ARTICLE

J. Cent. South Univ. (2019) 26: 353-360

DOI: https://doi.org/10.1007/s11771-019-4007-9

Rehabilitation of bauxite residue to support soil development and grassland establishment

Ronan COURTNEY¹, XUE Sheng-guo(薛生国)²

1. Department of Biological Sciences and Bernal Institute, University of Limerick, Co. Limerick, Ireland;

2. School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Rehabilitation (amendment and vegetation establishment) on bauxite residue is viewed as a promising strategy to stabilize the surface and initiate soil development. However, such approaches are inhibited by high pH, high exchangeable sodium (ESP) and poor nutrient status. Amendment with gypsum is effective in improving residue physical and chemical properties and promoting seed establishment and growth. Application of organics (e.g. compost) can address nutrient deficiencies but supplemental fertilizer additions may be required. A series of germination bioassays were performed on residue to determine candidate species and optimum rehabilitation application rates. Subsequent field trials assessed establishment of grassland species Holcus lanatus and Trifolium pratense as well as physical and chemical properties of amended residue. Follow up monitoring over five years assessed elemental content in grassland and species dynamics. With co-application of the amendments several grassland species can grow on the residue. Over time other plant species can invade the restored area and fast growing nutrient demanding grasses are replaced. Scrub species can establish within a 5 Yr period and there is evidence of nutrient cycling. High pH, sodicity and nutrient deficiencies are the major limit ing factors to establishing grassland on residue. Following restoration several plant species can grow on amended residue.

Key words: bauxite residue; substrate amendment; soil development; soil formation in bauxite residue; vegetation establishment

Cite this article as: Ronan COURTNEY, XUE Sheng-guo. Rehabilitation of bauxite residue to support soil development and grassland establishment [J]. Journal of Central South University, 2019, 26(2): 353–360. DOI: https://doi.org/10.1007/s11771-019-4007-9.

1 Introduction

Extraction of alumina from bauxite ore using the Bayer process (sodium hydroxide digestion under high temperature and pressure) results in the generation of bauxite residue as a by-product.Production rate of the residue generally ranges from 1 to 1.5 mg per mg of alumina yielded with approximately 200 MT of bauxite residue produced globally each year [1, 2]. Almost all bauxite residue is stored in land-based bauxite residue disposal areas (BRDAs) [3] as re-use attempts have resulted in limited application potential (ca. 2%–3%) [2, 4, 5].

Bauxite residue is typically highly alkaline, saline, sodic and may contain trace elements at elevated levels [3, 6]. As a result, its improper disposal and management may pose a risk to the environment and surrounding communities due to generation of dust and aqueous emissions [7–10].It is therefore a major challenge for the industry to develop robust strategies to rehabilitate residue that can fulfill the objectives of stabilization and support soil development and ecosystem development.

Weathering processes can decrease the alkalinity burden of the residue [11, 12], but during this time the material may be subjected to erosion resulting in contamination of surrounding lands [13]. Although chemical and physical techniques exist for stabilizing mine wastes and to reduce wind and water erosion, the process is limited by availability, costs, lack of permanency and need for regular monitoring [14]. The long-term objective of residue rehabilitation and restoration can only be realistically achieved by the use of vegetation as a basis for landscaping, stabilization and pollution control [14] and is an issue of concern to alumina producers worldwide [15–17].Thus, promoting vegetation establishment through transformation of residue to soil-like substrates with physical and chemical properties similar to soil is viewed as a realistic strategy for mitigating the environmental risks posed by BRDAs [18–21].

Establishing vegetation on bauxite residue is inhibited by its extreme chemical, physical, and microbiological properties. Residue pH is typically pH>10, exhibits high salinity (electrical conductivity-EC>30 dS/m, high exchangeable sodium percentage (ESP)>70% [22, 23]). In addition, concentrations of essential plant nutrients are low [24–26]. Physically, residue exhibits a high bulk density and high contents of silt- and clay-size particles and is low in organic matter.

