Buffering of Alkaline Steel Slag Leachate across a Natural Wetland

Abstract

Buffering of high-pH (>12) steel slag leachate is documented across a small, natural calcareous wetland. The alkaline leachate is supersaturated with respect to calcite upstream of the wetland (Sl(calcite) values +2.3) and becomes less saturated with progress across the wetland, to Sl(calcite) values of +0.27 at the wetland outlet. Reduction in pH across the wetland (to around pH 8 at the wetland outlet) was observed to be more pronounced over summer months, possibly due to increased microbial activity, possibly further assisted by greater flow baffling by emergent vegetation. Calculated calcite precipitation rates downstream of the leachate source, estimated from hydrochemical data, flow, and surface area, were on the order of 0.4-15 g m(-2) day(-1), while direct measurements (using immersed limestone blocks) showed calcite precipitation values in the range 3-10 g m(-2) day(-1). Precipitation rate was highest in the pH range where the carbonate ion is a dominant constituent of sample alkalinity (pH 9.5-11) and at the locations where wetland biota became established downstream of the leachate emergence. These data provide valuable insights into the potential for using constructed wetlands for the passive treatment of high pH steel slag leachates.

Biogeochemical Reduction Processes in a Hyper-Alkaline Leachate Affected Soil Profile

“…Microbial activity can influence the rate of calcite precipitation by increasing the CO 2 flux and by providing potential nucleation (Mayes et al 2006), however the reaction scheme above accounts for the formation of a large tufa deposit immediately where the spring water emerges and the gradual reduction in the stream's pH value down the site (in July 2009 the pH of stream just below the area of continuous surface precipitate was 8.6).…”

“…Some are entirely natural, e.g., soda lakes, hot springs, oceanographic cold seeps, deep mine waters (Takai et al 2001;Takai et al 2005;Pollock et al 2007;McMillan et al 2009;Brazelton et al 2010), but many are also due to human activities. These anthropogenic sites occur as a result of the presence of residues from a range of industrial processes, e.g., lime production waste, steelworks slags, coal combustion residues, Solvay process waste, chromite ore processing residues, bauxite processing wastes, borax wastes and cementitious construction wastes (Effler et al 1991;Carlson and Adriano 1993;Townsend et al 1999;Deakin et al 2001;Ye et al 2004;Mayes et al 2006;Mayes et al 2008;Hartland et al 2009;Mayes et al 2011). Weathering of these wastes typically produces highly alkaline leachate (pH 10-13) due to the ubiquitous presence of Ca, Na and K oxides (primarily CaO) that hydrolyze in natural waters to produce soluble metal hydroxides.…”

Environmental Compatibility of Lightweight Aggregates from Mine Tailings and Industrial Byproducts

“…The CaO in the aggregate generates alkaline leachates, and free CaO dissociates to form Ca 2+ and OH − to increase the pH. Similarly, steel slag contains free CaO and generates an alkaline leachate with a pH of 10 to 13 [35], containing heavy metals that are passively treated by storage in a wetland [36]. Proctor et al [37] analyzed the potential ecological risk of three types of steel-industry slags and concluded there is potential for impacts to aquatic life due to a high pH.…”

Effect of Basic Oxygen Furnace Slag used as Structural Filling Materials on the Subsurface Environment

“…로 사용하기 어려워 성·복토재로 사용되고 있고, 이 때 물과 만나면 생성되는 알칼리 화합물인 Ca(OH) 2 는 환경 오염원으로 간주되기도 한다 (Choi et al, 2007;Mayes et al, 2006). 기존 연구에서는 알칼리 용출수가 수계 내 화학적 산소 요구량(Chemical Oxygen Demand, COD), Mayes et al, 2006Mayes et al, , 2008 (Peek and Volk, 1985).…”

“…기존 연구에서는 알칼리 용출수가 수계 내 화학적 산소 요구량(Chemical Oxygen Demand, COD), Mayes et al, 2006Mayes et al, , 2008 (Peek and Volk, 1985). 전반적으로 전로제강슬래그의 영향을 받은 토양 내 오염우려물질 함량은 대체적으로 유의한 큰 변화는 없었 (Lekakh et al, 2008).…”