less available pool. Therefore, this nutrient might be released to soil slowly over a longer period of time. The results showed that temperature, along with residence time and heating rate, had a significant effect on the characteristics observed. Therefore, all these factors need to be carefully considered when preparing biochar for use as a soil amendment.

Combining phytoextraction by Brassica napus and biochar amendment for the remediation of a mining soil in Riotinto (Spain).

Soil contamination in mining areas is an important environmental concern. In these areas, phytoremediation is often impeded because of the low fertility and pH. Assisted phytoremediation is increasingly being used in polluted areas. Biochar could assist plant growth via enhanced soil fertility. An experiment was performed in a mining soil (RIII) from the mining area of Riotinto (Spain) contaminated with Cu, Pb, Zn and As in order to study: (i) The effects of biochar on soil fertility; (ii) Biochar temperature of preparation effect and (iii) Effect of biochar on phytoremediation potential. A mesocosm experiment was designed using Brassica napus as test specie. Soil (RIII) was treated with rabbit manure biochars prepared at 450 °C (BM450) and 600 °C (BM600) at a rate of 10% in mass and incubated for 60 days with or without Brassica napus. Results showed that the combination of BM450 or BM600 with Brassica napus growth decreased the amount of As, Cu, Co, Cr, Se and Pb in the soil. Values of bioaccumulation factor (BAF) for Cd were particularly elevated (>10) in the unamended soil and reached values higher than 1 for other elements, indicating the potential of Brassica napus to accumulate several heavy metals. Translocation Factor (TF) was reduced for Co, Cr, Cd, Cu, Ni, Zn, Pb and As after biochar addition indicating root accumulation of these metals. In all cases, biochar addition increased biomass production. Finally, the addition of BM450 increased GMea index indicating also an improvement on soil quality.

Hydrochars from industrial macroalgae "Gelidium Sesquipedale" biomass wastes.

Macroalgae wastes from the Agar-Agar industry were used as a feedstock to obtain hydrochars by means of hydrothermal carbonization. The effect of temperature (200 °C and 230 °C) and time (2 h and 6 h) on the yield, higher heating value (HHV) and chemical-morphological-textural properties of the hydrochars was studied. The carbon content and the higher heating value were observed to increase with the hydrothermal carbonization. The hydrochars yields (up to 60%) were much higher than yields obtained using conventional char (27.5-33.5%). The hydrochar obtained at 230 °C and after 6 h showed a HHV of 23.25 MJ/kg, which is similar to that of lignite HHV. The H/C and O/C atomic ratios decreased as a consequence of the dehydration and decarboxilation reactions. Hydrothermal carbonization barely changed the vegetal structure of the macroalgae waste. The hydrochars were found to be essentially meso-macroporous with average pore sizes of up to 110.5 nm.

Biochars and hydrochars prepared by pyrolysis and hydrothermal carbonisation of pig manure.

Pyrolysis of organic wastes for biochar preparation has been proved as a useful way of waste management. However, the elevated water content of some organic wastes precludes its use without a drying step before pyrolysis treatment. For this reason, hydrothermal carbonization (HTC) of wet biomass could be an inexpensive alternative management method. The main objective of the present work is to compare the properties of biochars and hydrochars obtained from thermal treatment of pig manure. Biochars were prepared at 300 °C (BPM300), 450 °C (BPM450) and 600 °C (BPM600) and hydrochars were obtained using a pig manure solution (ratio 30:70) that was heated at 200 °C (HPM200), 220 °C (HPM220) and 240 °C (HPM240) during 2 h. Characterization of biochar and hydrochar samples showed that pyrolysis led to chars with more aromatic structures and high thermal stability while HTC process originated chars with more aliphatic structures. HPM220 and HPM240 showed the highest values of field capacity water content and available water probably due to their higher O/C ratios and the macroporosity development in the range from 200 to 30,000 nm. These results suggested that HTC could be an interesting method to obtain soil growing media or green roof materials with adequate hydrophysical properties.