The approach to rehabilitating residue therefore typically requires a strategy improving the physical and/or chemical properties of the residue using amendments (e.g. gypsum, biosolids, compost) followed by direct vegetation of the residues surface [1, 21, 27, 28]. Grasses tolerant of saline and sodic soils may be used e.g. Agropyron elongatum [29] and Cynodon dactylon [30] or conditions improved sufficiently to support growth of less tolerant plants [22].

An overview of residue characterisation, amendment strategies and plant establishment at a European BRDA is provided. Effects of amendments on residue properties and plant growth and nutrition over a one-year establishment phase were examined.Evidence of ecosystem development such plant diversity and pedogenesis over a 5 year period is also assessed.

2 Materials and methods

A restoration research programme was undertaken on a European BRDA to develop a method for amending the residue to support the establishing of a vegetation cover that can be seeded directly on the surface under temperate climatic conditions (e.g. 5 year mean annual rainfall 930 mm; mean temperature is 12.6 °C).

Rehabilitation strategies were designed to evaluate the effect of gypsum to amend the high pH, sodicity and salinity of the residue and the effects on seed germination and seedling growth of candidate species for rehabilitation. The focus of the strategy was to develop a grassland system and several grass (Poaceae) and leguminous (Fabaceae) were selected. Relative seed germination, relative root growth and germination index for red clover (Trifolium pratense) and ryegrass (Lolium perenne) were assessed.

Following identification of suitable species field plots were constructed on an amended residue mix. Replicate plots (x3) of 2 m² received gypsum application at rates 0, 45 and 90 t/ha and incorporated to 0–20 cm.Following a suitable leaching period, organic waste (compost) was applied at 60 and 120 t/ha to evaluate the effect of amendment addition on plant growth and nutrient uptake under field conditions.Treatments thus amended were seeded with grassland species Agrostis stolonifera, Festuca rubra, Holcus lanatus, Lolium perenne, Trifolium repens, and Trifolium pratense at a rate of 100 kg/ha.

Herbage samples were collected after approximately one year’s growth and prior to inflorescence. Representative herbage samples were harvested by cutting at 2 cm above ground level.In the laboratory samples were rinsed with deionised water; oven dried at 70 °C and milled (<1 mm); digested in nitric acid on an open block digester and Na content determined by ICP-MS.Herbage N content was determined by the Kjeldahl method. Subsequent herbage analysis was conducted at the same stage of growth on both H. lanatus and T. pratense over a 5 year period.

Additionally, the species composition and dynamics of the emerging vegetation cover on the rehabilitated residue was assessed annually. Assessment was conducted during the month of July when florescence aided identification and the nomenclature for plants followed those of Stace (1991).

2.1 Substrate analysis

Representative samples of revegetated bauxite residue were taken at a depth of 0–10 cm using a soil corer.Residue samples were air-dried at 30 °C and ground to pass through a 2-mm sieve prior to analysis.Soil parameters determined were pH and EC (1:5 extract), exchangeable sodium percentage (ESP) using the equation ESP= (100Naex)/CEC as per Sumner and Naidu (1998) where Naex=exchangeable sodium and CEC is cation exchange capacity. Available P was extracted using Olsens reagent and micronutrients Cu, Zn and Mn by extraction with DTPA.

The bulk density (ρ_b) of the residue was determined using the method described by BLAKE (1965). The microaggregate stability of the amended residue was determined on samples that were mechanically shaken and dispersed using optical laser diffraction on a Malvern Mastersizer [31] and results expressed as percent fraction under 2 μm.

2.2 Statistical analysis

The effects of gypsum treatment on residue properties were analyzed using analysis of variance (ANOVA) and any differences between treatments tested using Tukey’s HSD. Statistical analyses were conducted using SPSS version 15 (SPSS) and Graphpad Prism 4.

