The use of mine impacted water and its treatment by-products in agriculture

Abstract

The Coalfields of the Republic of South Africa (RSA) discharge approximately 360 Ml d-1 of mine impacted water, referred to as Acid Mine Drainage (AMD), requiring neutralization to reduce risk to the environment. High Density Sludge Process (HDSP) is commonly used to treat AMD, and neutralization is typically with either limestone (CaCO3) alone to save costs, or with limestone plus hydrated lime (Ca(OH)2) to effectively reduce acidity and improve metal removal. This water either needs to be further treated to reduce metal content and salinity, or a potential option is to use it for irrigation. Since, it would be possible to lime a soil and irrigate directly with AMD as this would be potentially easy to manage than an HDS plant and save costs on the plant. The treatment process produces a circum-neutral mine water that requires further treatment with reverse osmosis to potable water. Suitability of these waters for irrigation can be evaluated with the Irrigation Water Quality Decision Support System (IWQDSS) for RSA. This study therefore evaluated the two specific mine impacted waters for irrigation. The treatment process also generates gypseous products, referred to as High Density Sludges (HDS), which may be classified as hazardous, based on metal (Mn, Ni, Pb) content, in which case expensive waste storage is required. However, these sludges may have some value for use in agriculture since they are gypseous. Four out of six sludges considered in this study were investigated for potential use in agriculture since their chemical characteristics depend on the quality of AMD and the treatment process. If hazardous, a potential approach was to add phosphate to them since this has been shown before to immobilize metals. The influence of phosphate on the availability of elements in sludges was therefore investigated. Hence, the objectives were to investigate; 1) the fitness for use of AMD and circum-neutral mine impacted waters for irrigation with IWQDSS, 2) chemical and physical properties of sludges, 3) hazardous status of sludges using the RSA waste classification system including those of the United States Environmental Protection Agency (USEPA), Australia, China and Canada, 4) phosphate potential in reducing the solubility of metals in sludges, 5) crop and soil response to sludges applied on their own as soil amendments and when co-applied with phosphate, 6) the influence of phosphate co-applied with sludges to the phyto-availability and uptake of Ni and Pb, including food safety. Assessment with IWQDSS showed that both waters were not fit for irrigation because of some quality issues. However, AMD can only be used if the soil can be limed and used as a reactor and further showed that there would be no leaf scorching. The circum-neutral mine water was found to be not effectively saline. Micro irrigation should not be considered for these waters due to suspended solids they contain.

Four of the six sludges assessed for agricultural use included; a Ferriferrous Gypseous sludge (GypFeMnNi) with Fe, Mn and Ni from a limestone process, and three others generated from three stages of a limestone plus hydrated lime process; Ferriferrous Gypseous sludge with Mn (GypFeMn), Gypseous sludge with Brucite (GypB) and Gypseous sludge (Gyp) with Fe removed. Chemically, the sludges, GypFeMn, GypB and Gyp showed pH values of 8.2, 9.4 and 9.5, exhibiting CaCO3 equivalents (CCE) of 510, 601, 617 mg kg-1. The sludge, GypFeMnNi, had a pH of 5.5 and a CCE of 250 mg kg-1. All four sludges showed to be largely gypsum (72 – 95 %) composed. Physically, all sludges had particle sizes falling between 0.4 to 906 µm. These four sludges were further considered for hazardous assessment, including two sludges; GypFeNi and GypFe from a different limestone process. USEPA rated all six sludges non-hazardous, while Canada and China found GypFeNi as hazardous based on Ni solubility, Australia found GypFeMn as hazardous. RSA considered GypFeMnNi and GypFeNi hazardous, based on Ni and Mn solubility. Limestone was therefore less effective in reducing the solubility of Ni and Mn in the sludges than limestone plus hydrated lime. The sludges found hazardous (GypFeMnNi and GypFeMn) were then phosphated to reduce Mn and Ni solubility. Their solubility was reduced in both sludges. GypFeMnNi and Gyp, were further considered for use as soil amendments and selection was based on differences in the treatments that generates them. A pot trial was conducted where both were applied at 10 and 20 t ha-1 each to a soil with pH 3.75 and co-applied with phosphate at application rates of 40 and 100 kg ha-1. Maize (Zea mays) was planted and harvested at physiological maturity. Effect on soil showed that both sludges marginally increased pH, with Gyp at 20 t ha-1 and 100 kg ha-1 P increasing it the most by 0.46 units. This pH was still not suitable for plant growth. The sludge, Gyp increased soil salinity the most from 7.8 mS m-1 to 728 mS m-1, suitable only for salt tolerant crops. The effect on the maize showed that both sludges on their own marginally increased plant height and biomass, but co-application with phosphate increased these parameters. Grain was present only in treatments where phosphate was co-applied with either sludge. The highest grain yield was obtained when Gyp was applied at 20 t ha-1 with 100 kg ha-1 P. With food safety, Ni and Pb concentrations in the grain were below thresholds regarded as toxic. It is suggested that irrigation with AMD may be possible on condition that the soil is limed and used as treatment reactor to prevent the reduction of soil pH. Also, micro irrigation systems are to be avoided when irrigating with AMD and circum-neutral mine impacted waters because they contain suspended solids that can clog them. Irrigation should be with an appropriate leaching fraction to reduce accumulation of salts in the soil profile. It can also be concluded that two of the sludges from a limestone only HDSP were found to be hazardous by the RSA waste classification system due to Mn and Ni solubility., whereas international systems felt these materials were non-hazardous. The RSA waste classification system was found to be overly cautious compared to international systems and should be revisited. Sludges from HDSP can rather be used as soil amendments instead of being classified hazardous and destined to expensive waste management sites. If certain trace elements are excessively available, the study demonstrated that phosphating reduces mobility and toxicity, ensuring the safety of produce from soils treated with HDS.

Keywords: AMD, HDS, Circum-neutral mine water, waste classification, amendment