Choice of pyrolysis parameters for urban wastes affects soil enzymes and plant germination in a Mediterranean soil.

The production of organic waste has steadily increased in recent years, with subsequent impact on the environment. The European Union committed to diminish the volume of biodegradable municipal waste disposed of in landfills by 2016-2020. The synthesis of biochar from urban waste and its application to improve soil quality can constitute a novel route for valorization. The aim of this paper was to study the effect of three biochars originated from pyrolysis of the organic fraction of urban waste at two different temperatures (300°C and 500°C) and two residence times (1h and 5h) on the biochemical properties of an agricultural soil. Soil was amended with biochars at a rate of 8% and incubated for 74days. A phytotoxicity assay, using garden cress as the test species, was conducted. CO2 emissions, microbial biomass C and the enzymes dehydrogenase, phosphomonoesterase and ß-glucosidase were measured in tested soils. Biochars prepared at 300°C resulted in lower germination index values, which could partly be ascribed to a higher bioavailability of heavy metals and higher soluble organic matter, while the biochar prepared as 500°C exhibited a phytostimulant effect. Biochars produced at 300°C (B300-1h, B300-5h) augmented soil CO2 emissions while there was no effect on microbial respiration in the soil amended with the biochar prepared at 500°C. Pyrolysis temperature and, for some enzymes, residence time, controlled soil enzymatic activity.

Use of magnetic biochars for the immobilization of heavy metals in a multi-contaminated soil.

Modified biochars, including magnetic biochars, have been tested in water for the removal of inorganic pollutants. However, at present it is unknown if they possess benefits over conventional biochar materials in relation to land remediation. A paddy soil was collected near Liantang village in Lechang Pb-Zn mine area in Guangdong Province (China). The soil was polluted with Cd, Cu, Zn and Pb, with total contents of 1.4mg/kg, 80mg/kg, 1638mg/kg and 2463mg/kg, respectively. We prepared magnetic and conventional biochar from two feedstocks (poultry litter and Eucalyptus) at a temperature of 300 and 500°C. A sequential extraction procedure for the speciation of heavy metals and a phytotoxicity test using rice were performed. Acid-soluble Cd in soils amended with PLB was 8 to 10% lower than in the control polluted soil. This figure was 27 to 29% for acid-soluble Zn and 59 to 63% for acid-soluble Cu. In some cases, differences were found between the heavy metal fractionation in samples amended with magnetic and conventional biochars. Plant biomass was unaffected by most treatments, but increased by 32% in the treatments containing magnetic poultry litter biochar. Our study shows that a careful choice of feedstock is of utmost importance for successful containment of heavy metals in a multi-contaminated mining area soil. An appropriate choice of feedstock (in the case of this study poultry litter vs. eucalyptus) was more determinant with respect to the mobility of pollutants than altering pyrolysis temperature or modifying surface properties through magnetization. However, surface modification through magnetization can have a significant impact on plant yield and offer comparative advantages in the management of some degraded landscapes.

Influence of pyrolysis temperature on composted sewage sludge biochar priming effect in a loamy soil.

Biochar is a carbon-rich solid product obtained by the pyrolysis of organic materials. The carbon stability of biochar allows that it can be applied to soil for long-term carbon storage. This carbon stability is greatly influenced by the pyrolysis temperature and the raw material used for biochar production. The aim of the present work is to study the soil carbon sequestration after the application of biochar from sewage sludge (SL) pyrolysis at two different temperatures (400 and 600 °C). For this purpose, soil CO2 emissions were measured for 80 d in an incubation experiment after soil amendment with the SL and each biochar at a dosage of 8 wt%. Biochar reduced the CO2 emissions during incubation between 11% and 32% relative to the SL treatment. The CO2 data were fit to a dual exponential model, and the CO2 emissions were simulated at different times (1, 5 and 10 yr). Additionally, the kinetics of the CO2 evolution from SL, two biochar samples, soil and amended soil were well fit to a dual first-order kinetic model with correlation coefficients greater than 0.93. The simulation of CO2 emissions from the soil by applying the proposed double first-order kinetic model (kg CO2-C ha(-1)) showed a reduction of CO2 emissions between 301 and 932 kg CO2-C ha(-1)with respect to the direct application of raw sewage sludge after 10 yr.