3 Results and discussion

Unamended residue displayed pH 11.5 and is typical of the range reported (10.9–12.8) [11, 23] (Table 1). Residue was also highly saline (14 mS/cm) and highly sodic (ESP 85). Physical working of the residue (e.g ploughing and rotovation) facilitated leaching and weathering, and this treatment (no gypsum) achieved reductions in key properties but not to desired levels. Amendment plus leaching decreased the pH, EC and ESP of the residue and again this beneficial effect has been recorded by others [1, 27, 31]. Encouragingly, the results also demonstrate that residue amendment strategies implemented in the field can effectively achieve rehabilitation goals that are standard in soil disciplines and proposed for bauxite reside [23].Some ESP values may be still over the desired range but should decrease with further weathering, as has been evidenced in other rehabilitation studies [32].

Table 1 Mean values for substrate parameters for bauxite residue treatments and desired range for rehabilitation

Unamended residue was deficient in nutrient content (Table 2). Nutrient assessment showed plant content to be with in normal ranges (Table 3) but other studies have highlighted deficiencies for plants growing in residue, even after fertiliser or organic addition [26, 33, 34]. High rates of organic additions raised soil amounts to above critical values for tested nutrients but this may not transfer to plant availability. Where residue is not sufficiently amended, there is potential for loss or adsorption of added nutrients [17]. A further difficulty in providing sufficient nutrient supply is the adequately of standard soil assessment strategies for determining nutrient plant availability in residue [25, 35, 36].

Table 2 Mean values for substrate parameters for unamended residue and 90 t/ha gypsum amended residue with different application rates of compost and desired range for plant growth

3.1 Plant germination and growth

Germination in saline and sodic soils is usually inhibited by excess salts, but seed germination was broadly similar across the residue treatments (no significant increase gypsum amended residue). This is likely due to the use of tillage practices in the amendment strategies enhancing leaching of excess salinity in the residue. The role of gypsum in improving residue as a soil medium is more apparent when developed root length is determined, with significant improvement following gypsum amendment.Supplemental Ca in saline and sodic solutions increased the growth rate of several test species in other residue studies [22]. High application rate (gypsum at 90 t/ha was effective in providing an appropriate environment for seed germination and plant establishment for T. pratense and L. perenne where the germination index was significantly greater than the 80%, indicating the disappearance of phytotoxicity) as shown in Figure 1. GI values of <80% in the other treatments illustrate the importance of lowering pH, EC and ESP in residue prior to seeding.

3.2 Residue physical properties

The sodic properties of residue result in it being difficult to cultivate.The lack of structural stability can result in soil erosion as conditions can promote seal and crust formation at the soil surface and can be detrimental to plant root structures and survival of plant seedlings. Amendment procedures supplying organic matter also decreased the high bulk density of unamended residue (exceeding 1.6 g/cm³) as shown in Figure 2(a), which would hinder root penetration and plant establishment and has been reported for other residue [1, 11]. The creation of an appropriate soil environment, such as capacity to resist structural degradation, is necessary for successful ecosystem development.

High ESP values cause swelling and dispersion of colloids and microaggregates and improvement of the soil structure requires the application of appropriate amendments in order to promote clay flocculation [37].Gypsum amendment effectively lowers residue ESP and has a concomitant effect in decreasing the amount of dispersible clay (Figure 2(b)).

3.3 Ecosystem and soil development

Although unamended bauxite residue inhibits plant establishment, amended favourable conditions can prevail. Improvement in residue properties beyond the amended zone persist deeper in the profile [38] and further reductions in pH occurs in the rhizosphere [31].

