NƯỚC CỨNG VÀ TÁC HẠI CỦA NƯỚC CỨNG

Chỉ số pH liên quan đến độ cứng và độ kềm của nước. Khi độ cứng và độ kềm ổn định, độ pH ít thay đổi. Nước tự nhiên chứa trên 3 mini đương lượng gam cation canxi (Ca2+) và magie (Mg2+) trong một lít. Nước nhiều Mg2+ có vị đắng. Tổng hàm lượng ion Ca2+ và Mg2+ đặc trưng cho tính chất cứng của nước. Ngày nay, người ta còn tính cả ion Fe2+ và Na+ vào độ cứng. Độ cứng của nước thiên nhiên dao động rất nhiều và đặc trưng lớn ở nước ngầm.

cấu tạo khoa học của nước cứng

Vôi cứng tạo mảng bám trên bề mặt làm hỏng các thiết bị chứa nước

1. Độ cứng của nước

Độ cứng của nước được quyết định bởi hàm lượng chất khoáng hòa tan trong nước, chủ yếu là do các muối có chứa ion Ca++ và Mg++. Độ cứng của nước được chia làm 2 loại:
Độ cứng tạm thời hay độ cứng carbonat: Tạo bởi các muối Ca và Mg carbonat và bicarbonat, trong đó chủ yếu là bicarbonat vì muối carbobat Ca và Mg hầu như không tan trong nước. Gọi là độ cứng tạm thời vì chúng ta có thể giảm được nó bằng nhiều phương pháp đơn giản. Trong tự nhiên, độ cứng tạm thời của nước cũng thay đổi thường xuyên dưới tác dụng của nhiều yếu tố, ví dụ như nhiệt độ …
Độ cứng vĩnh viễn: Tạo bởi các muối khác của Ca và Mg như sulphat, clorua… chỉ có thể thay đổi bằng các phương pháp phức tạp và đắt tiền.

Thông thường người ta chỉ quan tâm đến độ cứng tạm thời của nước vì nó có ảnh hưởng nhiều hơn là độ cứng vĩnh viễn. Có nhiều đơn vị đo độ cứng khác nhau, nhưng chủ yếu người ta dùng 3 đơn vị đo: độ dH, mg đương lượng/lít và ppm. Để đơn giản, khi đo độ cứng người ta thường quy về 1 loại muối là CaCO3.

Nước có độ cứng tạm thời lớn hơn 100 ppm được coi là nước cứng, dưới mức đó được coi là nước mềm.

2. Tác hại của nước cứng

Độ cứng vĩnh viễn của nước ít ảnh hưởng đến sinh vật trừ phi nó quá cao, ngược lại, độ cứng tạm thời lại có ảnh hưởng rất lớn. Nguyên nhân là vì thành phần chính tạo ra độ cứng tạm thời là các muối bicarbonat Ca và Mg: Ca(HCO3)2 và Mg(HCO3)2, chúng là các muối hòa tan hoàn toàn nhưng không ổn định, không bền. Chúng dễ dàng bị phân hủy thành CaCO3, MgCO3 là các muối kết tủa:
Ca(HCO3)2 => CaCO3 + H2O + CO2
Mg(HCO3)2 => MgCO3 + H2O + CO2
Khi phản ứng phân hủy xảy ra trong cơ thể sinh vật, các muối này kết tủa trong cơ thể sinh vật sẽ gây hại không nhỏ. Ở con người, chúng là nguyên nhân gây ra sỏi thận và một trong các nguyên nhân gây tắc động mạch do đóng cặn vôi ở thành trong của động mạch. Lưu ý là các muối CaCO3 và MgCO3 là các muối kết tủa và chúng không thấm qua niêm mạc hệ tiêu hóa của chúng ta được, chỉ các muối hòa tan mới thấm được thôi. Vì vậy nước cứng chỉ có tác hại do các muối bicarbonat.

