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.

通過溶解將物質從固體中除去，這叫作瀝濾�；瘜W加工業用到瀝濾，但這個過程通常被稱作淬取，而且常常使用有機溶劑。因為瀝濾被用在分離金屬與礦石以及從甜菜中抽取糖方面已經有很多年，所以其理論和實踐已得到高度發展。近來，環境工程師給于瀝濾更大的關注，因為垃圾傾倒場和垃圾填埋場含有有害有毒廢料。一種有毒化學品的自然分解有時產生一種毒性更強的化學品。從這些物質間流過的雨水會流入地下水、湖泊、河流、井、池塘等等。

雖然許多有毒物質在水中具有較低的溶解度，但產生危害的濃度也很低。而且，許多有毒物質被活細胞累積下來，在細胞內受到的濃縮程度比在細胞外更甚。這就是為什么長期的接觸這些物質是一個嚴重問題的原因。接觸低濃度的有毒物質次數不多，可能沒有危險。但是，飲用水里日復一日，年復一年地存在這些物質卻是致命的。

堆灘浸出是一種逆流過程，其中固體是一個固定堆，溶劑從其間穿流而過。垃圾傾倒場或垃圾填埋場就是一個例子。工業瀝濾中，將溶劑和固體混合，使其接近平衡，再將兩相分離。液體和固體逆向移動到下一工序。被稱為浸出物的溶劑相不斷接觸含有溶質的固體時，濃度增加。當淬余液流向新的溶劑相時，濃度降低。

大多數瀝濾理論忽視了固體對物質的固定作用。雖然吸附和離子交換可將物質牢固地附著在固體上，但我們仍將瀝濾簡單地看作是從不溶性固體中將可溶性組分溶解去除。