Potential exhaustion of applied nutrients through leaching and plant uptake is of concern in restoration of mine wastes. There is potential for N losses through leaching of bauxite residue [35] and over time nutrients such as P can be limiting in mine wastes. Results showed significant decreases in nitrogen content for H. lanatus in the second year with decreases not as pronounced for T. pratense.By Yr 5 all nutrients had recovered to Yr 1 values or greater and all were above critical levels. The decreases for the N in the second year’s growth are possible due to the rapid growth and high content in aerial portions during the first year.Recovery of values to Year 1 levels or higher indicates that plant available nutrients in the residue substrate increased.Reductions in plant available Na also occurred and are evidenced in plant content (Figure 3).

Improved soil conditions were also evidenced by the wider range of plant species that established in the rehabilitated residue (Figure 4). While the rehabilitation procedure involved seeding with grassland species (poaceae and fabaceae), the number of species in these groups increased. This is particularly encouraging for Fabaceae, as tested species in this group showed poor germination and seedling establishment potential in amended residue (data not shown). Overall, the increase in species diversity and support for species more characteristic of scrubland demonstrate the advances in the ecological processes that can be supported by disappearance of adverse conditions, improved soil conditions and establishment of processes such as nutrient cycling.

Table 3 Nutrient range in Holcus lanatus in gypsum and compost amended residue and typical range for grasslands year 1

Figure 1 Germination (a) and root length percentage (b), and germination indices (c) for Lolium perenne and Trifolium pratense seeds in extracts of gypsum amended bauxite residue

4 Conclusions

Unamended bauxite residue exhibits properties inhibitory to plant germination and growth.Application of gypsum with organic wastes is an effective method for overcoming the physical and chemical restraints and nutrient deficiencies exhibited by the residue.Evidence of soil development and support of soil functions is emerging from revegetated bauxite residue field treatments.

Figure 2 Relationship of bauxite residue bulk density with total carbon content (a) and relationship of percentage dispersed clay with substrate exchangeable sodium percentage (ESP) (b)

Figure 3 Concentrations of nitrogen (a) and sodium (b) in H. lanatus and T. pratense aerial portions growing on restored bauxite residue over a 5-year period

Figure 4 Vegetation composition of restored bauxite residue over a 5-year period

A range of grassland species can establish and grow on the amended residue surface.Fast- growing agricultural grasses experience a decrease in nutrient content after a second year growing season and species more tolerant of nutrient poor conditions replace them.Scrub species are capable of colonising the vegetated residue within a 5-year period.Nutrient cycling is occurring but this process requires monitoring.

Future rehabilitation practices should consider amendment procedures to optimize conditions for the necessary soil microbial and faunal communities required to promote sustainable ecosystem establishment.

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(Edited by YANG Hua)

中文导读

赤泥堆场修复促进土壤发育和植被重建

摘要：赤泥是氧化铝工业生产过程排放的强碱性固体废物，盐碱性强和营养元素匮乏是影响赤泥堆场植物生长的主要限制因素。对赤泥堆场的长期野外研究，分析基质改良对赤泥理化特性和植物多样性的影响，结果表明：施用石膏后，赤泥pH和可交换钠明显降低，黑麦草和红牛轴草发芽指数分别由22%和42%提高到100%；施用堆肥显著提高赤泥碳、氮、磷等养分元素含量；赤泥改良1年后，绒毛草主要元素含量与普通草地植物元素含量相似；基质改良5年后，绒毛草和红牛轴草钠含量显著降低，分别由0.6%和0.58%降低到0.3%和0.1%，赤泥堆场优势物种为菊科、豆科和禾本科植物。研究结果对赤泥土壤化研究及堆场生态修复实践具有重要的参考价值。

关键词：赤泥堆场；基质改良；土壤发育；赤泥土壤化；植被恢复

Foundation item: Projects(41877551, 41842020) supported by the National Natural Science Foundation of China；Project supported by the Science Foundation Ireland 17/CDA/4778

Received date: 2018-11-05; Accepted date: 2018-12-10

Corresponding author: Ronan COURTNEY, PhD, Lecturer; Tel: +61-202427; E-mail: ronan.courtney@ul.ie; ORCID: 0000-0003-4247- 2877