Removal of materials by dissolving them away from solids is called leaching .The chemical process industries use leaching but the process is usually called extraction, and organic solvents are often used. The theory and practice of leaching are well-developed because for many years leaching has been used to separate metals from their ores and to extract sugar from sugar beets. Environmental engineers have become concerned with leaching more recently because of the multitude of dumps and landfills that contain hazardous and toxic waste. Sometimes the natural breakdown of a toxic chemical results in another chemical that is even more toxic .Rain that passes through these materials enters ground water, lakes, streams,wells, pond, and the like.

Although many toxic materials have low solubility in water, the concentrations that are deemed hazardous are also very low. Furthermore, many toxic compounds are accumulated by living cells and can be more concentrated inside than outside a cell. This is why long-term exposure is a serious problem; encountering a low concentration of a toxic material a few times may not be dangerous, but having it in your drinking water day after day and year after year can ge deadly.

Heap leaching is a countercurrent process where the solid is in a stationary heap and the solvent percolates through the solid. An example is a dump or landfill. In industrial leaching, solvent and solid are mixed, allowed to approach eqilibrium, and the two phase are separated. Liquid and solids move countercurrently to the adjacent stages. The solvent phase ,called the extract, becomes more concentrated as it contacts in stagewise fasion the increaseingly solute -rich solid. The raffinate becomes less concentrated in soluble material as it moves toward the fresh solvent phase.

The main theory of leaching neglects mechanisms for holding the materials on the solid. Although adsorption and ion exchange can bind materials tightly to solids, we will simplify the analysis and consider only dissolving a soluble solid. An example is removing salt from sand by extraction with water.

通过溶解将物质从固体中除去，这叫作沥滤。化学加工业用到沥滤，但这个过程通常被称作淬取，而且常常使用有机溶剂。因为沥滤被用在分离金属与矿石以及从甜菜中抽取糖方面已经有很多年，所以其理论和实践已得到高度发展。近来，环境工程师给于沥滤更大的关注，因为垃圾倾倒场和垃圾填埋场含有有害有毒废料。一种有毒化学品的自然分解有时产生一种毒性更强的化学品。从这些物质间流过的雨水会流入地下水、湖泊、河流、井、池塘等等。

虽然许多有毒物质在水中具有较低的溶解度，但产生危害的浓度也很低。而且，许多有毒物质被活细胞累积下来，在细胞内受到的浓缩程度比在细胞外更甚。这就是为什么长期的接触这些物质是一个严重问题的原因。接触低浓度的有毒物质次数不多，可能没有危险。但是，饮用水里日复一日，年复一年地存在这些物质却是致命的。

堆滩浸出是一种逆流过程，其中固体是一个固定堆，溶剂从其间穿流而过。垃圾倾倒场或垃圾填埋场就是一个例子。工业沥滤中，将溶剂和固体混合，使其接近平衡，再将两相分离。液体和固体逆向移动到下一工序。被称为浸出物的溶剂相不断接触含有溶质的固体时，浓度增加。当淬余液流向新的溶剂相时，浓度降低。

大多数沥滤理论忽视了固体对物质的固定作用。虽然吸附和离子交换可将物质牢固地附着在固体上，但我们仍将沥滤简单地看作是从不溶性固体中将可溶性组分溶解去除。