Complementing compost with biochar for agriculture, soil remediation and climate mitigation

We are racing to manage a phenomenally increasing volume of organic wastes from urban, industrial and agricultural entities. Composting is one of the preferred ways to convert biodegradable wastes into nutrient-rich soil conditioners. The age-old technique of composting process is being improved with innovative scientific means. Biochar, a widely studied soil amendment, is a carbonaceous material that can hold nutrients from endogenic/exogenic sources. Biochar-compost, a biochar-complemented compost, may provide a wide range of benefits expected from both materials. Compost and biochar can improve physicochemical and microbiological attributes of soils by supplying labile and stable carbons, and nutrients. Compost may also supply beneficial microbes. This means biochar-compost is a synergic soil amendment that can improve soil quality, increase crop production, and remediate contaminated soils. Having stable carbon, large reactive surface with nutrient loads, biochar can interact widely with organic biomass and modify physicochemical and-microbial states during a composting process while making biochar-compost. Production and application methods of biochar, compost and biochar-compost are covered for agricultural and contaminated soils. Metal and organic contaminations are also discussed. A case study on making and field-testing a mineral-enhanced biochar and a biochar-compost to improve rice yield, is presented at the end

Neutralization and cation dissolution characteristics of bauxite refining residue

The neutralization reaction of red mud with various concentrations of HCI acid and the dissolution of cations were studied in order to evaluate the mechanism of Na release from red mud. The changes in the mineralogy of the desilication products (DSP) was simultaneously followed by XRD and SEM. From the neutralization curve, four buffer regions were identified: pH 10.0-8.3 resulting from the neutralization of carbonate and free hydroxyl; pH 7.8-6.2 due to the dissolution of carbonates of Ca and Mg and accompanied by the release of Na from the desilication products; pH 35-3.2 resulting from the dissolution of the aluminum hydroxide precipitated from the decomposition of DSP taking place between pH about 7.0 and 5.4; and pH 0.8-0.4 resulting from a further dissolution of aluminum and iron oxide. The mineralogical study of the DSP particles of red mud revealed it to be a zeolitic mineral akin to natrodavyne consisting of crystalline rods and blobs forming a spherical ball-like network. The rods curved and wrapped on top of each other in a meridian orientation producing what appears to be poles from which rods radiated. The X-ray diffraction analysis only showed the major peaks of natrodavyne at pH 6.2. Scanning electron micrographs on the DSP particles at pH 6.2 showed no observable change in the external morphology of DSP with reduction in pH. In addition, the maximum release of Na was achieved at pH 5.4, thus Al and Si must have been released at a pH higher than 5.4 but immediately re-precipitated and therefore not observed in solution. It can be concluded that Na in the DSP particles were released at pH 7.0- 5.4 by acid hydrolysis. Hence, neutralization of red mud to a pH above 5.4 would produce a maximum release of Na without the possibility of excess dissolution of Al, although in practice we would not necessarily reduce the pH to as low as 5.4

Combined effect of nonionic surfactant Tween 80 and DOM on the behaviors of PAHs in soil–water system

Batch experiments were performed to examine the desorption behavior of phenanthrene and pyrene in soil–water system in the presence of nonionic surfactant Tween 80 and dissolved organic matter (DOM) derived from pig manure or pig manure compost. Addition of 150 mg l−1 Tween 80 desorbed 5.8% and 2.1% of phenanthrene and pyrene from soil into aqueous phase, respectively, while the addition of both Tween 80 and DOM derived from pig manure compost and pig manure could further enhance the desorption of phenanthrene to 15.8% and 16.2%, respectively, and 6.4% and 10.9%, respectively, for pyrene. In addition, our finding also suggested that subsequent addition of Tween 80 into the soil–water system could further enhance PAHs desorption. The enhancement effect of the co-existence of Tween 80 and DOM was more than the additive effect of the Tween 80 and DOM individually. It is likely that the formation of DOM–surfactant complex in the soil–water system may be a possible reason to explain such desorption enhancement phenomenon. Therefore, it is anticipated that the coexistence of both Tween 80 and DOM derived from pig manure or pig manure compost in soil environment will enhance the bioavailability of PAHs as well as other hydrophobic organic contaminants (HOCs) by enhancing the desorption during remediation process