3. Các phương pháp làm mềm nước

Có rất nhiều phương pháp làm giảm độ cứng của nước, từ đơn giản đến phức tạp.

a. Làm nóng nước
Đun nóng nuớc sẽ làm giảm đáng kể độ cứng của nước

b. Làm nước lưu động liên tục
Khuấy liên tục hoặc bơm tuần hoàn liên tục cũng có tác dụng, tuy rằng khá chậm và trong nhiều trường hợp, sự phân hủy bicarbonat chậm hơn sự hòa tan bicarbonat mới từ các nguồn khác vào nước

c. Chưng cất nước
Về nguyên tắc, nước cất có thể coi là H2O tinh khiết hoàn toàn.

d. Lọc RO (Thẩm thấu ngược)
Công nghệ lọc RO cho phép loại bỏ gần như tất cả các chất hòa tan và không hòa tan ra khỏi nước, nước lọc RO có thể coi là H2O tinh khiết (tuy không bằng nước cất)

e. Trao đổi ion bằng sử dụng máy làm mềm nước
Đây là phương pháp được dùng phổ biến nhất vì có giá thành rẻ kể cả chi phí đầu tư lẫn chi phí vận hành. Nguyên lý của nó là đưa nước qua 1 vật liệu chứa các ion dương hoạt động mạnh hơn Ca++ và Mg++, vật liệu này sẽ hấp thụ các ion Ca++ và Mg++ trong nước và nhả ra các ion mạnh hơn kia, do đó tạo ra các hợp chất carbonat không kết tủa. Vật liệu đó được gọi là Cationit (hay cation-exchange resyn). Thông thường người ta dùng 2 loại cationit là Na-Cationit và H-Cationit tương ứng với các ion là Na+ và H+ và các hợp chất tạo ra tương ứng là Na2CO3 và H2CO3 (H2CO3 sẽ bị phân tích ngay thành H20 và CO2)

Khi các Cationit đã hết khả năng trao đổi, người ta phải “hoàn nguyên” tức là phục hồi lại các ion dương cho nó. Đối với Na-Cationit người ta dùng muối ăn NaCl, đối với H-Cationit người ta dùng axit.

Thiết bị trao đổi Na-Cationit thông thường có thể hạ độ cứng của nước xuống đến dưới 10 ppm, nếu được thiết kế đặc biệt có thể hạ xuống dưới 2 ppm. Mỗi lít hạt Na-Cationit có khả năng trao đổi khoảng từ 2-6 gam đương lượng tùy loại, tức là có thể làm hạ độ cứng từ 300 đến 1000 lít nước có độ cứng 6 mgdl/lít (300 ppm hay 16,8 dH) xuống 0 trước khi phải hoàn nguyên.

H-Cationit có khả năng trao đổi mạnh hơn Na-Cationit và cũng triệt để hơn vì nó loại bỏ hoàn toàn gốc carbonat ra khỏi nước. Tuy nhiên nó có giá thành cao hơn nhiều lần, chi phí vận hành cũng cao hơn (axit đắt hơn muối ăn) và yêu cầu vận hành cũng nghiêm ngặt hơn do phải dùng axit nên ít được sử dụng

Ngoài cách trao đổi ion dương người ta còn dùng cả trao đổi ion âm (chất trao đổi gọi là Anionit) để loại bỏ các ion HCO3- ra khỏi nước. Phương pháp này thường chỉ được sử dụng trong các nhà máy nhiệt điện, ở đó họ dùng cả Na-Cationit, H-Cationit và Anionit nối tiếp nhau trong 1 hệ thống.

Phương pháp trao đổi ion này được sử dụng rất rộng rãi và là phương pháp cơ bản để làm mềm nước trong công nghiêp. Kể cả đối với các thiết bị RO, để tăng tuổi thọ và giảm tải lên các thiết bị RO, người ta cũng làm mềm nước bằng trao đổi ion trước khi đưa nước vào lọc RO.

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