# Remediation Opportunities for Arsenic-Contaminated Gold Mine Waste
## 1 Introduction  
Arsenic (As) is an element with metalloid characteristics which occurs naturally and is the 20 th most abundant element in the Earth's crust [1]. Due to anthropogenic activities, such as gold mining, As concentrations in the environment are elevated above naturally occurring levels [2]. In legacy gold mining areas, abandoned As-rich mine waste dumps act as ongoing sources of As to the surrounding environment. Mine waste such as tailings and grey sands have polluted landscapes through mechanisms such as soil erosion as a result of a lack of vegetation cover and As leaching [3]. This is a concern because arsenic is toxic to all living organisms, is carcinogenic, and disrupts ecosystem functions, for example those involved in soil properties, geochemical cycles, and water acidification [4; 5]. As such, to reduce the burden posed by arsenic from mine waste, there is a need to identify and explore sustainable remediation techniques that alleviate environmental pollution whilst also restoring ecosystem functioning. It is vital to restore ecosystem functions because natural fertile soil takes thousands of years to form [6]. Currently, in some regions of the world, 50 tonnes (t) per hectare of fertile soil has been lost to degradation [6]. In India, mined land equates to approximately one-third of the agricultural land [4]. It is essential to restore degraded soil with increased population growth and increasing demand for fertile agricultural soil [6]. In addition, soil organic matter stores more organic carbon (C) than the atmosphere and global vegetation combined [6; 7]. Increasing soil organicmatter and sequestering soil carbon in degraded soils will improve key soil fertility parameters, such as nutrients, microorganisms, and soil structure, which supports living organisms and biodiversity [7; 8; 9; 10; 11; 12]. Biological remediation techniques involving plants and soil amendments are environmentally sustainable methods; revegetation of barren mine waste sites also reduces soil erosion, mitigating further contamination. Phytoremediation is the use of plants to remediate soil. Plants can either bioaccumulate As (phytoextraction) or stabilise As in the rhizosphere (phytostabilisation) [13; 14]. However, As is toxic to plants; for example, arsenic-contaminated soil and pore water can have detrimental impacts on the agriculture sector, such as rice production [15]. Rice is a staple food for many. A study by [PERSON] et al. [15] investigated rice production under climatic and As stress. Climatic stress (38 \({}^{\circ}\)C/850 ppmv CO\({}_{2}\)) and As stress caused a 16% and 39% yield reduction, respectively [15]. Soil amendments are vital to reduce toxicity and improve plant productivity [13]. Discovering new and effective remediation techniques for legacy mine waste has never been more important for the environment and world food security.  
This review aims to identify plant species compatible with an arid to semi-arid climate, which can survive elevated As concentrations ranging from 1000 to 2300 mg/kg, a common As concentration for grey sands [16; 17]. This review also investigates soil amendments and their abilities to enhance plant survival, phytoremediation and their soil-ameliorating capabilities. A novel approach to this review is to also investigate the role of As speciation in combination with arsenic remediation, soil amendments, toxicity, and restoring ecosystem functions. There are many reviews addressing total reclamation of mine waste sites, but additional assessment of As speciation is limited. The information obtained from this review will be used to identify the knowledge gaps in soil amendment phytoremediation studies and future research opportunities.