Use of waste gypsum in the revegetation on red mud deposits: A greenhouse study

The disposal of fine bauxite refining residue (red mud) has occupied huge areas of land which remain derelict and subject to wind and water erosion. The major constraints in rehabilitation are the high alkalinity, salinity and sodicity of red mud. This paper describes a greenhouse study to evaluate the use of waste gypsum as ameliorants for red mud. Red mud was amended with gypsum at 0, 2, 5 and 8% (w/w) and its effect on the growth of two grass species, Agropyron elongatum (Tall wheat grass) and Cynodon dactylon (Bermuda grass), was evaluated. Gypsum amendment significantly reduced the pH, electrical conductivity (EC), and Na and Al content of red mud and provided a continuous supply of Ca2+, which resulted in a lower soil exchangeable sodium percentage (ESP). The improved soil conditions at ≥ 5% gypsum amendment significantly increased seedling emergence and dry weight yields. The enhanced plant growth and seedling emergence show significant negative correlations with pH, Al and ESP of soil. Tissue analyses indicated that the availability of P, K, Mg and Ca also increased for ≥ 5% gypsum amended red mud, while Na and Fe were less available. Results from the present study indicated that waste gypsum is an effective ameliorant for red mud revegetation and no topsoil amendment is required

Effectiveness of acidic industrial wastes for reclaiming fine bauxite refining residue (red mud)

Land disposal of bauxite refining residues produced by the aluminum industry has resulted in areas devoid of plants because of the high salinity and alkalinity of the residues. In addition, the fine fraction (red mud) of these residues is prone to wind and water erosion, which can pollute the surrounding soils and surface waters. A glasshouse pot-leaching study was performed to evaluate the effectiveness of waste gypsum (CaSO4) and copperas (FeSO4) as ameliorants for the red mud fraction. Red mud was amended with the wastes at rates of 0, 2, 5, and 8% (w/w), and leached with 1200 ml (126 min rainfall) of deionized water before conducting a seedling emergence test using Agropyron elongatum. Leachate and soil analyses indicated that gypsum and copperas amendments were able to reduce the pH, electrical conductivity (EC) and Na content of red mud significantly. Copperas was more effective in reducing soil Na contents and maintaining a lower soil EC. On the other hand, gypsum-amended red mud maintained a lower pH and a higher Ca content because of the low solubility of gypsum. Both amendments also resulted in significant reductions in soluble Al content and exchangeable sodium percentage (ESP). The reduced pH, ESP and Al content were correlated to the improved seedling emergence of Agropyron at application rates of >2%. The results demonstrate that both gypsum and copperas are effective ameliorants for red mud. However, gypsum appears to provide a more persistent pH buffering capacity and lower ESP for red mud

Sewage sludge as organic ameliorant for revegetation of fine bauxite refining residue

The rise in aluminium demand in the world has significantly increased the generation of bauxite residue which occupies huge areas of land worldwide. Direct revegetation of residue storage areas has been unsuccessful because of the high alkalinity and salinity, and poor nutrient contents of the fine residue (red mud). This paper describes glasshouse and field experiments evaluating the potential use of sewage sludge as an organic ameliorant for gypsum amended red mud. The growth of Agropyron elongatum in red mud receiving gypsum (0 and 38.5 t ha-1) and sewage sludge (0, 38.5 and 77 t ha-1) amendment was assessed in a glasshouse study. Leachate and soil analyses revealed that gypsum was effective in reducing the pH, EC and ESP of red mud, while sewage sludge gave additional reductions in EC, Na and ESP. No evidence of any significant increases in heavy metal contents were observed in the leachates following sewage sludge amendment. However, soil Al contents were more available in red mud receiving only sewage sludge treatment. Sewage sludge amendment significantly increased dry weight yield and provided sufficient nutrients for plant growth except K which was marginal. No heavy metal accumulation was observed in Agropyron. Following that, a field experiment was performed having red mud amended with sewage sludge (38.5, 77 and 154 t ha-1) and gypsum (38.5 and 77 t ha-1) to evaluate their effects on soil physical properties of red mud. Sewage sludge significantly reduced soil bulk density (25%) and particle density (9%) and increased the total porosity of red mud (8%). Hydraulic conductivity also increased from 1.5 to 23 x 10-5 m s-1. Plant cover percentage and dry weight yield of Agropyron increased with an increase in gypsum and sewage sludge amendment. The results confirm that sewage sludge is effective in improving both soil structure and nutrient status of gypsum amended red mud. The use of sewage sludge for red mud revegetation provides not just an option for sludge disposal, but also a cost effective revegetation strategy for bauxite refining industry

Effects of gypsum and sewage sludge amendment on physical properties of fine bauxite refining residue

Fine bauxite refining residue (red mud) was amended with gypsum and sewage sludge to evaluate their effects on soil physical properties in a field experiment. Sewage sludge amendment significantly reduced soil bulk density (25%) and particle density (9%), and increased the total porosity of red mud (8%). Both sewage sludge and gypsum contributed to the increases in hydraulic conductivity after one growing season. Enhanced plant growth was due to improved soil structure and hydraulic conductivity. An application of 77 t/ha gypsum and 144 t/ha sewage sludge exerted the maximum effect on soil physical properties of red mud and should ensure the initial establishment of plants