Effects of sewage sludge biochar on plant metal availability after application to a Mediterranean soil.

Pyrolytic conversion of sewage sludge into biochar could be a sustainable management option for Mediterranean agricultural soils. The aim of this work is to evaluate the effects of biochar from sewage sludge pyrolysis on soil properties; heavy metals solubility and bioavailability in a Mediterranean agricultural soil and compared with those of raw sewage sludge. Biochar (B) was prepared by pyrolysis of selected sewage sludge (SL) at 500°C. The pyrolysis process decreased the plant-available of Cu, Ni, Zn and Pb, the mobile forms of Cu, Ni, Zn, Cd and Pb and also the risk of leaching of Cu, Ni, Zn and Cd. A selected Mediterranean soil was amended with SL and B at two different rates in mass: 4% and 8%. The incubation experiment (200 d) was conducted in order to study carbon mineralization and trace metal solubility and bioavailability of these treatments. Both types of amendments increased soil respiration with respect to the control soil. The increase was lower in the case of B than when SL was directly added. Metals mobility was studied in soil after the incubation and it can be established that the risk of leaching of Cu, Ni and Zn were lower in the soil treated with biochar that in sewage sludge treatment. Biochar amended samples also reduced plant availability of Ni, Zn, Cd and Pb when compared to sewage sludge amended samples.

Adsorbent materials from paper industry waste materials and their use in Cu(II) removal from water.

This paper deals with the removal of Cu(2+) from water using adsorbent materials prepared from paper industry waste materials (one de-inking paper sludge and other sludge from virgin pulp mill). Experimental results showed that de-inking paper sludge leads to mesoporous materials (V(mic)/V(T)=0.13 and 0.14), whereas the sludge from virgin pulp mill produces high microporous adsorbents (V(mic)/V(T)=0.39 and 0.41). Adsorbent materials were then used for Cu(2+) removal from water at acid pH. During water treatment, heavy metals lixiviation from adsorbent materials was not produced. However, important Ca and Mg leaching was observed. Final pH significantly increases after treatment of water with adsorbent materials probably due to their elevated CaCO(3) content. In general, highest Cu(2+) removal was obtained using adsorbent materials from de-inking paper sludge. This result could be due to their higher content in oxygenated surface groups, high average pore diameter, elevated superficial charge density, high CaCO(3) amount and high Ca and Mg exchange content.

Composition of a Spanish sewage sludge and effects on treated soil and olive trees.

The effects of sewage sludge (SL) application on the soil and olive trees (Olea europaea L., cultivar: cornicabra) were studied. The plants were grown in 8.5L pots and subjected to the following treatments: 0, 3.66, 7.32, 14.65, 29.3, 58.6, and 117.2 g SL kg(-1) soil that corresponded, respectively, to 0, 4, 8, 16, 32, 64 and 128 M g ha(-1) dry weight of sewage sludge. The application of SL at the rates 64 and 128 M g ha(-1) produced leaf tip burning and leaf drop after 120 days, although cumulative metal pollutant loading rates was below USEPA and European regulations. This toxicity symptom could be caused by the high sodium levels in the leaves (over 0.19%), which can damage olive tree development. The Na contents of leaves were well correlated with soil Na content (r2: 0.91). In general, SL rates significantly increased the level of Cr, Ni, Cu, Zn, Cd and Pb in soil and plants, but these concentrations were in the normal ranges, except for the Zn concentration, which was over the critical soil content for the rates of 32, 64, 128 Mg ha(-1) but not in the leaves. Results suggested that regulations about the utilization of sewage sludge on agricultural land should consider the limit values for salt, and not only metals, that may be added to soil, in order to minimize the risk of negative effects to plant health.